Institute of research into the fundamental laws of the Universe

The Euclid satellite, to be launched in 2022, will observe the sky in the optical and infrared, and will be able to map large scale structures and weak lensing distortions out to high redshifts. Weak gravitational lensing is thought to be one of the most promising tools of cosmology to constrain models. Weak lensing probes the evolution of dark-matter structures and can help distinguish between dark energy and models of modified gravity. Thanks to the shear measurements, we will be able to reconstruct a dark matter mass map of 15000 square degrees. Mass mapping entails the construction of two-dimensional maps using galaxy shape measurements, which represent the integrated total matter density along the line of sight. Small- field mass maps have been frequently used to study the structure and mass distribution of galaxy clusters, whereas wide-field maps have only more recently become possible given the broad observing strategies of surveys like CFHTLenS, HSC, DES, and KiDS. Mass maps contain significant non-Gaussian cosmological information and can be used to identify massive clusters as well as to cross-correlate the lensing signal with foreground structures.

A standard method to derive mass maps from weak-lensing observations is an inversion technique formulated by Kaiser & Squires [2]. It has many limitations, however, including the need to smooth the data before (and often after) inversion, thereby losing small-scale information. An alternative method called GLIMPSE has been developed in the CosmoStat laboratory based on sparse reconstruction that avoids this problem and improves the recovery of non-Gaussian features [3, 4]. The algorithm has been tested on simulations and was also recently used to study the A520 merging galaxy cluster with Hubble Space Telescope data [5]. More recently, machine learning has emerged as a promising technique for mass map recovery [6].

The goal of this thesis is to i) compare this technique to the state of the art and investigate if it can be used in practice, ii) extend the method for spherical data, and iii) develop a new machine learning approach to estimate the cosmological parameters. At the core of this new statistical framework will be the development of fast and differentiable cosmological simulations capable of emulating the Euclid survey under various cosmologies. This simulation tool will be based on the FastPM N-body simulation code [7] and implemented directly in the TensorFlow machine learning framework, yielding a differentiable physical forward simulation pipeline which can be directly interfaced with deep learning components or with inference techniques relying on having access to the derivatives of the simulation.

As part of the CosmoStat Laboratory, located at CEA Saclay, the successful candidate will be embedded in a leading French research group, heavily involved in the preparation of the Euclid space mission, and with a long tradition of developing cutting-edge statistical tools for the analysis of astronomical and cosmological data.

1. Bartelmann, M. & Schneider, P. 2001, Phys. Rep., 340, 291. ?

2. Kaiser, N. & Squires, G. 1993, ApJ, 404, 441. ?

3. Leonard, A., Lanusse, F., & Starck, J.-L. 2014, MNRAS, 440, 1281.

4. Lanusse, F., Starck, J.-L., Leonard, A., & Pires, S. 2016, A&A, 591, A2.

?5. Peel, A., Lanusse, F., & Starck, J.-L. 2017, ApJ, 847, 23.

6. Niall Jeffrey et al, submitted. https://arxiv.org/abs/1908.00543

7. Y. Feng, M. Yat Chu, U. Seljak, and P. McDonald. MNRAS, 463(3):2273–2286, 2016.

Are cosmic rays actors or passengers in galaxy evolution? In the current models of galaxy evolution stars form too efficiently and too early in the history of the Universe. High-energy processes such as jets from supermassive black holes and supernova explosions can modify how the gas and magnetic fields cycle in and out of a galaxy, but their impact fails to explain key observations such as galactic outflows. Cosmic rays can play a particular role in galaxy evolution as they mediate energy transfers from supernovae to the interstellar medium over thousands of parsecs and tens of millions of years around their source. They also increase the gas buoyancy and add anisotropic pressures along magnetic field lines and off galactic discs. To evaluate their impact, it is central to understand how cosmic rays propagate through a galaxy and how their transport properties vary with the ambient interstellar conditions. To gain insight into this problem, we propose to compare for the first time the distribution of cosmic rays obtained in numerical simulations of interstellar clouds with measurements obtained from multi-wavelength observations in comparable regions of the Milky Way. A team of well-known experts in the Astrophysics Department will advise the PhD student on high-performance computing simulations and on multi-tracer observations of the interstellar medium, magnetic topology, and cosmic rays. He or she will also work within the broad international collaboration for the Fermi Gamma-ray Space Telescope.

Space telescopes focusing hard X-rays up to 100-200 keV would bring breakthrough in our understanding of the most violent and energetic phenomena of the Universe, like those in the Active Galaxy Nuclei, the supernovae, or closer to us in the solar flares. Techniques of super-mirrors are emerging for that prospect. In parallel, technological developments in rupture shall be led to realize large focal planes with high pixel density and efficient up to 200 keV to be placed at the focal plane of these optical systems. This PhD thesis in space instrumentation consists in setting up and studying innovative hybrid detectors for the imaging spectroscopy in the 1-200 keV energy range, based on 250 µm-pitch pixelated cadmium telluride (CdTe) semiconductor detectors, point to point connected to spectroscopic readout channels of dot-matrix application specified integrated circuits (ASIC) designed in our institute. By means of experimental characterizations coupled to modern data analysis methods, and to tests in accelerators coupled to numerical simulations, the candidate will demonstrate and optimize the spectral resolution, the spatial resolution and the counting capabilities of these new devices.

Very recently a fundamentally new domain of astronomy and astrophysics has shown its first results: multi-messenger and real-time astrophysics. The simultaneous detection of various new astrophysical messengers (gravitational waves, high-energy gamma rays and high-energy neutrinos) and the exchange and combination of data from very different observatories allows to open new windows and provides unprecedented insights into the most violent phenomena ever observed.

New and significant conclusions can be obtained by combining these new messengers. Joint analyses of archival observations in different wavelengths have brought enormous insights in the past and, as this technique provides an assured and certain scientific return, it will also be used in the proposed thesis project. At the same time it has becomes clear that another important step does greatly enhance the sensitivity of multi-messenger searches: the need to gain full access to the wealth of information provided by analyzing and combining the data in real-time. The proposed thesis project will allow opening this new window to the high-energy universe: real-time multi-messenger astronomy at very high energies. The combination of the various particles and radiations in a truly multi-messenger online alert system will resolve several challenges faced in high-energy astrophysics and especially allow detecting and studying violent transient phenomena that are supposed to be at the origin of high-energy cosmic rays. The project will introduce the time domain to high-energy astrophysics and has the potential to cause a paradigm shift in how observations and data analyses are performed.

The core of the proposed project will be H.E.S.S., currently the world’s most sensitive gamma-ray instrument, and CTA, the next generation, global high-energy gamma-ray observatory. We’ll combine their data with events recorded by IceCube, the world’s largest neutrino telescope and the advanced Virgo and Ligo gravitational wave interferometers. The detection of a transient high-energy gamma-ray source in coincidence with gravitational waves or high-energy neutrinos will provide the long sought evidence for their common origin and may resolve the century old quest for the origin of high-energy cosmic rays.

We’ll also collaborate with the world’s most sensitive radio observatories (e.g. the SKA precursors MeerKAT and ASKAP) to search for counterparts to Fast Radio Bursts and in general study a large variety of messengers like Gamma-Ray Bursts or flares from active galactic nuclei. By scanning the data acquired with high-energy gamma-ray observatories in real-time, it will also possible to send alerts to the wider astronomical community to ensure simultaneous observations at other wavelengths.

Very high energy gamma ray astronomy observes the sky above a few tens of GeV. This emerging field of astronomy has been in constant expansion since the early 1990s, in particular since the commissioning of the H.E.S.S. array in 2004 in Namibia. IRFU/CEA-Paris Saclay is a particularly active member of this collaboration from the start. It is also involved in the preparation of the future CTA observatory (Cherenkov Telescope Array), which should come into operations by 2024. The detection of gamma rays above a few tens of GeV makes it possible to study the processes of charged particles acceleration within objects as diverse as supernova remnants or active galactic nuclei. Through this, H.E.S.S. aims in particular at answering the century-old question of the origin of cosmic rays.



