Particle Physics Division

One of the coolest thesis you may do today: Higgs boson physics through one of its rarest and not yet observed decays, standard model processes at their best to fully understand it, and a detector part completely related to it and to the CMS detector upgrades for the High-Luminosity phase of the LHC.



In the quest for understanding our universe, the standard model of particle physics is believed to be a low-energy approximation of a more comprehensive theory. The discovery of the Higgs boson has inserted a new tile in the puzzle, but many questions remains open (naturalness, numbers of lepton generations, asymmetry matter-antimatter, etc.). A precise characterization of the Higgs boson through all of its decays can improve our comprehension of the puzzle.



This thesis proposes a search for the Higgs boson decay into a Z boson and a photon (Zgamma). Almost as rare as the golden decay into two photons, it is harder to observe because of the additional branching fractions of the Z bosons into final state particles: electrons and muons (but also neutrinos and other final states can possibly be exploited to some extent). The decay has not been observed yet -limits on its likelihood have been set so far- but some evidence of it might be found if exploiting the full dataset provided by the Run2 of the LHC (2015-2018) and by the just started Run3, expected to almost double the Run2 dataset by 2025.

The Zgamma decay is related to and can be constrained by other Higgs boson decays: the direct one into two muons plus an additional photon radiated in the final-state, the one into two Z bosons, Dalitz decays in electrons and muons. The standard model production of Z bosons and pairs of vector bosons such as ZZ, ZW, WW can dissimulate a final state as the Zgamma decay and have to be considered in the analysis: searching for the Zgamma decays implies playing with and understanding numerous fundamental SM processes and Higgs boson decays.



The thesis also consists of an experimental part to optimize the time resolution of the electromagnetic calorimeter of CMS (ECAL). While designed for high precision energy measurements, the ECAL also provides an excellent resolution on the arrival time of the photons (about 150 ps in collision events, but 70 ps have been achieved in test beams).

In an environment populated by photons from overlapping events (pileup), the arrival time of a final-state photon can help constraining its provenance to be the same vertex of the Higgs boson decay, i.e. the one of the Z decay products. This feature will be a key for the High-Luminosity LHC phase (2029-) when the ECAL electronics will be upgraded to offer even better time resolution (30 ps for high energy electrons and photons) and the luminosity of the LHC -and the number of overlapping events and photons in the final state- will be a factor of 5 higher than today.



The thesis also proposes the participation at CERN to CMS/ECAL shifts and to laboratory tests foreseen for the newly developed ECAL electronics.

Very-high-energy (VHE, E>100 GeV) gamma-ray observations are crucial to probe the most-violent non-thermal phenomena in the universe. The central region of the Milky Way is very complex and active in VHE gamma rays. Among the VHE gamma-ray sources are the supermassive black hole Sagittarius A* lying at the centre of the Milky Way, supernova remnants and star-forming regions. The diffuse emission detected by the H.E.S.S. telescopes allowed to discover the first Galactic Pevatron - a cosmic accelerator up to PeV energies. The Galactic Center region habours the base of the Fermi bubbles- bipolar structures extending over several ten degrees, possibly linked to an enhanced past activity of Sagittarius A*. The Galactic Center region should also be the brightest source of annihilating dark matter particles VHE gamma rays.

The H.E.S.S. observatory located in Namibia is composed of five atmospheric Cherenkov telescopes. It is designed to detect gamma rays from a few ten GeV up to several ten TeV. The Galactic Centre region is observed by H.E.S.S. for 20 years. These observations allowed to detect the first Galactic Pevatron and to derive the strongest constraints to date on the annihilation cross section of dark matter particles in the TeV mass range.

The proposed PhD work will be focused on the data analysis and interpretation of all the observations carried by H.E.S.S. in the Galactic Center region over the last 20 years.The first part of the work will be dedicated to the analysis of the low-level data and the study of the systematic uncertainties in this massive dataset. In a second part, the student will combine all the available data obtained from the phase 1 and phase 2 of H.E.S.S. to search for diffuse emissions and dark matter signals using multi-component template fitting techniques. The third part will consist in the development and implementation of a new analysis method using Bayesian neural networks to search for new astrophysical emissions in H.E.S.S. data and study the detection potential of CTA. The student will participate to the data taking with the H.E.S.S. telescopes.

Over the last two years the Imaging Air Cherenkov Telescopes (IACTs) H.E.S.S. and MAGIC were able to detect very-high-energy gamma-ray emission from gamma-ray bursts (GRBs). These breakthrough results have triggered renewed discussions of the particle acceleration and emission mechanisms that can be found in these violent explosions [1].



Complementing the detections of GRBs via X-ray satellites, the detection of gravitational waves allows to provide new and complementary insights into the pre-explosion phase, the initial conditions, the geometry of the system, and much more. The proposed thesis project will exploit the exciting possibilities of combining the detection of GWs and the detection of the resulting GRB by VHE gamma-ray observatories in truly multi-messenger observations and analyses.



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 also collaborate closely with partners from around the world including obviously the gravitational wave instrument Advanced VIRGO, the SVOM satellite to detect GRBs, various radio telescopes in Australia and South Africa, optical observatories, and many more. The group at IRFU, CEA Paris-Saclay is leading observations of transient phenomena by both H.E.S.S. and CTA and has long-standing experience with these challenging observations. The group is also driving changes and modernizations of the communication in the astroparticle community (e.g. via the Astro-COLIBRI web/smartphone application, [2]).



