15 sujets IRFU/DPhP

Dernière mise à jour : 05-12-2020

##### Measuring the Higgs-top coupling CP properties in the multilepton channel at the ATLAS experiment

SL-DRF-21-0365

 Location : Service de Physique des Particules (DPHP)Groupe Atlas (ATLAS)Saclay

Contact :

Henri BACHACOU

Starting date : 01-10-2021

Contact :
 Henri BACHACOUCEA - DRF/IRFU +41227675650
Thesis supervisor :
 Henri BACHACOUCEA - DRF/IRFU +41227675650
The proposed PhD is aiming at measuring the CP properties of the Higgs-top coupling in the multilepton channel within the ATLAS experiment. Discovering new sources of CP violation is one of the most pressing questions in particle physics today. The Yukawa-like interactions in the Higgs boson sector could provide a particularly attractive way for additional sources of CP violation. The goal of PhD is to measure the ttH process in two innovative ways. First new reconstruction algorithms to identify the heavy particles in ttH events will be developed, which is made very challenging by the multilepton final state. Then the analysis will be designed exclusively based on pure CP observables. New observables based on ratios or angles will be explored, and measurements in regions of phase space where the separation between the various Higgs-top processes is less dependent on the CP properties of the Higgs-top-quark coupling will be studied. This new analysis should also have increased sensitivity to other rarer top quark associated Higgs production modes such as tHq.
##### Probing new sources of CP violation in the Universe using Higgs boson production at the LHC

SL-DRF-21-0364

 Location : Service de Physique des Particules (DPHP)Groupe Atlas (ATLAS)Saclay

Contact :

Laurent SCHOEFFEL

Starting date : 01-10-2021

Contact :
 Laurent SCHOEFFELCEA - DRF/IRFU/DPHP 01.69.08.25.83
Thesis supervisor :
 Laurent SCHOEFFELCEA - DRF/IRFU/DPHP 01.69.08.25.83
The proposed PhD is aiming at probing new sources of CP violation in the Universe by studying the Higgs boson properties at the LHC, in particular through a modification of the coupling of the Higgs boson with the heaviest elementary particle: the top quark. The goal of this PhD is to develop a new data analysis strategy within the ATLAS collaboration probing the CP properties of the ttH coupling in pp collisions. The idea would be to design an analysis based exclusively on pure CP observables, meant to complement the existing model-dependent approach relying on machine learning methods that mix observables specific and non-specific to CP.

This new analysis should also have increased sensitivity to other rarer top quark associated Higgs production modes such as tHq.

New observables that will need to be developed will be made flexible enough to be applicable to a large panel of Higgs and top-quark decay channels. They will first be tested on the multilepton channel. A focus will be put on the reconstruction of the heavy particles (Higgs and top quark) in the final state, which is quite challenging in this channel.

##### High-energy multi-messenger astrophysics with H.E.S.S. and CTA

SL-DRF-21-0237

 Research field : Astroparticles Location : Service de Physique des Particules (DPHP)Groupe Astroparticules (GAP)Saclay

Contact :

Fabian Schussler

Starting date : 01-10-2021

Contact :
 +33169083020
Thesis supervisor :
 Fabian SchusslerCEA - DRF/IRFU/DPHP/HESS 2 +33169083020

Personal web page : http://irfu.cea.fr/Pisp/fabian.schussler/index.html

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.

##### Impact of the density of galaxies in the analysis of the large spectroscopic survey DESI

SL-DRF-21-0281

 Research field : Astrophysics Location : Service de Physique des Particules (DPHP)Groupe Cosmologie (GCOSMO)Saclay

Contact :

Etienne Burtin

Vanina RUHLMANN-KLEIDER

Starting date : 01-10-2021

Contact :
 Etienne BurtinCEA - DRF/IRFU/DPHP/GCOSMO 01 69 08 53 58
Thesis supervisor :
 Vanina RUHLMANN-KLEIDERCEA - DRF/IRFU/DPHP/GCOSMO 01 69 08 61 57
In the last 30 years, the study of the Universe has seen the emergence of a standard model of the Universe based on general relativity. In this model, our Universe is made of ordinary matter, dark matter and a mysterious component called “dark energy”, responsible for the recent acceleration of the expansion of the universe. The upcoming large surveys, such as DESI in the US, will provide a map of the distribution of the galaxies 10 times more precise than the current state-of-the-art. The scientific community is gearing up to define new analysis techniques in order to extract the maximum of information from these surveys and hence enter the era of precision cosmology especially as far as growth of structure measurements are concerned. In this thesis, we propose to study a novel approach based on using the density at large scales to improve the precision on those measurements 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 10 physicists, 4 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