HESS allows measuring the direction, the energy and the arrival time of each detected photon. The time measurement makes it possible to highlight sources which present significant temporal or periodic flux variations. The study of these variable emissions (transient or periodic), either towards the Galactic Center or active nuclei of galaxies (AGN) at cosmological distance allows for a better understanding of the emission processes at work in these sources. It also helps characterizing the medium in which the photons propagate and testing the validity of some fundamental physical laws such as Lorentz invariance. It is possible to probe a wide range of time scales variations in the flux of astrophysical sources. These time scales range from a few seconds (gamma ray bursts, primordial black holes) to a few years (binary systems of high mass, active galaxy nuclei).

One of the major successes of the first decade of data collection of H.E.S.S. was to conduct the first Galactic Plan survey of sources in this energy range. This survey, comprising more than 10 years of data, combines observations dedicated to known sources, such as the Galactic Center or some supernova remnants, as well as blind observations for the discovery of new sources. The subject of the thesis proposed here deals with one aspect of the study of the Galactic plane that remains to be explored: research and study of the variability and periodicity of gamma-ray sources throughout this dataset.

X-ray data are multidimensional by nature. For each photon the energy and position is recorded by the X-ray satellite. Here we propose to develop novel techniques to fully exploit the multidimensional nature of the data by combining blind source separation technique with feature learning.

The intergalactic magnetic field pervading the cosmic voids is suspected to be a relic field originating from the very first epoch of the cosmic history. The goal of this PhD is to look for signatures of this field in the high-energy data of gamma-ray bursts, and to predict the ability of the future CTA observatory to constrain its properties. This work combines both theoretical modelling and analysis of simulated CTA data.

The discovery, by the LIGO-Virgo collaboration on Sept. 14th 2015, of gravitational waves (GW) from the merger of two stellar-mass black holes, applauded by the whole scientific community, was unexpected in terms of astrophysical sources: two such heavy stellar-mass black holes (~30 solar masses) had never been seen before, although they likely constitute the tip of the iceberg. From this detection, several questions immediately arose: how can such black holes form, and how many are there in our local Universe and beyond? The second breakthrough came with the detection of a kilonova associated with the merger of two neutron stars, on Aug. 17th 2017. Further questions arose, such as the nature of the outcome of such a merger. More generally, one of the most fundamental questions in terms both of astrophysics and physics, concerns the nature of the progenitors for this type of system. Finally, we now know that many such mergers will be detected by current and future GW observatories, but we do not know the exact rate.



Stellar binaries hosting compact objects (especially neutron stars and black holes) constitute the best progenitors, evolving until eventually merging in binary black holes (BBH), binary neutron stars (BNS) or black hole and neutron star binaries (BH/NS), and emitting GW. The overall evolution of such binaries is still subject to many uncertainties about some parameters of binary evolution, such as: the natal kick received during each supernova event, metallicity effect on stellar wind, common envelope phase, which condition the survival of the stellar binaries, the spin of each component, etc…

Galaxy clusters, located at the node of the cosmic web, are the largest gravitationally bound structures in the Universe. Their abundance and spatial distribution are very sensitive to cosmological parameters. Galaxy clusters thus constitute a powerful cosmological probe. They have proven to be an efficient probe in the last years (Planck, South Pole Telescope, XXL, etc.) and they are expected to make great progress in the coming years (Euclid, LSST, CMB-S4, etc.).



Theoretical predictions of the cluster abundance and spatial distribution depend on cosmological parameters and cluster mass. To determine cosmological parameters from cluster surveys, one needs to be able to measure accurately cluster mass. The error on the mass estimation is currently the main systematic error for the determination of cosmological parameters with galaxy clusters. This is the reason why it is crucial to improve on the measurement of the cluster mass and to control associated errors.



The most direct method to measure cluster mass is based on gravitational lensing. It is now used routinely in optical surveys: a cluster induces distortions of the shapes of background galaxies. Using these distortions, it is possible to reconstruct cluster mass. It was shown recently that it is also possible to detect these distortions at millimetre wavelengths in the cosmic microwave background (CMB) instead of using background galaxies, and reconstruct the mass of galaxy clusters. The main advantage of using the cosmic microwave background is because it is located at very high distance allowing for mass measurement of distant clusters; it is not possible to do this measurement with background galaxies, which are too few for distant clusters.



Irfu/DPhP has developed the first tools to measure galaxy cluster masses using gravitational lensing of the cosmic microwave background for the Planck mission. The PhD thesis work will consist in taking hands on the tools and improve them to make them compatible with ground-based data. They will then be applied to public SPT-SZ (https://pole.uchicago.edu) and SPT-SZ+Planck data jointly.



In the second part of the thesis, the tools will be used to find observation strategies and compute integration times to measure cluster masses for high resolution ground based experiments such as NIKA2 (http://ipag.osug.fr/nika2/), alone and jointly with Planck.



The current methods are optimal for maps in total intensity and in the low signal-to-noise regime. The future experiments will have lower noise levels and will be very sensitive to polarization. The third part of the thesis will be dedicated to development of new methods to extract the masses for the future low noise cosmic microwave background experiments such as CMB-S4 (https://cmb-s4.org), PICO (arXiv:1902.10541) or CMB Backlight (arXiv: 1909.01592).



Finally, we will study the precision on cosmological parameters that can be reached from galaxy cluster catalogues, given the precision on the mass expected from these future experiments.

JWST will be launched in 2021, with an important participation from ESA, CNES and CEA for the mid-infrared instrument MIRI. JWST capabilities are revolutionary, compared to the existing state of the art, in terms of resolution and sensitivity over the 1- 30?m wavelength range, where stars and (warm) dust emit their light from galaxies at high redshift. For the first time JWST will provide spatially resolved photometry up to the mid-IR (at least 10?m, with NIRCAM and MIRI) with sub-arcsec resolution. The competitive exploitation of the data for scientific endeavors will require the mastering of the data, deeply understanding the reduction, treatment and developing tools to foster the analysis. I propose a PhD thesis in Saclay as a collaborative effort between experts from ’MICE, the Centre of Expertise for MIRI’, developed at CEA/Irfu/DAp, and with researchers in galaxy formation and evolution. The student will be responsible for developing new high level software for the analysis of resolved imaging data from MIRI and NIRCAM, modeling and understand the resolution, ’pixelization’ and PSF convolution effects. This will include high-level software to create spatially resolved maps of physical parameters (stellar mass, dust attenuation, stellar age, star formation rate) and pixel-by-pixel spectral energy distributions. The student will work on testing and improving the existing MIRI simulator, adapting it to the case of resolved observations of distant galaxies. The results of the efforts will be shared with several of the CEA Saclay groups in the spirit of fostering our expertise and efficiency in the early use of the groundbreaking JWST data. This work will be based on data from our recently approved Early Release Science (ERS) project observing with a suite of JWST instruments (NIRCAM, NIRSPEC, and MIRI) on well-studied cosmological fields. This ERS project is lead by S. Finkelstein at the University of Texas and includes E. Daddi and D. Elbaz from CEA-Saclay among the international teams of proposers. These observations will be among the first delivered by JWST, in parallel with those from GTO teams.

The student will ultimately use the Early Release Science data on cosmological fields to search for ongoing hidden merger events and AGN components resolved inside galaxies, by distinguishing them from the whole galaxy (e.g., nuclear events, or similar), and constraining the growth of inner bulges with passive and/or active stellar populations. This research is based on recent discoveries from our team at the peak of galaxy formation z=1-4. Eventually this research will lead to the first realistic estimates of the relevance of these widely discussed and hot topics.

Stellar couples are very common in our Galaxy: more than 70% of massive stars live as a couple during their stellar life. This PhD-Thesis aims at studying how these systems form, evolve and have an impact on their environment.



Massive stars live in couples...

Several revolutions have occurred in recent years in the stellar domain. The first is the realization that most (over 70%) massive stars live within a stellar pair (Sana et al., 2012). This binarity has major consequences on the evolution of stars, strongly influenced by the presence of a "companion", particularly via the transfer of matter and kinetic momentum (Chaty 2013). The fate of these stellar pairs is determined by the evolution of each component, with the most massive star collapsing first during the supernova explosion, giving rise to a neutron star or a black hole (Tauris et al. 2017). A stellar couple, composed of a compact star orbiting its companion, is among the most fascinating celestial objects of our Universe. The companion star, massive, is characterized by an ejection of wind more or less intense according to its metallicity, and the compact star, bathed in this wind, attracts a part of this matter, which, accreted, accumulates to the surface, heated to temperatures of several million degrees, emitting mainly in the field of X-rays. These stars regularly give rise to extreme variations in luminosity, several orders of magnitude over the entire electromagnetic spectrum, on scales time from the second to the month.



