The central region of the Milky Way is a very complex region at very-high-energy (VHE, E>100 GeV) gamma rays. Among the VHE gamma-ray sources are the supermassive black hole Sagittarius A* lying at the the centre of the Galaxy, supernova remnants and pulsar wind nebulae. The detected diffuse emission at TeV energies revealed the detection of the first Galactic Pevatron - a cosmic-ray accelerator up to PeV energies. At the ten-to-hundred GeV energy range, the Galactic Center region harbors the base of the Fermi bubbles - giant bipolar structures extending on ten-degree spatial scale, possibly related to past activity of Sagittarius A*. Beyond the rich VHE astrophysics, the GC region is expected to be the brightest source of particle dark matter annihilations in VHE gamma rays. The H.E.S.S. observatory located in Namibia is composed of five imaging atmospheric Cherenkov telescopes. It is designed to detect gamma-rays in the ten GeV up to several ten TeV energy range. The observation of the Galactic Center region is one of a long-term key science observational program carried out by H.E.S.S. The four-telescope observations performed by H.E.S.S. led to the detection of the first Galactic Pevatron and provide the strongest constraints so far on the annihilation cross section of dark matter particles in the TeV mass range. The PhD work will be focused on the data analysis and interpretation of the H.E.S.S. observations of the inner Galaxy survey program with the full H.E.S.S. array. In the first part, the PhD student is expected to characterize the spatial and spectral properties of the Galactic Center TeV diffuse emission. Second, she/he will develop a novel analysis method to search for new diffuse emissions connected to the Galactic Center outflows, using a multi-component template-fitting technique. The third part of the work will be dedicated to the search for dark matter in the Galactic Center region and in the complementary dwarf galaxy satellites of the Milky Way. The PhD student will be involved in the data taking with the H.E.S.S. telescopes towards these objects and will participate to the detection prospects of dark matter with the next ground-based gamma-ray observatory CTA.

Very high energy astronomy observes the sky above 50 GeV. It is a relatively recent part of astronomy (under 30 years old). After the success of the H.E.S.S. in the 2000s, an international observatory, the Cherenkov Telescope Array (CTA), should come into operation by 2024. This observatory, whose construction began in 2018, will include two sites equipped with about fifty telescopes. The IRFU is involved, in partnership with the CNRS and Spanish and German partners in the construction of NectarCAM, a camera intended to equip CTA's "medium-sized" telescopes (MST). A prototype of NectarCAM is being installed at the IRFU. After extensive tests to check that NectarCAM is capable of achieving the required performance, astronomical observations are planned at one of CTA's candidate sites. These observations will fully validate the operation of the camera. The thesis has two independent parts: on the one hand the tests in darkroom at the IRFU, then the preparation and realisation of the astronomical observations with the prototype of NectarCAM on the site of CTA.

In parallel, it is planned to participate in the analysis of HESS collaboration data on astroparticle subjects (search for primordial black holes, constraints on Lorentz Invariance using distant AGNs).

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 by the end of 2019. 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 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.

The matter distribution on cosmological scales can be predicted within the standard cosmological model, and it depends among others on the (yet unknown) absolute neutrino mass and on the properties of dark matter, whose nature is still a great mystery. The IRFU-DPhP cosmology team strongly contributes to the DESI large spectroscopic sky survey which will provide in the next years an unprecedented cartography of cosmological structures.

This thesis proposes to use observations of the so-called Lyman-alpha forest, which measures the absorption by the intergalactic medium of light from quasars, and provides as of now the best measurement of the matter distribution at "small" (~megaparsec) cosmological scales. State-of-the-art numerical tools will be used to develop a new set of cosmological simulations which include both the hydrodynamics of the intergalactic medium and the hypothetical properties of neutrinos and dark matter. The PhD student will then analyze the first Lyman-alpha forest data from DESI, and by making use of the simulations he/she will be able to derive new measurements on the neutrino mass as well as several dark matter scenarios ("warm dark matter", "fuzzy dark matter",...)

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 and emits two back-to-back 511 keV photons. These photons are used to reconstruct the biological activity in the

body.

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). The time-of-flight (TOF) technique is used to improve the signal-to-noise ratio in full body scans and therefore the image quality. This technique measures the difference in time between the two annihilation photons in addition to their position.

The ClearMind project at IRFU develops a novel detection technology, which combine the possibility to acquire images with a high spatial precision of several mm3 and time-of-flight capability with precision better than 50 ps (RMS). To obtain these performances, it will be necessary to reconstruct properties of the interaction in the detector volume, from the data acquired on the surface. For this we work on artificial intelligence techniques (Neural Network, Deep Learning etc ...)

After that it is necessary to estimate the quality of images of a foreseen scanner based on our technology.

In this thesis, we propose, first, to work on the algorithms of artificial intelligence necessary for the event reconstruction in the detector, and then

measure its performances. After that contribute to a Monté-Carlo simulation of the foreseen scanner and to the image reconstruction.



PROPOSED WORKS

The foreseen detector registers the 2D spatial coordinates and arrival time of the scintillation and Cherenkov photons produced by the gamma-conversion in the crystal. The Ph.D. study will consist in the development of the detector simulation with Geant 4 software, development and optimization of the coordinates and time reconstruction of the gamma-conversions using artificial intelligence algorithms (neural network, boosted decision tree, etc). Then he/she will contribute to the simulation of a complete scanner using GATE software and estimation of the it performances following NEMA standards.