##### DETECTORS FOR TIME-OF-FLIGHT PET IMAGING WITH HIGH SPATIAL RESOLUTION

SL-DRF-21-0221

 Research field : Nuclear physics Location : Service de Physique des Particules (DPHP)Groupe Santé et Energie (GSE)Saclay

Contact :

Dominique YVON

Viatcheslav SHARYY

Starting date : 01-10-2021

Contact :
 01 6908 3625
Thesis supervisor :
 Viatcheslav SHARYYCEA - DRF/IRFU/DPHP 0169086129

Personal web page : http://irfu.cea.fr/Pisp/dominique.yvon/

DESCRIPTION

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) as well as for full body PET imaging. 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 should allow one to produce PET scanner 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.

SUPERVISION

The successful candidate will work in the Department of Particle Physics of IRFU in close collaboration with the Department of Electronics Detectors and Computer Science for Physics. The CaLIPSO group includes two physicists and two students. Two Postdoc will join the project next spring. We collaborate closely with CNRS- IJCLabs on fast readout electronic, with CPPM of Marseille and CEA-SHFJ, and CEA-DES for simulations of medical imaging devices and image reconstruction algorithms, and with the University of Munster (Germany).

THE PROPOSED WORK

You will calibrate and optimize the detector prototypes and analyze the measured data. Your work 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, particle interactions 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.

##### Z boson precision physics with the Atlas detector at LHC

SL-DRF-21-0105

 Research field : Particle physics Location : Service de Physique des Particules (DPHP)Groupe Atlas (ATLAS)Saclay

Contact :

Fabrice Balli

Starting date : 01-09-2021

Contact :
 +33169081715
Thesis supervisor :
 Fabrice BalliCEA - DRF/IRFU/DPHP/Atlas +33169081715

The thesis will start in Autumn 2021. ATLAS, one of the major experiment at the LHC, is preparing for the expected increase of luminosity for Run3 and HL-LHC. The first part of the thesis is dedicated to a qualification task that could either consist in participating to the commissioning of the new muon detectors which are integrating the experiment, or taking part in the muon momentum calibration effort in view of Run3, that will start in 2022. Both options are closely related to the main thesis subject. The thesis will be followed by a measurement of precision physics in the field of the Z boson with ATLAS data.

The subject is focused on electroweak precision physics in ATLAS. The aim is to measure with the best possible precision the electroweak mixing angle, as well as the mass of the Z boson, using Run2 and Run3 data. The explored channel is that of the Z boson decaying into a muon-antimuon lepton pair. The student will work on muon momentum calibration using the J/Psi resonance as a standard candle, and will also reduce, through advanced fitting methods, the uncertainties related to the parton distributions functions (PDFs). These measurements should lead to a high improvement in the electroweak fit and thus significantly constrain the Standard Model, as well as Beyond Standard Model physics.

The CEA ATLAS group is part of the Department of Particle Physics (DPhP) of the Institute of Research into the Fundamental Laws of the Universe (IRFU) at CEA Paris-Saclay.

DPhP comprises about 110 physicists. DPhP scientific themes include elementary components of matter at the highest energies at the CERN LHC collider, R&D for future accelerators, study of antimatter, neutrino physics, gamma ray astronomy, study of gravitational waves, observational cosmology and instrumentation for medical applications. The group has a world-leading expertise in electroweak physics, namely with measurements of Z, W and Higgs boson cross sections and measurement of the W boson mass, achieved for the first time at the LHC. It builds on competences in muon reconstruction and muon spectrometer alignment and electron/photon identification.