... until they merge ...

The second revolution is the detection, by interferometers of the LIGO / Virgo collaboration, of gravitational waves coming from the fusion of two black holes (first detection in September 2015) and two neutron stars (August 2017). This fusion occurs at the end of the life of certain stellar pairs, depending on their mass, their orbital separation, and several other parameters involved in their evolution. The fusion of neutron stars is accompanied by an emission of electromagnetic waves, called kilonova, and spectroscopic observations have shown that heavy atoms were created during this event, via the "fast process" of nucleosynthesis (r-process).



... with an impact on their environment!

It is now established that the collapse of massive supernova stars plays a key role in the enrichment of the interstellar medium - from heavy atoms to complex molecules - and in triggering the formation of new stars. On the other hand, the impact of the wind of these massive stars on their environment, throughout their life, was long neglected. However, this ejected material disperses in the surrounding environment, until it collides with a dense interstellar medium, potentially triggering new star formations, as suggested by observations from the Herschel satellite (Chaty et al. 2012). Finally, the recent observations of r-process concomitant with the detection of a kilonova show that the fusion of two neutron stars is an important (or even majority) element of nucleosynthesis in the galaxy.



This PhD-thesis, covering various fields of astrophysics, proposes to study how these formidable couples of massive stars form, whose role is primordial in the cycle of matter, how they evolve, and what is their impact on their environment, based on multi-wavelength observations (ESO, Gaia...).

Gamma bursts, discovered by chance in the late 1960s, are the most violent explosions in the universe. Their study is complex because it requires the deployment in space of a gamma telescope to detect and locate them. All the data collected made it possible to establish a global scientific scenario, the main lines of which are as follows: gamma-ray bursts are stellar explosions that result in the formation of a black hole and the ejection of material jets propelled at speeds very close to that of light. When the jet is directed towards the Earth, an observer sees an extremely bright source, which decreases rapidly over time. A typical gamma-ray burst includes a prompt emissive phase that lasts a few seconds, followed by a persistent emission when the jet hits the surrounding environment violently.

The main objective of the Franco-Chinese SVOM mission, which will be launched at the end of 2021, will be to establish a sample of 30 to 40 gamma-ray bursts per year with as complete a description as possible. Thanks to its instruments deployed in space and on the ground, for the first time, the prompt emission of the burst will be observed over more than three decades in energy and the associated persistent emission will be studied in X, visible and near infrared.

The proposed thesis topic consists in jointly studying, from the catalogue of GRB detected by the SVOM mission, the prompt phase and evolution of remanence over a period of a few days. The interpretation of these observations will provide us with information on the nature of the jet, the acceleration of particles, the production of radiation in the jet and in the surrounding environment shocked by the jet, the properties of the same medium (stellar wind, interstellar medium, molecular cloud) and the nature of the star that exploded.

The recent direct detections of gravitational waves (GW) from mergers of

massive compact objects has opened a new window to our Universe. The

electro-magnetic (EM) counterpart of the event GW170817 started a new

multi-messenger era for astronomy. Joint GW and EM observations provide a way

to better understand the physics and rate of violent processes of black hole

and neutron star mergers, and the properties of their host galaxies and stellar

populations.



To identify GW transients via quick follow-up observations across the EM

spectrum, galaxy surveys from ultra-violet (UV), optical, to infrared (IR)

wavelengths are of great importance. This PhD project will explore the synergy

and complementarity of two upcoming space missions, the ESA satellite Euclid

(launch in 2022), and CSST, the Chinese Space Station Telescope (planned for

2024). Both missions will cover a large fraction of the extra-galactic sky

with a common area of 15,000 deg^2.

Gaseous giant planets like Jupiter and Saturn in our solar system and “hot” Jupiters orbiting around other stars with very short periods are complex and fascinating objects. Indeed, they are turbulent rotating magnetic bodies that have strong interactions with their environment: their moons in the case of Jupiter and Saturn and their host stars in the case of “hot” Jupiters/Saturns. In such gaseous giant planets’ systems, tidal forces, the tidal waves they excite and their dissipation shape the orbital architecture and the rotational dynamics of the planets. During the last decade, several revolutions have occurred for our understanding of tides in these systems. On the one hand, high precision astrometry and the CASSINI (NASA/ESA) space mission have measured dissipation stronger by one order of magnitude than expected in Jupiter and Saturn. On the other hand, the large space-based photometric surveys Kepler/K2 and now TESS (NASA) are observing a broad diversity of orbital architecture for exoplanetary systems while “hot” gaseous giant planets seems to host a weakest tidal dissipation than Jupiter and Saturn. Finally, the space mission JUNO (NASA) and the grand finale of CASSINI have revealed the internal structure and dynamics of Jupiter and Saturn: the intense zonal flows observed at their surface are confined in their external layers because of the action of the magnetic field in their deepest regions while the heavy elements contained in their core are mixed in their deep gaseous envelope that modifies the global structure of the planet. The objective of this PhD project is thus to build the new coherent models of tidal dissipation in gaseous giant (exo-)planets mandatory to understand the evolution of their systems. They will take into account all these complex phenomena and new observational constraints. These models will be applied to predict the evolution of planetary systems in support of ongoing and forthcoming space missions in which the CEA-IRFU Department of Astrophysics is strongly involved (JWST, PLATO, ARIEL).

One of the major challenges of astrophysics is to understand how galaxies assemble their mass, give birth to their supermassive stars and black holes over time, and how this assembly depends on their internal evolution and/or external factors such as dark matter haloes and galactic fusions. To date, our understanding of the cosmic history of star formation remains largely incomplete over the key period of the 3 billion years following reionization, i.e. between z~6 - when galaxies had formed less than 1% of their current stars - until the time of the star formation peak around z~1.5. ?During this thesis, the student will benefit from a unique set of data - mainly from the ALMA interferometer and the James Webb Space Telescope (JWST) - that will allow him to quantify this period in the history of star formation activity, mass growth and the morphological evolution of galaxies. Overcoming the uncertainty about this era will therefore have a major impact on our understanding of the formation of structures in the universe, and even our cosmology, and this is one of the major objectives of our JWST program.

The study of exoplanets is booming. Since the detection of the 1st exoplanet in 1995 by Mr. Mayor and D. Queloz (2019 Nobel Prize in Physics), more than 4000 exoplanets have been detected. The domain is now faced with a new challenge: the characterisation of the atmosphere of exoplanets. Knowledge of the atmosphere brings unique information to constrain the formation and evolution of the exoplanet, its interior, even the presence of biological activity, etc. This characterisation will take a considerable step forward with the launch of two space missions: the JWST in 2021 and the ARIEL mission, entirely dedicated to exoplanet atmospheres, in 2028. The atmosphere is studied from spectroscopic infrared observations; the level of instrumental stability required for these studies is very high (up to 10 ppm over 10 hours).

The JWST was not designed to have the required stability. During his/her thesis the student will determine the stability in flight of the JWST MIRI instrument, to which CEA has made a strong contribution, will compare it with the predicted one and will analyse different methods to improve the stability during data reduction. CEA is also strongly involved in the ARIEL mission (mastery of the main instrument of ARIEL: the AIRS infrared spectrometer; realization and testing of the detection chain). The student will participate in the studies of the instrumental stability (laboratory tests of the detection chain, analysis of results, determination of the best operating modes, system analysis) in order to maximize the instrumental stability upstream to the launch.

Key words : space missions, infrared detectors, exoplanets

Astrophysical research requires the development of very high performance cameras embedded in space observatories. The observation of the universe in the X-ray range (X-ray spectro-imagery) needs detectors made of matrices of micro-calorimeters operating at very low temperature (50 mK). The absorption by the detector of an X-ray photon coming from the observed celestial object causes a micro-rise in the temperature of the detector. The measurement of this temperature rise, which makes possible to determine the energy of the photon, requires ultra-sensitive micro-thermometers, and a cryogenic electronics, with very low noise, capable of reading them.

