Particle Physics Division

The late acceleration of the Universe expansion, revealed at the end of the 1990s and confirmed since then with more precise cosmological data, remains unexplained. Modifications to General Relativity at large scales offer a promising explanation. Among the measures, that of the growth rate of structures is the most direct way to test the predictions of General Relativity, since gravity is the driving force behind this growth. This PhD proposes to use observations of the recently commissioned DESI spectrograph to measure the clustering of emission line galaxies, the main tracer of DESI. The aim is to derive a measurement of the growth rate of structures and to compare it with General Relativity predictions in order to search for possible deviations.

The thesis will take place at Irfu, the Institute for Research on the Fundamental laws of the Universe. The PhD student will join the cosmology group of Irfu/DPhP, composed of 9 physicists, 6 PhD students and 2 post-docs. Actively involved in the eBOSS and DESI experiments, the group also participates in Euclid and has in the past had a strong contribution in the SNLS, Planck and BOSS international collaborations. The future PhD student will be integrated into the DESI collaboration and will benefit from all the group’s expertise acquired on BOSS and eBOSS.

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.

Time domain and high-energy astrophysics is dealing with the most violent phenomena in the universe. Rapid exchange of information is crucial to detect these transient, i.e. short-lived, events with multiple observatories covering the full multi-wavelength domain and all cosmic messengers. Victim of its own success, the current way of manual reading, analyzing and classifying information shared by astrophysicists in Astronomers Telegrams or circulars within the Gamma-ray Coordinates Network is approaching saturation. One of the most promising novel approaches is to build on the recent progress in artificial intelligence and especially natural language processing (NLP) and feature extraction.

This thesis will bring together leading experts in two exiting domains: Artificial Intelligence and time domain, multi-messenger astrophysics. It will be part of the UDOPIA program of the Paris-Saclay university and benefit from a rich AI ecosystem with strong links to industry.

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.

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.

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.

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.

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).

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

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.

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.

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.

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.

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).