##### LHC luminosity measurement with the ATLAS Liquid Argon Calorimeter and search for long lived massive particles

SL-DRF-21-0321

 Research field : Particle physics Location : Service de Physique des Particules (DPHP)Groupe Atlas (ATLAS)Saclay

Contact :

Philippe Schwemling

Starting date : 01-10-2021

Contact :
 Philippe SchwemlingCEA - DRF/IRFU/DPHP/Atlas 33 1 69 08 85 85
Thesis supervisor :
 Philippe SchwemlingCEA - DRF/IRFU/DPHP/Atlas 33 1 69 08 85 85
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,

a factor two or three better than the precision that has been reached up to now.

After the LHC restart, foreseen in 2022, 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.

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. A very promising analysis technique would be to use a neural net, that

could be implemented in the core of the FPGA that processes the data.

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.

This long term memory feature gives the possibility to compensate real time the effect of charge space accumulation,

which will be crucial for data taken after 2025, at very high luminosity. More importantly, this also 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.
##### 3D imaging of a nuclear reactor during its decommissioning using muon tomography

SL-DRF-21-0372

 Research field : Particle physics Location : Service de Physique des Particules (DPHP)Groupe Santé et Energie (GSE)Saclay

Contact :

Hector GOMEZ

Sébastien Procureur

Starting date : 01-10-2021

Contact :
Thesis supervisor :
 Sébastien ProcureurCEA - DRF/IRFU/DPhP (+33)(0)1 69 08 39 22
The final goal of this PhD thesis is to obtain the very first 3D image of a nuclear reactor with a non-invasive method, namely the muography. This penetrating imaging technique has shown a rapid development during the last years, with major technological improvements including several innovative contributions from the CEA. Muon telescopes of unprecedented resolution thus unveiled the 2D structures of very large objects like Khufu’s Pyramid or a nuclear reactor. Recently, an algorithm was developed to combine these 2D images in a 3D tomography, despite the small number of available projections and the huge size of the corresponding matrix system. This PhD will then be dedicated to the use of this algorithm to ongoing muography measurements on a nuclear reactor in decommissioning phase. The PhD student will actively participate to the data taking, data analysis and to the corresponding simulations. He/She will first apply the algorithm to smaller objects, in particular nuclear waste containers in various environments, in order to understand and optimize its performance. These intermediate steps, beyond their own interest, will help to better tune the algorithm parameters but also to determine the future measurements (positions, orientations, acquisition times, etc.). The overall goal of this work is thus the development of a 3D, generic, innovative imaging tool in the field of decontamination & decommissioning, with certainly many more applications in the societal and academic domains.
##### MEASUREMENT OF VECTOR-BOSON SCATTERING WITH THE ATLAS DETECTOR AT THE LHC

SL-DRF-21-0369

 Research field : Particle physics Location : Service de Physique des Particules (DPHP)Groupe Atlas (ATLAS)Saclay

Contact :

Maarten Boonekamp

Starting date : 01-09-2021

Contact :
 Maarten BoonekampCEA - DRF/IRFU/SPP/Atlas
Thesis supervisor :
 Maarten BoonekampCEA - DRF/IRFU/SPP/Atlas
The vector-boson scattering process, pp?VV+jj+X, where V=W,Z, is characterized by the presence of

leptons from the W or Z decay, and high-energy forward jets in the final state. WW, WZ and ZZ scattering all have

been observed during the LHC Run2, with partial

datasets. The next step for the community in this field, using improved event selections and event classification, and larger data samples, is to enhance the visibility of the signal and confront the SM predictions with more precise analysis results.

This measurement constitutes a fundamental test of the coupling between the Higgs and the vector bosons, and of the Standard Model as whole.
##### Design, characterisation and exploitation of resistive MICROMEGAS for the near detector of DUNE

SL-DRF-21-0291

 Research field : Particle physics Location : Service de Physique des Particules (DPHP)Groupe Neutrinos Accélérateurs (GNA)Saclay

Contact :

Guillaume Eurin

Samira Hassani

Starting date : 01-10-2021

Contact :
 Guillaume EurinCEA - DRF/IRFU/DPHP 0169085925
Thesis supervisor :
 Samira HassaniCEA - DRF/IRFU/DPHP/TK2 0169087226
Description and context :

The discovery of neutrino oscillations proved that neutrinos are not massless. This cannot be explained in the framework of the Standard Model of Particle Physics.