Two technologies of thermometers have been used so far : high-impedance silicon-doped metal insulator sensors (MIS), and very low impedance transition edge sensors (TES). Each requires a very specific electronics, either based on HEMT transistors for adapting to high impedances, or based on SQUIDs for adapting to very low impedances. The high impedances have the advantage of an extremely reduced heat dissipation on the detection stage, which allows a large number of pixels, while the very low impedance TES, more sensitive than the MIS, make easier to obtain excellent spectral resolutions.

A few years ago, a new type of thermometer has been developed by the CNRS/CSNSM : this is high impedance TES, potentially allowing to combine the advantages of one and the other types of detectors. A first thesis was carried out in our laboratory (2016 - 2019), with the aim of evaluating this new path by implementing it for the first time, and by associating it with an innovative readout electronics architecture that performs an active electro-thermal feedback. This thesis has highlighted the extremely promising nature of the device, by obtaining very interesting first experimental measurements.

The objective of the new thesis, proposed here, is to continue this exploratory work by going one new major stage further : validate from this new technology the feasibility of a matrix of several thousand pixels. For this, the work will focus on two parallel axes : on the one hand carry out a complete work of improvement and optimization, in order to draw from the device its best performances, and on the other hand design and test the integrated electronic system (ASIC) essential for the realization of the future large matrices.

The main difficulty lies in the conditions of implementation of the system : the detector must be placed in a cryo-generator to be cooled to very low temperature (50 mK), and equipped with a cryogenic electronics, to be designed, operating at 4 K. This one will have to ensure not only the multiplexing and the amplification of the signal but also, despite this multiplexing, the maintenance of the active electro-thermal feedback of the detectors, and this while satisfying the extremely severe noise and thermal dissipation constraints required by space cryogenics.

Euclid is the major cosmological mission led by the European Space Agency, planned to be launched in 2022. The AIM laboratory, initiator of the project, occupies a number of key Euclid positions in management, instrumental development, ground segment and related science. The sensitivity of Euclid should allow blind detection of clusters through their lensing signal i.e. directly through their total projected mass. Combined with the sky coverage, this will allow the construction of a significant galaxy cluster catalogue that is for the first time truly representative of the true cluster population. Indeed, up to now all galaxy cluster catalogues rely on detection through their baryonic signal (e.g. through the intra-cluster gas content in X-rays and the Sunyaev-Zeldovich effect (SZE) at millimetre wavelengths, or through the optical light in the galaxies). This will provide new constraints on galaxy cluster abundances in the Universe, which has important implications for cosmology. In this context, AIM is also deeply involved in the ongoing CFIS survey (PI: Jean-Charles Cuillandre) that has to provide some of the ground-based data necessary for the Euclid mission. CFIS data in hand are sufficient for testing blind detection of clusters through lensing signal particularly at high masses. AIM is also deeply involved in the ongoing XMM-Heritage project (PI: Monique Arnaud) that is a multi-year programme to obtain X-ray observations with XMM-Newton of 118 SZ-selected galaxy clusters at 0.05 < z < 0.6. A key project goal is to obtain X-ray data with homogeneous quality for the first time for such a large number of objects. Crucially, object selection was tailored specifically to the CFIS and Euclid survey areas, allowing comparison of clusters detection methods through both dark and baryonic signals. The thesis project takes place in this stimulating context and aims at exploring innovative methods to build a weak lensing selected galaxy cluster catalogue. This will be undertaken using initially numerical simulations, and subsequently CFIS data. Scientific exploitation will be enhanced by combining the resulting catalogue with the XMM-Heritage sample. The ultimate goal will be to apply the methods to Euclid.

Measurements of the statistical properties of the large-scale structure (LSS) of the Universe can provide information about the physics that generated the primordial density fluctuations. In particular, they offer the possibility to distinguish between different models of cosmic inflation by measuring primordial non-Gaussianity (PNG), the deviation from Gaussian random field initial conditions.



Our plan to study the PNG is to use a spectroscopic survey, DESI, starting its observations in fall 2020. The LSS will be measured with two different tracers of the matter : Emission Line Galaxies (ELG), which are star-forming galaxies and quasars. These two tracers allow us to cover a large redshift range from 0.6 to 2.5.



DESI will perform a 3D survey of tens of millions of galaxies and quasars in 5 years over 14 000 squared degrees. The observations will take place at the 4-m Mayall telescope in Arizona.



During its first year of thesis, the PhD student will participate to the commissioning of the new instrument and to the survey validation. In particular, he/she will be in charge of the validation of the ELG and quasar target selection. Then he/she will study the correlation function at large scale of these tracers in order to measure the PNG. With the first year of DESI, we should achieve an sensitivity better than all the previous measurements with LSS.

We propose a PhD thesis which builds on the tools and expertise available within the lab, and aims at providing key Tools and results that will be used for the Euclid collaboration and beyond.

The hired PhD candidate within this project will be at the interface between theory and observations to get the best scientific return out of the big investment done in space missions like Euclid, in particular in Europe and by CNES.

The main objectives are:

1) learn how to use existing XC codes (such as COSMOSIS, developed by Martin Kilbinger) and use available data (such as real or simulated data for Euclid) to test modified gravity models beyond LCDM (with supervision of Valeria Pettorino, expert in the field);

2) investigate how large the contribution of XC with spectroscopic galaxy clustering would be, potentially using 3D WL (for which a code has been validated by A. S. Mancini & V. Pettorino);

3) investigate synergies with other probes, such as data from BOSS/eBOSS (of which Vanina Rulhmann-Kleider is expert) and the Cosmic Microwave Background from Planck (of which V.Pettorino is a CORE2 team member and Planck scientist) or next to come ground space / balloon experiments which will provide (during the time of the PhD) polarisation spectra with a better resolution at small scales.

The formation of stars is a fundamental aspect of the evolution of the Universe. Strangely it is still poorly understood as it results from an intricate combination of complex physical processes : several instabilities (dynamical, chemical, thermal), magneto-hydrodynamic turbulence, gravity and energy injection by stars themselves. Because of this complex, multi-scale and multi-phase physics, this problem is now study using numerical simulations of increasing sophistication. Important progress has been made to a point that we are now in a situation where there is a lack of constraints from observations to identify the relevant physical scenarios. This is in part due to the difficulty to find the right metric to compare numerical simulations and observations. In this context, machine learning tools open new ways of exploration. In particular they allow the automatic definition of new comparison and evaluation metrics making possible the direct comparison of observations and simulations.

The subject of this Ph.D. thesis is to define a framework that would allow to estimate physical parameters (magnetic field intensity, temperature distribution, density distribution, power spectra of density and velocity) of different regions of the interstellar medium by applying machine learning tools on hyper-spectral observations (21 cm and CO). The setup of the tools would be first done by a training on a set of numerical simulations that are already available. This project is made possible thanks to a combination of expertise present in the Hyperstars collaboration, a joint effort by experts of the star formation process (Marc-Antoine Miville-Deschênes - hyper-spectral data and Patrick Hennebelle - numerical simulations) and experts of several aspects of data science.

Quasar Lyman-alpha forests probe the hydrogen density along the lines of sight of the quasars, providing a measure of the baryonic acoustic oscillation (BAO) scale in the hydrogen correlation function. This BAO scale is a standard ruler that provides a measurement of the expansion rate of the Universe, therefore constraining dark energy models. The BAO scale was measured for the first time in the Lyman-alpha forest by SDSS III / BOSS.

The DESI survey should be three times more accurate than SDSS IV. Data taking begins in 2020 and will end in 2025. The doctoral student will participate in the measurement and in the study of quasar spectrum simulations to estimate systematic effects. Then the BAO scale should be extracted from the correlation function and the result used to study dark energy models.

The student will be in a favorable situation, since the French teams (LPNHE and CEA Saclay) play a leading role in the Lyman-alpha analysis in SDSS and DESI. He will develop his knowledge and capabilities in cosmology, statistics, data analysis and fitting, systematic effects studies, and computer science: Python and possibly C and C++, dealing with a large amount of data and use of CPU farms. He will present his work in English in weekly teleconferences and collaboration meetings in USA and Europe.