REQUIREMENT

Excellent academic profile. General knowledge in physics of particle interaction with matter, radioactivity and particle detector principles. Good background in computational mathematics, be comfortable in C++ programming and working

in Unix environment. Background in simulation and artificial intelligence algorithms will be considered an asset.

On 14 August 2017, the ATLAS experiment at CERN has published in Nature Physics [1] the first direct evidence of high-energy photon-photon elastic scattering, thereby confirming one of the oldest predictions of quantum electrodynamics (QED). The experimental observation of this process is difficult. Indeed, this phenomenon is only possible in intense electromagnetic (EM) field with an electric field of at least 10^18 V / m. The ATLAS experiment used 2.5 TeV high energy ion beams per nucleon to produce these intense fields. The interaction of the EM fields of the 2 lead ion beams (of opposite directions) makes it possible to carry out the appropriate experimental conditions. Then, the publication of 14 August 2017 has reported 13 events identified as elastic collisions of photons (with 2.7 events of background). From mid-November 2018 to the beginning of December 2018, a new data taking is underway which should allow this number of events to be increased by a factor of 4 to 5, allowing more detailed studies than a simple observation.

The aim of the thesis will be to participate in this new analysis. The student will contribute to the general understanding of the data and will focus on a few specific points: for example, to study in depth the systematic uncertainties on the cross section of the 4-photon interaction as a function of the invariant mass of the 2 incident photons, such as the uncertainties on the photon reconstruction identification efficiencies. The mass spectrum of photon-photon elastic scattering events is also essential for another innovative aspect of the thesis. Thus, at first, the student will work on measuring the cross section of the photon-photon elastic scattering. With more data, it will be possible to search for new particles that can be produced in photon-photon collisions, in particular a scalar particle called Axion [2] which plays an important role in the theory of strong interactions and which could also to be a source of dark matter in astrophysics. With a good understanding of the mass spectrum described above, we can start to search for resonances in this spectrum (for each accessible Axion mass) and, failing that, to extract limits for the effective cross sections of the spectrum. 'Axions. This approach, the feasibility of which has been demonstrated [2], will have to be undertaken by the student on the data recorded at the end of 2018. To conclude, let us mention that the contribution of the CEA Saclay group to the first publication of ATLAS [1] was decisive. and that, on this momentum, the environment within the group is favorable so that the student can play an important role in what will follow.



[1] Nature Physics 13, 852–858 (2017)

[2] ATL-COM-PHYS-2018-1113

Neutrino physics has made enormous progress in the last decades, in particular with the discovery of neutrino oscillations. This indicates that neutrinos have non-zero masses, requiring ingredients beyond the particles and interactions presently known. Moreover, the relatively large value of the theta13 angle open the possibility to probe CP violating phenomena in the leptonic sector. This might be related to the observed matter-antimatter asymmetry in the Universe.

These studies require very intense neutrino beams and large underground detectors, like the project DUNE, now in construction in USA. Within DUNE, the team at IRFU is building a large demonstrator of a new technology for liquid argon Time Projection Chamber, offering superior detector performance. This 300-tonnes demonstrator will be exposed to cosmic rays in 2019 and to a charged-particle beam in 2021. The thesis work will consist in the analysis of the data to understand the detector response and to develop the algorithms for track and shower reconstruction. The student will also participate to a R&D of the gas detectors (Large Electron Multipliers, LEM) to improve the performance toward DUNE.

The proposed thesis focuses on the detection of neutrinos emitted by nuclear reactors according to the coherent diffusion process, first demonstrated in 2017 with neutrinos from a spallation source of a few tens of MeV[1]. The Nu-Cleus experiment[2], which is the subject of the thesis, is currently at the end of its design phase. It will be deployed at the Chooz nuclear power plant from 2019. A new detection site, located between the two cores of the plant, will be used for the measurement. Detection will be done with mini-bolometers with an extremely low detection threshold, in the order of 10 eV, in order to observe small nuclear retreats induced by neutrinos[3]. A first phase of the experiment, between 2020 and 2021, will use a detector mass of around 10 grams. This reduction in fiduciary mass will represent a technological breakthrough in neutrino physics. A network of around 100 bolometers will then be used to conduct an innovative physics programme: standard model tests and research into new low-energy physics (including sterile neutrinos), weak nucleus shape factors, application to reactor monitoring.



The main thesis work will involve the development of a simulation chain and analysis of experience data. This chain will be used to optimize detection, to carry out sensitivity studies, and finally to analyze and publish the results of the measurements. The student will also participate in the integration of the on-site experience, scheduled for 2020, followed by the commissioning of the detectors. This work will be done in close collaboration with the University of Munich, the Irfu Nuclear Physics Department, and the CNPE Chooz teams.



Irfu is firmly involved in the theme of low-energy neutrinos, with the measurement of theta-13 mixing angle by the Double Chooz experiment[4], non-proliferation applications by the Nucifer experiment[5], and sterile neutrino research by the Stereo[6] and KATRIN[7] experiments.



REFERENCES



[1] D. Akimov et al., Science 03 Aug 2017

[2] Strauss, R., Rothe, J., Angloher, G. et al. Eur. Phys. J. C (2017) 77: 506

[3] J. Bill J.Phys. G44 (2017) no.10, 105101

[4] M. Kaneda, Phys. Part. Nucl. 49 (2018)

[5] M. Pequignot, Nucl. P. P. Proc. 265-266 2015

[6] H. Almazan et al., arXiv:1806.02096

[7] M. Kleesiek, arXiv:1806.00369