Several experiments are currently investigating neutrino oscillations properties such as T2K or NOvA. These experiments study a beam of muon neutrinos produced at particle accelerators.

DUNE is one of the main projects under development in this field. The target mass of DUNE of 4$\times$17 kt and a larger beam power and longer baseline will allow a very precise measurement of the CP violating phase (deltaCP) and of the neutrino mass hierarchy. The measurement of CP violation in the leptonic sector would be a major discovery. It is connected, in several models for new physics, to the matter-antimatter asymmetry observed in the Universe. T2K has already set the first constraint on the value of (deltaCP), excluding at 90\% the conservation of CP in the leptonic sector (deltaCP=0).

The concept of DUNE is very similar to existing experiments. Neutrinos are produced in the interaction of a beam with a target and detected at two locations: the near detector site at ~210 m and the far detector site at ~1300 km. The comparison between the spectra observed at both sites allows the parameters governing neutrino oscillations to be measured. The synergy between the detectors on both sites enables the reduction of the uncertainties coming from the flux or the neutrino interaction cross-sections.

The construction of the near detector of DUNE should start at the end of 2026 or at the beginning of 2027. This makes the future years crucial for the development of the technologies to be deployed in the detectors and of the data analysis techniques necessary to their scientific exploitation.

The beam monitoring for DUNE will be performed by the SAND detector, largely inspired from the one of T2K. The student will take part in the development and the optimisation of the resistive Micromegas detectors for the charge readout of the time projection chambers performing track measurements in SAND. The work will take place inside the DUNE international collaboration with a possible contribution to the tests and commissioning of the detectors for the near detector of T2K: ND280-Upgrade.

Description of the group, institute and supervision :

The DUNE collaboration is an international collaboration constituted of more than a thousand members with a strong contribution in Europe. The accelerator neutrino group at IRFU/DPhP has been actively involved in the collaboration for several years, namely thanks to the developments for the charge readout of the dual-phase technology for the far detector.

The activities on the near detector SAND directly benefit from the expertise acquired from the design of the resistive Micromegas detectors for the near detector of T2K. In this context, the accelerator neutrino group is constituted of 6 permanent scientists and 2 students involved in the resistive Micromegas activities.

The student will also benefit from the collaboration with the detector and technical services at IRFU/DEDIP, one of the international leaders in the development of micro-pattern gaseous detectors. Very advanced tools on detectors, DAQ, slow-control and electronics will therefore be available.

Proposed activities :

The student will take part in the optimisation on test bench data and with simulations of the design of the resistive Micromegas detectors for the TPCs of SAND. Cosmic rays tests and beam tests will be necessary to characterise the detectors in real conditions. These tests will be performed at the beam lines of CERN and/or DESY and on the equipment at IRFU. A large contribution to the detectors installation on site and to their operation is foreseen. The analysis of the data acquired during these tests, which will be done in collaboration with a post-doctoral researcher, will be one of the main activities and could result in a publication. The construction of the near detector of T2K is also supposed to be completed by Summer 2022. This will give the opportunity to the student, already during the internship, to take part in the testing of the resistive detectors in the context of the production of a final detector.

The other main activity proposed will be the production and analysis of simulation data of the entire SAND detector. This work will allow the theoretical systematic uncertainties to be reevaluated using new neutrino-nucleon interaction models, an important contribution to the sensitivity of SAND for the oscillation measurements with DUNE. This analysis on the nuclear models, currently being developed in the group for T2K, will need to be adapted to DUNE while incorporating the specific instrumental developments based on the various prototypes available. The development of the track reconstruction algorithms for the TPCs of SAND will also be carried out in tight collaboration with the members of the group currently involved in this activity for the near detector of T2K.

Education and skills required :

A Master's degree in particle physics with knowledge of the Standard Model is a requirement for this thesis. A strong interest for instrumental activities is expected and a motivation for neutrino physics is a very positive point. Knowledge of C++ and ROOT will also be very useful.

Acquired skills :

The student will have by the end of his/her PhD a good understanding of detectors and computing tools used in a particle physics collaboration thanks to his/her involvement in their developments. He/she will be able to promote his/her technical skills on detectors and on data analysis methods in other contexts.