The theoretical description from first principles, i.e. in a so-called ab initio fashion, of atomic nuclei containing more than ~12 nucleons has become possible only recently thanks to crucial developments in many-body theory and the availability of increasingly powerful high-performance computers. Such ab initio techniques are being successfully applied to study the structure of nuclei starting from the lighter isotopes and reaching today mid-mass nuclei containing up to about 80 nucleons. The extension to even heavier systems requires decisive breakthroughs regarding the storage and running time induced by any of the available many-body methods. In this context, the goal of the thesis is to formulate and apply the recently proposed Importance Truncation (IT) techniques within the frame of Gorkov Self-consistent Green’s function calculations, a specific ab initio technique devised at CEA Saclay over the last 9 years, as a way to select a priori and systematically many-body basis states that do contribute significantly to many-body correlations. The project will exploit the latest advances in nuclear theory, including the use of nuclear interactions from chiral effective field theory and renormalisation group techniques, as well as high-performance computing codes and resources. The work will consist in formal developments, computational tasks and application of the new technology to cases of experimental interest. International collaborations constitute an integral part of the project.

Antiproton-nucleus reactions can occur at rest or in flight. The reactions at rest, or almost (~100 eV - 1 keV), are notably used at the Antiproton Decelerator (AD) at Cern by different experiments (GBAR, ASACUSA, AEgIS, ALPHA, ATRAP,). At FAIR, the PANDA project (antiProton ANnihilation at DArmstadt) aims to study hypernuclei with in-flight reactions (~GeV). In both cases, reliable simulations are necessary for a good analysis of the results. This is where we propose to make our contribution.



The INCL calculation code (IntraNuclear Cascade Liège) developed at the CEA (Irfu/DPhN) processes hadron-nucleus reactions for energies up to 20 GeV. Recognized for its reliability, it has recently been extended to the production of strange particles and hypernuclei (with the help of the ABLA de-excitation code), and can also be used within the Geant4 transport code. The extension to antiproton-nucleus reactions will therefore make it possible to participate in PANDA’s studies on hypernuclei, with a first step of tests on data already available (obtained at LEAR), as well as in the experiments of the Antiproton Decelerator where the behaviour of anti-hydrogen atoms is studied.



In addition, INCL is able to treat nucleus-nucleus reactions when one of the nuclei is light (A <= 18). This could also be used to treat by extension reactions with anti-deuteron and anti-helium. The GAPS experiment (General Anti-Particle Spectrometer) aims precisely to measure the fluxes of these particles in cosmic radiation. Simulations are obviously necessary in this case and INCL could thus contribute to this.

Understanding the limits of the nucleus cohesion, and in particular its mass limit, is currently one of the major areas of research in nuclear physics. The study of heavy nuclei, located in the upper part of the nuclear chart, has recently benefited from a new experimental approach: the laser spectroscopy. This is a method from atomic physics, allowing the properties of the nucleus to be deduced through the study of atomic levels, independent of nuclear models.

In this region of heavy nuclei, neutron-deficient actinides are of particular interest. Indeed, several theoretical calculations predict strong octupole deformations (pear shape).

The objective of the thesis is to study octupole deformations in neutron-deficient actinides. It will be done in collaboration with the University of Jyväskylä. The thesis is divided into two parts: i) an experiment at Jyväskylä to study the production of neutron-deficient actinides by a new reaction mechanism, ii) the development of a detector that will couple laser spectroscopy and delayed spectroscopy at S3/LEB at GANIL-Spiral2.

One of the most fundamental properties of the atomic nucleus is its shape, which is governed by the interplay of macroscopic, liquid-drop like properties of the nuclear matter and microscopic shell effects, which reflect the underlying nuclear interaction. In some cases, configurations corresponding to different shapes may coexist at similar excitation energies, which results in the wave functions of these states mixing. Experimental observables such as quadrupole moments and the electromagnetic transition rates between states are closely related to the nuclear shape. The experimental determination of these observables, therefore, represents a stringent test for theoretical models. This thesis is integrated in our ongoing programme to study nuclear shapes by means of Coulomb excitation and more specifically such an experiment is planned on 100Zr. This method allows to extract the excitation probability for each excited state and to extract a set of electro magnetic matrix elements, and in particular the quadrupole moment which determines the shape of the nucleus. The radioactive 100Zr beam is provided by the ATLAS-CARIBU facility at Argonne National Laboratory (ANL), which is currently the only facility world-wide able to deliver beams of such refractory elements. The programme advisory committee has already accepted the experiment with high priority and we expect it to be scheduled in Q4/2020. The PhD student will participate in the preparation and setting-up of the experiment. It would be advantageous if he/she started already working on the subject already during the stage M2. He/she will be responsible for the data analysis, the presentation of the scientific results (at conferences or workshops) and their publication in a scientific journal. During the thesis work the PhD student may also participate in other experiments of the research group. All experiment(s) take place in international collaborations and may require prolonged stays at foreign laboratories (e.g. 4-6 weeks at ANL, USA).

The Quark Gluon Plasma is a state of matter created under extreme conditions at temperatures of the few hundreds MeV where nucleon constituants are deconfined long enough to produce sizeable effects when interacting with the plasma.

Thermodynamical conditions required to form this plasma can be reproduced in ultra relativistic heavy-ion collisions at the LHC (CERN).

The goal of the proposed thesis is the first measurement in the ALICE experiment of hadrons made of bottom quarks down to zero transverse momentum in Pb-Pb collisions from their decay into J/psi resonances. This measurement adds a very important constraint in the understanding of the initial state of heavy-ion collisions and in the heavy-quarks transport within the plasma, in particular their hadronisation.

The proposed thesis will be initially concentrated on the commissioning of a new high-precision silicon detector, called Muon Forward Tracker, necessary for the detection at forward rapidities of muon pairs coming from the J/psi decay. In a second stage, the analysis of the first Pb-Pb data that will be taken in 2021 with this new detector will allow studying the separation between prompt J/psi produced during the collisions from those coming from the B-mesons decay. This study, novel within the ALICE experiment, represents a very important test of heavy quarks properties inside the Quark Gluon Plasma.

Most of the visible mass of the universe is contained in nucleons. However, the origin of this mass remains mysterious, with the portion from the Higgs mechanism in standard renormalization schemes corresponding to only a few percent of the total mass. The answer is to be found in the dynamics of strong interaction, described by the theory of quantum chromodynamics (QCD) in terms of quarks and gluons. Thus, the interaction between quarks and gluons is responsible for the emergence of known and measured properties of hadrons such as their masses or spins.

There is now a strong theoretical and experimental dynamic to determine the 3D structure of hadrons in terms of quarks and gluons. From a theoretical point of view, the classical tools of quantum field theory, namely perturbative expansion, do not allow the study of the emerging properties of hadrons. The latter are inherently non- disruptive.

The aim of this thesis is to develop and use a non-perturbative formalism based on the Dyson-Schwinger and Bethe-Salpeter equations to determine the 3D structure of hadrons, in particular the nucleon. Different dynamic assumptions will be used to obtain a 3D mapping of the charge, mass and orbital angular momentum effects. A comparison of the results obtained with the experimental data will be carried out in collaboration with the other LSN members.

In nuclear physics, atomic nuclei are described in the framework of shell models, with formal analogies to atomic physics. Orbitals characterized with angular momentum are successively filled from the bottom of the nuclear well. « Magic nuclei » correspond to to the full filling of orbitals before a major energy gap is crossed, with a net gain of stability. Not only stable nuclei were successfully described this way, but it also provides a framework for the evolution of nuclear properties versus the neutron/proton asymmetry parameter. Evolution of orbitals and associated magic numbers is under study with increasing neutron number for exotic nuclei far from stability. For calcium isotopes with a magic proton number Z=20, the relative ordering of proton orbitals 0d3/2 and 1s1/2 evolves from 40Ca (N=20) to 48Ca (N=28) and filling the neutron 0f7/2 orbital as an effect of the neutron-proton interaction in nuclei [1]. It was evidenced by the measurement of energies and spins of ground states and first excited states of potassium isotopes Z=19, which can be described as a proton hole in a calcium core. We propose to extend this study of low-energy spectroscopy to the neighbor chlorine isotopes Z=17 and a valence proton in 0d3/2 and 1s1/2 orbitals.

This study will be the aim of an experiment to be done in 2020 at the RIBF facility (Tokyo). An intense secondary beam including 46,48Ar isotopes will be obtained from fragmentation of a primary 70Zn beam at 345 MeV/u. In beam spectroscopy will be studied from one proton removal reaction in inverse kinematics 46,48Ar (p,2p) 45,47Cl and a cryogenic thick liquid hydrogen target surrounded by a time-projection chamber used for proton tracking and reconstruction of the reaction vertex in the target [2]. In flight emitted photons from 45,47Cl at target will be detected with the high resolution HiCARI array of Ge detectors, dedicated to this 2020 experimental campaign. We will measure the momentum distribution of 45,47Cl fragments to determine the angular momentum (s or d wave) of the knocked proton, giving access to the spin of the final state.

The experiment will be examined in 2019 December for an expected realization in 2020. A close collaboration with nuclear structure theoreticians will be an opportunity to analyze the results and develop the theoretical skills of the student.

Although known and studied for more than 80 years, nuclear fission remains a very active research topic both for its fundamental aspect of the study of nuclear matter and for its applications, including nuclear energy.

Prompt gamma rays allow probing the structure and properties of fragments emitted during the fission process. Their use therefore offers a new perspective for its study. In particular, it makes it possible to explore effects that have not yet been studied experimentally, such as the influence of the shape of the fragments on the fission process or the sharing of excitation energy between the fragments. On the other hand, the measurement of fission-prompt gamma rays also provides valuable data for the simulation of gamma heating in nuclear reactors.

The thesis work will consist in the analysis of the data from the new gamma spectrometer, FIPPS, installed at the Grenoble research reactor. This spectrometer is composed of an array of Germanium detectors arranged around a fissile target placed in an intense flux of thermal neutrons. Experimental results will allow the student to test recent models on the fission and fragment de-excitation processes.

Description

This thesis takes place in the GBAR experiment at CERN, which aims to measure the acceleration of gravity on antihydrogen. The specificity of the experiment is to use positive antihydrogen ions to produce ultra-cold antihydrogen atoms, allowing an accurate measurement of their free fall. To produce antihydrogen ions, antiprotons ( p ) must interact on a cloud of positronium (Ps), the bound state of an electron and a positron. Two successive reactions are used:



p + Ps -> H + e- et H + Ps -> H+ + e- .



Only the material counterpart of the first reaction,

p +Ps -> H + e+, was observed with 211 events. Therefore, there are only theoretical estimates of the antihydrogen ion production rate for the experiment. These estimates are subject to significant uncertainties with respect to low-energy three- and four-body interactions.

The first objective of the thesis is to manufacture and use a beam of hydrogen atoms to measure the effective cross-section of the 2nd reaction where incident antihydrogen is replaced by these hydrogen atoms. These measurements will validate the theoretical calculations and optimize the experimental conditions for GBAR (incident energy, Ps excitation level) with high statistics. The second objective is to carry out these measurements with antihydrogen atoms, with much less statistics, but with the aim of producing for the first time H + anti ions.



Group/lab/supervision

The host team includes six physicists, including the spokesperson for the collaboration, and a thesis student. GBAR is an international collaboration involving about 50 physicists from 18 laboratories.

Work proposed

At the start of the thesis, the experiment will be at the end of its installation at CERN. The student will participate in the measurements with the hydrogen beam (October 2020 - mid 2021). This work will cover the understanding of detectors as well as simulation of the experiment, estimation of efficiencies, background noise, calculation of effective cross-sections and comparison with theoretical predictions. Systematic measurements with antiprotons will start from the second half of 2021 when the antiproton beam is operational. The results will be presented at conferences and published.



Training and skills required

The student must have training in experimental corpuscular physics. He will need to acquire programming and instrumentation skills, and will be able to work in a team environment.



Skills acquired

At the end of the thesis, the student will have acquired knowledge in fundamental physics (particle and atomic physics), instrumentation (ultra-high vacuum techniques, trace detectors, lasers, electronics) and programming. He will have worked in a very competitive international environment.



Collaborations/Partnerships

The realization of the experiment is conducted as an international project. Much of the work will be done at CERN.



Contacts

Scientist :

P. Debu (thesis director), L. Liszkay (responsible for the production of positrons and positronium), P. Pérez (spokesman for the experiment).

The neutrino masses and flavor mixing are a direct evidence of new physics Beyond the Standard Model (BSM): the study of neutrino oscillations is thus a royal road to the search of new, unexpected phenomena. In particular, the analysis 8of neutrino and antineutrino oscillations at T2K and NOVA are providing first exciting hints of CP violation in the leptonic sector. This would be a major discovery related with one of the most fundamental questions in High Energy Physics: why there is an asymmetry between matter and antimatter in the Universe?



The long-baseline experiments T2HK and DUNE, will measure neutrino oscillations with high statistics, requiring a control of the systematic uncertainties at 1-2% level, the most complex ones being the modeling of the neutrino flux and of neutrino-nucleus interactions. Thus, the role of the near detectors and the neutrino-nucleus cross-section measurements are becoming crucial. T2K experiment is opening the way to the next generation, by improving the understanding of the systematic uncertainties and exercising new performing detector and analysis techniques. The final aim is the discovery of CP-violation in the leptonic sector, the definitive identification of the mass ordering through matter effects and the precise measurement of atmospheric oscillation parameters.



The T2K collaboration is preparing an upgrade of the Near Detector (ND280), to be installed in 2021, in order to improve the Near Detector performance to measure the neutrino interaction rate and thus constrain the neutrino interaction cross sections so that the systematic uncertainty in the number of predicted events at the T2K far detector, Super-Kamiokande, will be reduced to about 4% (from about 8% as of today). This will allow to improve the physics reach of the T2K project, enabling a 3sigma exclusion of CP conservation.

The upgrade of the ND280 detector consists in the addition of a highly granular scintillator detector, the Super-FGD, sandwiched between two High-Angle Time Projection Chamber (TPC). The new TPCs will be readout by resistive Micromegas detectors and instrumented with a compact and light field cage. The SuperFGD will enable much lower threshold for the particle reconstruction (notably for protons) and, for the first time in T2K, the measurement of neutrons. The TPC will measure charge, momentum and directions of tracks produced by charged particles and will provide particle identification through dE/dx measurement with excellent efficiency and precision. Detector prototypes of the new TPCs have been successfully tested in Summer 2018 and 2019 at CERN and DESY test-beams validating the detector technologies and their performances.



The IRFU group is heavily involved in the TPC project, especially in Micromegas detectors production and tests. The first part of the thesis will be devoted to TPC data analyses. The student will contribute to the test-beam data taking and analysis foreseen in October 2020. The work will focus on the characterization of the Micromegas resistivity and the consequent spatial and energy (dE/dx) resolution. The IRFU group is developing a quantitative model of the resistivity calculation which will be included in the simulation. A precise reconstruction algorithm will be tuned, based on the so-called Pad Response Function approach. This will be the first detailed measurement of the resistivity, uniformity and the corresponding resolution of resistive MicroMegas in a complete detector with large surfaces and thus will have an important impact for the validation of such technology for further development.

The detector construction for the ND280 Upgrade will be performed in 2019-2020, for an installation in Japan in 2021. The student will contribute also to the installation and the commissioning of the ND280 detector.



The second part of the thesis will be dedicated to the analysis of the first T2K neutrino beam data, collected with the ND280 Upgrade detector, in order to extract a new, most precise, measurement of neutrino oscillations. Thanks to the increased statistics and the improved control of systematic uncertainties with ND280 upgrade, the project has the potential to achieve the best worldwide constrains on CP violation in the leptonic sector. The work will focus on the definition of the selection of the new ND280 data samples for the analysis, the evaluation of the corresponding experimental systematic uncertainties and the modification of the analysis framework for the fit to the neutrino oscillation parameters. The extraction of near detector constraints must be deeply modified to include the information of outgoing detected protons and neutrons from neutrino interactions on nuclei, which are completely missing in the present analysis. In parallel, the theoretical systematic uncertainties will need to be reevaluated based on the new exclusive models of neutrino-nucleus interactions.

Recent developments in computer hardware and deep-learning algorithms, combined with large datasets, lead to impressive progress in artificial intelligence in the past few years. Image processing with computer vision techniques, in particular, emerged as dynamic and prolific field of research with many applications.



Although only marginally studied in high energy particle collisions, deep-learning algorithms already demonstrated the ability to perform particle and event classification, estimation of kinematic variables and anomaly detection. Those abilities are extremely useful in the analysis of the unprecedented amount of proton-proton collisions expected in the next running phases of the Large Hadron Collider (LHC) at CERN. The CMS detector will undergo major upgrades to deal with the increasing number of additional collisions per LHC bunch crossing (pileup), benefiting from more finely segmented detectors and precise timing information. Fast, robust and adaptive event processing and analysis will be the key to explore those upgrades.