Collaboration/Partenariats :

The student will work within a large international collaboration. This will provide a very good experience in particle physics and an important visibility through the participation to physics schools, workshops and conferences where he/she will present his/her results.
##### MEASUREMENT OF THE W-BOSON MASS WITH THE ATLAS DETECTOR AT THE LHC

SL-DRF-21-0367

 Research field : Particle physics Location : Service de Physique des Particules (DPHP)Groupe Atlas (ATLAS)Saclay

Contact :

Maarten Boonekamp

Starting date : 01-09-2021

Contact :
 Maarten BoonekampCEA - DRF/IRFU/SPP/Atlas
Thesis supervisor :
 Maarten BoonekampCEA - DRF/IRFU/SPP/Atlas
The goal of the thesis is a measurement of a fundamental parameter in the Standard Model of particle physics, the W boson mass, with the ATLAS detector at the LHC, using leptonic W boson decays. The analysis will be based on dedicated low-pile-up data samples, which have limited integrated luminosity but optimal experimental resolution in the reconstruction of missing transverse energy, which is a requirement in the analysis of final states with neutrinos.

The candidate will participate in the installation and the commissioning of the New Small Wheel, an upgraded muon detector for the ATLAS endcaps. IRFU has played a leading role in its construction and will get strongly involved in its scientific exploitation. In addition, the candidate will calibrate the muon momentum with sufficient precision for the measurement. The second phase of the project consists in improving the QCD aspects of the modelling of W-boson production and decay, and optimizing the analysis to minimize the resulting measurement uncertainty. After completion, the measurement will be interpreted in terms of compatibility with the Standard Model or as a hint of New Physics.

##### Coherent Elastic Neutrino-Nucleus scattering and search for new physics with the NUCLEUS experiment

SL-DRF-21-0298

 Research field : Particle physics Location : Service de Physique des Particules (DPHP)Groupe Sources et Réacteurs (GNSR)Saclay

Contact :

Matthieu Vivier

Starting date : 01-10-2021

Contact :
 0169086626
Thesis supervisor :
 Matthieu VivierCEA - DRF/IRFU/DPHP/Double Chooz 0169086626
This PhD topic is about the NUCLEUS experiment, which aims at precisely measuring coherent elastic neutrino-nucleus scattering (CEvNS) at the Chooz nuclear power plant (France). Although at ~MeV energies, CEvNS is the predominant interaction process of neutrinos with matter, it has remained unobserved for a very long time because of the difficulty to measure the very low energy nuclear recoils it induces. It was only 40 years after its first prediction that this process was observed in 2017 with neutrinos of a few tens of MeV at the Oak Ridge laboratory (Tennessee). The first detection of CEvNS at a nuclear reactor remains to be achieved, especially because the corresponding nuclear recoils lie in an energy regime (~100 eV) which is difficult to measure with conventional detection technologies, and also because of the unfavorable background conditions nuclear power plant environments generally offer. The NUCLEUS collaboration is therefore working on the design of an innovative detection system using two cryogenic calorimeter arrays capable of reaching ~10 eV energy thresholds, and surrounded by a twofold system of instrumented cryogenic vetoes. This set of cryogenic detectors will be protected by an external passive shielding and by a muon veto to improve the identification and discrimination of backgrounds. With this system, NUCLEUS aims at a precise measurement of CEvNS in order to push the study of the fundamental properties of the neutrino as well as the search for beyond standard model physics towards the low energy frontier. Interestingly, CEvNS also exhibits a cross-section 10 to 1000 times larger than the usual ~MeV neutrino detection channels (inverse beta decay reaction, neutrino-electron scattering process), making it possible to miniaturize future long-range neutrino detection setups. The first phase of the NUCLEUS experiment will for instance deploy an array of cryogenic calorimeters made of sapphire (Al2O3) and calcium tungstate (CaWO4) crystals, totaling 10 g of detector.