The proposed PhD thesis topic focuses on the development of global and sophisticated algorithms using state-of-art machine learning techniques for particle identification, pileup rejection and finally event classification applied to the measurement of Higgs boson properties. The objective is to transfer the algorithmic knowledge gained in simple tasks to more complex ones. The successful candidate will contribute to those studies and participate in the development and testing of detector electronics for precise timing measurements with of the various CMS subsystems.

Since the first detection of the neutrino in 1956 at the Savannah River power plant (USA), nuclear reactors keep playing a central role in the understanding of the fundamental properties of the neutrinos. Neutrinos are electrically neutral elementary particles which can be of three different flavors, each of them being associated to the electron, muon and tau particles. Neutrinos can spontaneously transition from one flavor to another. This phenomenon, called neutrino oscillations, questions the validity of the standard model of particle physics and call for new physics. Because neutrinos are weakly interacting particles, detectors larger than the ton scale are often necessary to study them. This PhD thesis focuses on the development of a new technique for the detection of reactor antineutrinos, using the coherent elastic neutrino-nucleus scattering (CEvNS) process. Depending on the target nucleus, the CEvNS cross-section can be up to a factor 1000 larger than those of the other neutrino detection channels (inverse beta decay, neutrino-electron scattering), making it an attractive process to reduce the size of neutrino detectors and to perform precision neutrino physics. CEvNS is a promising process offering the possibility to carry out a rich experimental program, ranging from the study of the fundamental properties of the neutrino (magnetic moment, existence of additional sterile species) and the nucleus internal structure (weak charge distribution) to precision tests of the standard model of particle physics at low energies (measurement of the Weinberg angle, search for new couplings, etc). Finally, exploiting CEvNS for long range neutrino detection could also lead to useful applications, such as the detection of supernovae and the surveillance of nuclear reactors.

This PhD thesis takes place within the NUCLEUS experiment, which aims at detecting and studying CEvNS for the first time at a nuclear reactor. The CEvNS experimental signature is a very low energy nuclear recoil (< 0.1-1 keV), making conventional detection techniques ineffective to study this process. The NUCLEUS collaboration is therefore developing a new concept of cryogenic detectors achieving ultra-low energy thresholds down to 10 eV. The collaboration targets their deployment and commissioning at the Chooz nuclear power plant (France) by 2022/2023. The proposed PhD work consists in leading a detailed study and characterization of the backgrounds in this yet uncharted energy regime. This work is the main challenge to address in order to guarantee the success of the experiment, and it will lead to the first detection of CEvNS at a nuclear reactor.

In further details, the PhD student will first contribute to the development a full analysis chain for the processing of the NUCLEUS data. Background data and calibration data collected during the different stages of the experiment (cryogenic detector prototyping stage, blank assembly of the full experimental setup and integration of the full experiment at the Chooz nuclear plant) will be analyzed and compared to the predictions of a background modeling using the Geant 4 MC simulation package. The ultimate goal of this work is to quantify the background rejection performances of the NUCLEUS setup, and to end up with a detailed and comprehensive background modeling in the CEvNS region of interest (E < 1 keV). In the course of this work, a special attention will be paid to neutrons. Because they can elastically recoil on nuclei and hence perfectly mimic a CEvNS signal, neutrons are the ultimate type of background to fight against. Additionally to this work, the PhD student will also be expected to contribute to various service tasks for the collaboration: blank assembly of the NUCLEUS setup in Munich, integration and commissioning of the NUCLEUS setup at Chooz, calibration and data taking shift, etc. He/she will be hence expected to regularly travel to Chooz and Munich.

The PhD student will integrate the “low energy neutrino” group, which gather physicists from the particle physics and nuclear physics departments of the Institute of Research into the Fundamental laws of the Universe (Irfu) at CEA Paris-Saclay. Over the past decades, the team acquired a solid expertise in reactor antineutrino physics (Double Chooz, Nucifer and STEREO experiments) and in low energy antineutrino physics (CUORE, CUPID and KATRIN experiments). The PhD student will also work in an international environment, as the NUCLEUS collaboration gathers foreign partners in Germany, Austria and Italy.

Description

This thesis takes place in the GBAR experiment at CERN, which aims to measure the acceleration of gravity on antihydrogen. The specificity of the experiment is to use positive antihydrogen ions to produce ultra-cold antihydrogen atoms, allowing an accurate measurement of their free fall. To produce antihydrogen ions, antiprotons ( p ) must interact on a cloud of positronium (Ps), the bound state of an electron and a positron. Two successive reactions are used:

p +Ps* -> H* + e- et H + Ps -> H+ + e- .



The amount of anti atoms and anti ions produced depends not only on the flux of antiprotons, but also on the density of the positronium target. In particular, the amount of anti ions produced by the second reaction is proportional to the square of the density of Ps. Positronium is produced by implanting positrons in a nanoporous silica in an amount proportional to the flux of positrons. The positron flux currently obtained is about 5 x 1E8 in 10 minutes, already representing a factor of 10 improvement over the world record.

The purpose of the thesis is to gain up to another factor 100 on this flux and measure the density of the corresponding positronium target in order to measure the effective section of the first reaction.



Group/lab/supervision

The host team includes six physicists, including the spokesperson for the collaboration, and a thesis student. GBAR is an international collaboration involving about 50 physicists from 18 laboratories.



Work proposed

Important factors can be gained in the accumulation of positrons in Penning-Malmberg traps. The technique to be used will take into account the fact that the initial positron beam in GBAR is pulsed, and will do without the buffer gas used in the first trap. The density of the Ps target can be measured either by using gamma detectors or by characterizing this target with the Ps excitation laser beam. The measurement of the cross-section will follow a thesis during which the signal and background noise were studied by simulation. The restart of the antiproton beam is scheduled for mid 2021 for periods of about six months per year. During periods without antiprotons, a proton beam will be used to study the symmetric reaction producing hydrogen. This work will cover the optimization of trapping, the understanding of detectors, the excitation laser, as well as simulations of the experiment, estimation of efficiencies, background noise, calculation of cross-sections and comparison with theoretical predictions. The results will be presented at the conference and published.



Training and skills required

The student must have training in experimental corpuscular physics. He will need to acquire programming and instrumentation skills, and will be able to work in a team environment.



Skills acquired

At the end of the thesis, the student will have acquired knowledge in fundamental physics (particle and atomic physics), instrumentation (ultra-high vacuum techniques, trace detectors, lasers, electronics) and programming. He will have worked in a very competitive international environment.



Collaborations/Partnerships

The realization of the experiment is conducted as an international project. Much of the work will be done at CERN.



Contacts

Scientific :

P. Pérez (thesis supervisor), L. Liszkay (responsible for positron and positronium production), D. van der Werf (trapping).

The neutrino, as the only particle of mater (fermion) without electrical charge, could be a Majorana particle, i.e. identical to its antiparticle. In this case a new phenomenon should appear for a few radioactive atomic nuclei: the neutrinoless double beta decay. The violation of the leptonic number which follows, forbidden by the Standard Model, would be a major discovery and one of the required conditions to explain the mater-antimatter asymmetry in the Universe.



The PandaX-III experiment aims to measure the kinematics of double-beta decays of Xenon 136 in a large volume of 10 bar gaseous Xenon. This experiment could detect double-beta decays without emission of neutrinos and distinguish them from backgrounds like regular double-beta decays with neutrino emission, gammas from radioactive contamination, or cosmics. These rare processes will be detected in gaseous Xenon inside large Time Projection Chambers (TPC) with a detection of ionization electrons based on Micromegas Microbulk micro-pattern gaseous detectors. The TPC will operate under a pressure of up to 10 bar. An excellent resolution of electron energy measurement and a very good reconstruction of the event topology is required to separate neutrinoless double-beta decays from the various backgrounds. A high radiological purity of the experimental set-up is also necessary to limit the gamma background contamination. This experiment will take place in the Jinping underground laboratory (Sichuan province, China) which presents the lowest residual cosmics rate in the world. A first 145kg-Xenon TPC chamber will be installed in 2020, and 5 modules will be installed in the following years to reach a level of 1t of Xenon.