In addition to the characterization and preparation of the experimental site at Chooz, our team at Irfu is taking a leading role in the project through several hardware and software developments. In particular, the DPhP is strongly involved in the realization of one of the instrumented cryogenic shielding of the experiment, here called the cryogenic outer veto. This detector consists of an arrangement of high-purity Germanium crystals, erected around the two cryogenic calorimeter arrays, and operated in ionization mode. This detection system will play a central role in the identification and discrimination of external backgrounds, such as ambient gamma radioactivity or atmospheric muons resulting from the interaction of primary cosmic rays in the atmosphere. The exploitation of the data delivered by this detector is then a natural entry in the global analysis effort to extract a first CEvNS signal at a reactor, with first background data collected in 2021/2022 during the blank assembly phase at the Technical University of Munich, and with data collected during the first physics run planned in 2023 at Chooz.

The work proposed in this PhD thesis is focused on the external cryogenic veto of the experiment, with the ultimate goal of achieving a comprehensive understanding of the backgrounds in the CEvNS region of interest, between 0.01 and 1 keV. The priority at the beginning will be given to the realization and commissioning of the external cryogenic veto system during the blank assembly phase in Munich. This work includes the assembly of the different detector elements (crystals, support mechanics, readout electronics, etc.) in the cryostat of the experiment, and includes all the necessary tests to validate and quantify the performances of this detector. In a second step, the student will ramp up in the collaboration analysis effort by contributing to the development of analysis and simulation tools. These tools will be used to interpret the background and detector calibration data acquired during the blank assembly phase and during the first physics run. He (she) will focus on the study of a specific source of external background, and quantify its impact on the physics potential of the experiment. This work will require a good understanding of the processes governing radiation interactions in matter and of the solid-state physics driving the behavior of cryogenic detectors (e.g. phonon propagation). Finally, the student will use the first data from the physics run at Chooz to conduct a search for new physics beyond the standard model (measurement of the weak mixing angle at low energies, search for new neutrino couplings, constraints on the electromagnetic properties of the neutrino, etc.). This work will require the implementation of advanced statistical methods for interpreting the data, in order on the one hand to understand the impact of the various sources of uncertainty on the constraints obtained, and on the other hand to guarantee the reliability of the results.

##### Measuring four and three top-quark production in the multilepton channel at the ATLAS experiment

SL-DRF-21-0366

 Research field : Particle physics Location : Service de Physique des Particules (DPHP)Groupe Atlas (ATLAS)Saclay

Contact :

Frédéric DELIOT

Starting date : 01-10-2021

Contact :
 Frédéric DELIOTCEA - DRF/IRFU 0169086628
Thesis supervisor :
 Frédéric DELIOTCEA - DRF/IRFU 0169086628
The proposed PhD is aiming at observing for the first time the production of four top quark (tttt) at the LHC using the multilepton signature in data collected with the ATLAS experiment. The production of four top quarks is one of the most spectacular final states which became accessible at the LHC. While expected to be small in the Standard Model, the tttt cross section is predicted to be strongly enhanced in many new physics scenarios. The tttt process is also sensitive to the detailed properties of the Yukawa-like interaction between the top quark and the Higgs boson. Probing further this class of events over the next years will therefore be crucial to understand better the true nature of the Higgs-top-quark interaction and potentially to pinpoint subtle deviations from the Standard Model.

Several innovative ways will be pursued to reach the observation of this tttt process. First a better separation between the signal and the different background processes will be studied by designing various multivariate discriminants and by a better understanding of the modeling of the ttW process. Another path towards observation will be to study the possibility to reconstruct the different top quarks in the final state, which is particularly challenging in this channel.

Achieving a measurement of the tttt production will allow to probe a key property of the top-Higgs interaction, i.e. the CP nature of the top-Higgs coupling. Ultimately, the tttt process should also be measured separately from the production of three top quarks, which is currently totally unexplored experimentally.

##### STUDY OF THE RARE DECAY OF THE HGGS BOSON TO A PAIR OF MUONS WITH THE ATLAS DETECTOR AT THE LHC

SL-DRF-21-0352

 Research field : Particle physics Location : Service de Physique des Particules (DPHP)Groupe Atlas (ATLAS)Saclay

Contact :

RODANTHI NIKOLAIDOU

Starting date : 01-09-2021

Contact :
 RODANTHI NIKOLAIDOUCEA - DRF/IRFU/SPP/Atlas 0169086157
Thesis supervisor :
 RODANTHI NIKOLAIDOUCEA - DRF/IRFU/SPP/Atlas 0169086157
In July 2012, the ATLAS and CMS collaborations announced the discovery of a new particle with a mass of about 125 GeV, at the Large Hadron Collider (LHC) at CERN. Since this discovery, the two collaborations have been actively studying the properties of this new particle, so far consistent with those of the Standard Model Higgs boson.