Associated with the PandaX-III team at IRFU the student will participate to the development of data reconstruction algorithms by taking into account and compensating detectors imperfections like missing channels, performance inhomogeneities, etc... The compensation methods will include calibration methods and interpolations of missing data, and neural network methods of data corrections will be also evaluated. As soon as the data of the first TPC module are available the student will participate in collaboration of the other PandaX-III members to the analysis and the extraction of the physics results. In parallel he will participate to the detector R&D conducted on several types of Micromegas detectors, in order to reach 1% energy resolution at 2.5 MeV. This work will include performance measurements of the different prototypes in high pressure gaseous environment.

After the observation of a Higgs boson which is compatible with the predictions of the Standard Model of particle physics at the ATLAS and CMS detectors in 2012, the precise measurements of its properties is now one of the major goals of the high energy physics. The Higgs boson decaying into two photons (H -> GG decay channel) provides a final state with an invariant mass peak that can be reconstructed with high precision. As a consequence, despite the small branching ratio predicted by the SM (approximately 0.2%), H -> GG was one of the two most important channels involved in the discovery of the Higgs boson together with its decay to four leptons. The proposed PhD thesis topic aims at measuring the different Higgs boson couplings using Artificial Intelligence (AI) with a state-of-the-art deep learning model, where a multi-classifier will be designed to make use of all possible ingredients of the H -> GG analyses to provide the most optimal separation with the other non-Higgs SM processes, and hence, will achieve the utmost sensitivity for this particular decay channel in the CMS experiment. Furthermore, the LHC will undergo a High Luminosity (HL) upgrade which will deliver around ten times more integrated luminosity with a downside of imposing harsher conditions for the CMS detector. An accompanying upgrade of the CMS detector (Phase II upgrade) is foreseen to not only cope with these harsher conditions but also significantly improve the performance of the detector. One of the most important aspects of this upgrade is the ability to tag events with very high timing resolution, which will also improve reconstruction of Higgs particles in the H -> GG decay channel. The successful candidate is expected to contribute to the timing upgrade of the CMS detector, particularly to the fast monitoring and calibration of the high-precision clock distribution.

At the start of the project, the successful candidate will have a chance to explore possible improvements in the H -> GG analysis with ~140 fb-1 of data collected with the CMS detector in Run II with an established framework. The candidate will introduce a state-of-the-art machine learning technique for the Run III and foreseen upgrade of the detector. They will perform the H -> GG analysis using this technique with the first Run III data collected in 2021 and 2022 (expected to reach an integrated luminosity of ~115 fb-1). The precision timing distribution calibration and monitoring module will be employed in multiple timing detectors in the CMS experiment. Overall, both aspects of the project will provide a high visibility for the candidate within the collaboration and in the field.

The neutrino is currently the only particle in the standard model whose description is not entirely contained in it. His study therefore opens the way to the exploration of a new physics and to addressing very fundamental questions such as the preponderance of matter over antimatter in the universe. Future experiments with accelerator neutrinos (DUNE and T2HK) will measure its oscillation properties with unprecedented accuracy, which will require a high degree of control of uncertainties at the percent level.

One of the dominant uncertainties today is the one associated with the modelling of the neutrino interaction inside the detector. A decrease of this uncertainty would immediately imply an increase in the sensitivity of those experiments.

In this thesis work, we propose to improve the description of the neutrino-nuclei interaction, mainly the modelling of the final state of the interaction, and to evaluate its impact on the sensitivity of current and future experiments. The work will be based on the use and development of a nuclear cascade code coupled with measurement results. The results, coupled with an improvement of the near detector, would be used in the ongoing T2K experiment to improve the measurement of neutrino oscillations.

This work will also benefit to define the characteristics of the near detector in the DUNE experiment, whose components will be tested and validated at IRFU.

The discovery of the Standard Model Higgs boson in 2012 is undoubtedly a bright success for the Standard Model of particle physics ? This discovery however does not bring any answer to many of the questions that are still open in cosmology and particle physics. Among others, there is the nature of drak matter and dark energy, the origin of the Higgs potential, and the fact that the Standard Model does not provide an explanation for the very small masses of the neutrinos. Natural solutions to these problems could come from the existence of new interaction types or new particles.

This is why since the discovery of the Higgs boson efforts are focused on the search for new phenomena, beyond the Standard Model. One of the important aspects of the comparison between experimental measurements and theory is the need to normalize as precisely as possible experimental results to theory. This means in practice being able to measure as precisely as possible the luminosity of the LHC. The goal is to reach a precision better than 1% within the next few years. This is a factor two or three better than the precision that has been reached up to now.

LHC experiments are equipped with dedicated luminosity measurement subsystems, and several observables can be used to measure the luminosity. However, the techniques used have various stability and linearity issues, that complicate their exploitation.

After the LHC restart, foreseen in 2021, it is planned to increase the luminosity by a factor of about two. To make the best out of this luminosity increase, the calorimeter trigger system is being significantly modified and upgraded. The upgraded trigger system is based on real time analysis by FPGAs of the digitized detector signals. Irfu is one of the key contributors to the design and the production to the necessary hardware elements, as one of the instituts in charge of the design and production of the LTDB (LAr Trigger Digitizing Board), i.e. the board that digitizes the detector analog signals and transmits them to the back-end FPGAs

An essential feature of the upgraded trigger system is its ability to measure the energy deposited in the calorimeter bunch crossing by bunch crossing. Combined with the stability, excellent linearity and response uniformity of the ATLAS Liquid Argon calorimeter, the upgraded trigger system offers the potential to measure the luminosity with excellent linearity and stability performances. Preliminary studies performed on a prototype trigger chain show that the 1% precision level should be reachable.

An other feature of the upgraded trigger system is its ability to keep track of all the interactions taken place in the detector over a much longer period of time than the main readout. The main readout system is able to keep in memory only up to four or five consecutive interactions. The trigger system has the capability to keep track of each individual bunch crossing over a period of time corresponding to several tens of consecutive bunch crossings. The spatial granularity of the information is however somewhat coarser than the granularity available to the main readout. This opens up the possibility to detect particles reaching the detector long (several tens or even hundreds of ns, to be compared to the 25 ns between two consecutive bunch crossings) after their production. Such particles are slow and very heavy, and can be detected almost up to the kinematic limit of 7 TeV. This is significantly higher than the limits reachable by more classic techniques. Such particles typically appear in many classes of supersymmetric models.

In this thesis we propose to contribute to an ambitious detector based on Cherenkov/Scintillating crystals. We have selected technologies that are particularly effective for PET imaging. The principles of the detector are patented. They will allow one to produce neurological PET with highly improved performances. The device uses advanced particles detector technologies: a dense scintillator crystal, micro-channel plate photomultipliers, gigahertz bandwidth amplifiers and fast data acquisition modules (WaveCatcher, SAMPIC). Data processing will involve Monté-Carlo simulations and data analysis based on GATE/Geant4 and Root C++ software libraries.



Context:

Positron emission tomography (PET) is a nuclear imaging technique widely used in oncology and neurobiological research. Decay of the radioactive tracer emits positrons, which annihilate in the nearby tissue. Two gamma quanta of 511 keV energy are produced by positron annihilation and allow one to reconstruct the annihilation vertex and distribution of the tracer activity in the body of the patient.

The precise determination of the position of the positron annihilation vertex is important for an accurate image reconstruction with a good contrast. In particular, it is useful for neuroimaging studies of brain and for pre-clinical studies with animal models (rodents).



Supervision:

You will calibrate and optimize the detector prototypes and analyze the measured data. Your will be focussing on detector time and spatial resolution optimization. This will involve many skills of an instrument scientist : fast photo-detection, fast electronics read-out (analog and digital) with picosecond precision, hardware and detector simulations with GEANT4 and GATE software.



Requirements:

Knowledge in physics of particle interaction with matter, radioactivity and particle detector principles, a vocation for instrumental (hardware) work, data analysis are mandatory. Being comfortable in programming, having a background in Gate/Geant4 simulation and C++ will be an asset.



Acquired skills:

You will acquire skills in particle detector instrumentation, simulation of ionizing radiation detectors, photo-detection, implementation, operation of fast digitizing electronics, and data analysis.



Contacts:

Dominique Yvon : dominique.yvon@cea.fr

Viatcheslav Sharyy : viatcheslav.sharyy@cea.fr