In the Standard Model, the Brout-Englert-Higgs mechanism predicts that the Higgs boson will interact with particles of matter (quarks and leptons, called fermions) with a force proportional to the mass of the particle. It also predicts the Higgs boson will interact with the force carrier particles (W and Z bosons) with a strength proportional to the square of the particle's mass. Therefore, by measuring the Higgs boson decay and production rates, which depend on the interaction strength to these other particles, one can perform a fundamental test of the Standard Model.

The ATLAS and CMS collaborations have already observed the decay of the Higgs boson into tau lepton, belonging to the third “generation” of fermions. Since muons are much lighter than tau leptons, the decay of the Higgs boson into a muon pair is expected to occur about 300 times less often than that of a lepton-tau pair. Despite this scarceness, the H ? µµ decay provides the best opportunity to measure the interaction of the Higgs boson with second generation fermions at the LHC, providing new information on the origin of mass for different generations of fermions. The ATLAS and CMS collaborations recently presented results on this decay using the dataset during the 2nd phase of the LHC (Run-2 from 2015 to 2018). Figure 1 shows the mass distribution of muon pairs while Figure 2 shows an example of a candidate event to be a Higgs boson decaying into two muons as recorded by the ATLAS detector. The study of this process is one of the main objectives of the third phase of the LHC (Run-3).

The aim of this thesis is the search for a Higgs boson decaying into two muons by analyzing the whole dataset from Run-3 and combining them with the previous data from the second phase (Run-2) in order to establish the discovery of the decay of the Higgs boson into two muons and constrain possible theories of physics beyond the Standard Model that would affect this decay mode of the Higgs boson. The thesis will also include work on the performance evaluation of the ATLAS muon spectrometer. Particular interest will be paid to the understanding, analysis and operation of MicroMegas type gas detectors. The purpose of phase-I of the ATLAS detector upgrade is to prepare for the high luminosities that will supply the LHC. In this context, the 2 large detection planes called NSW (New Small Wheel) will be equipped with new MicroMegas type detectors and will replace part of the ATLAS muon spectrometer and be operational for the restart of the LHC in 2022.

##### ARTIFICIAL INTELLIGENCE TO SIMULATE BIG DATA AND SEARCH FOR THE HIGGS BOSON DECAY TO A PAIR OF MUONS WITH THE ATLAS EXPERIMENT AT THE LARGE HADRON COLLIDER

SL-DRF-21-0478

 Research field : Particle physics Location : Service de Physique des Particules (DPHP)Groupe Atlas (ATLAS)Saclay

Contact :

RODANTHI NIKOLAIDOU

Starting date : 01-09-2021

Contact :
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Thesis supervisor :
 RODANTHI NIKOLAIDOUCEA - DRF/IRFU/SPP/Atlas 0169086157
New artificial intelligence techniques are attracting growing interest in handling the massive volume of data collected by particle physics experiments, particularly at the LHC collider. This thesis proposes to study these new techniques for the simulation of the background noise of rare events originating from the decay into two muons of the Higgs boson as well as to set up a new artificial intelligence method to extract these rare events from the gigantic dimuon background noise.

In 2012, the Higgs boson, a fundamental part of the Standard Model of particle physics, was discovered at the LHC. The demonstration of its decay in dimuon is now at the heart of the LHC program to measure the coupling of the Higgs boson to 2nd generation particles.

Simulating the dimuon background with sufficient statistics is the first challenge of this analysis. The thesis proposes to test, for the first time, the use of very promising artificial intelligence models as a simulation method using "Generative Adversarial Networks (GANs)" with an architecture of two competing networks. In addition, the thesis also foresees a complete redesign of the analysis in order to implement new data processing methods (Deep Neural Networks) to optimize the extraction of the weak signal.