The PhD student will first have the opportunity to participate in the development and improvement of the framework that allows to optimize the schedule of follow-up observations of astrophysical transients. Some of the most interesting event are being detected only with large localization uncertainties (i.e. especially GWs, but also GRBs, neutrinos and others). We therefore need specialized tools and algorithms that allow to point the follow-up instruments like H.E.S.S. into the right direction to rapidly catch the associated emission [3]. A year-long observation period by the GW interferometers (called O4) is scheduled to start in spring 2023. This timing is perfectly matching the PhD project presented here, as the selected student will have the opportunity to lead the H.E.S.S. and CTA/LST-1 follow-up observations searching for GRBs and other VHE gamma-ray counterparts to the GWs detected by LIGO/VIRGO/KAGRA during that period. A sizeable amount of observation time with both the H.E.S.S. and CTA/SLT-1 IACTs has been reserved for these exciting searches. We’ll thus have ample opportunities to optimize our follow-up procedures, lots of data to analyze, results to present at international conferences, and papers to publish.



The core of the proposed thesis project will be the real-time search for transient high-energy gamma-ray emission linked to the detection of a gravitational wave (and other multi-messenger astrophysical transients like high-energy neutrinos, gamma-ray bursts, fast radio bursts, stellar/nova explosions, etc.). The combined observations will unequivocally prove the existence of a high-energy cosmic ray accelerator related to these violent multi-messenger phenomena and will allow to derive novel insights into the most violent explosion in the universe.



References:

[1] H.E.S.S. Collaboration: “Revealing x-ray and gamma ray temporal and spectral similarities in the GRB 190829A afterglow, Science, Vol. 372 (2021);

[3] P. Reichherzer, F. Schüssler, et al. : “Astro-COLIBRI-The COincidence LIBrary for Real-time Inquiry for Multimessenger Astrophysics”, ApJS 256 (2021);

[2] H. Ashkar, F. Schüssler, et al. : “The H.E.S.S. gravitational wave rapid follow-up program”, JCAP 03 (2021);

The goal of this PhD project is to develop and apply methodologies for large-scale Bayesian inference over a differentiable physical simulator, with application to the cosmological analysis of the DESI (Dark Energy Spectroscopic Instrument) galaxy survey.



DESI is a multi-object spectrograph mounted on the Mayall telescope at Kitt Peak, Arizona, which will enable redshift measurements of 35 millions of galaxies and quasars between 0.05 < z < 3.0, yielding a tenfold increase in statistics compared to previous spectroscopic surveys (e.g. BOSS, eBOSS). The first year of DESI data taking, corresponding to one fifth of the total statistics, has been completed, thereby constituting the largest spectroscopic dataset ever assembled.



We propose to develop a theoretically lossless approach to extracting cosmological information from modern galaxy surveys, in particular DESI, which consists in reproducing the observed galaxy density with simulations of the Universe large scale structure. This new approach requires methodological developments to make high-dimensional (~ 10^10) Bayesian inference tractable, and to accelerate numerical simulations with hybrid physical and machine learning modelling. The PhD candidate will apply the developed methodology to the analysis of the DESI first year data to produce state-of-the-art cosmological constraints. This project will lead to three first-author publications.

The Large Scale Structures (LSS) of the Universe come from the growth, under the effect of gravitation, of small primordial fluctuations of density created by inflation. The measurement of the statistical properties of LSS allow us to study the inflation at very large scales (~Gpc), the Dark Energy at smaller scales (~100 Mpc) with Baryonic Acoustic Oscillations (BAO) and the gravity at even smaller scales (~tens of Mpc) with Redshift Space Distortions (RSD).



Our strategy for studying the LSS is to use a spectroscopic survey, DESI that will observe tens of millions of galaxies and quasars. The observations take place at the 4-meter Mayall telescope in Arizona .Since spring 2021, the project has started an uninterrupted observation period that will last 5 years and that will cover a quarter of the sky.



For this PhD, LSS are measured with a single tracer of the matter: the quasars, very distant and very luminous objects. This tracer allows us to cover a wide redshift range from 0.9 to 3.5 and to the Universe clustering at all scales, from a few tens of Mpc to Gpc.



During the first year, the PhD student will participate in the analysis of the first observation year. The PhD student will be able to devote to a global measurement of the cosmological parameters which will simultaneously cover all the scales. The thesis will end with the study of the first three years of observation of DESI.

Positron emission tomography (PET) is a medical nuclear imaging technique widely used in oncology and neurobiology. The decay of the radioactive tracer emits positrons, which annihilate into two back-to-back photons of 511 keV. These pairs of photons are detected in coincidence and used to reconstruct the distribution of the tracer activity in the patient's body.

In this thesis, we propose to contribute to the development of the cutting-edge patented technology ClearMind. The first prototype is currently being tested in the laboratory. The proposed detector uses a monolithic lead tungsten crystal in which Cherenkov and scintillation photons are produced. Those photons are converted to electrons by the photo-electric layer and multiplied in a microchannel plate. The induced electrical signals are amplified by gigahertz amplifiers and digitized by the fast acquisition modules SAMPIC. The opposite surface of the crystal will be equipped with a matrix of the silicon photo-multiplier. Machine-learning techniques will be applied for processing the complex acquired signals in order to reconstruct the time and coordinates of the gamma-conversion in the crystal.

The candidate will work on the development of high-efficient machine learning algorithm for the reconstruction of the gamma-conversion vertex in the monolithic crystal. In particular, this work consists in the evolution and improvement of the existing Geant4 detector simulation for its adjustment to the prototype performances as measured in the laboratory. This simulator will be used to feed a training database for the development and optimization of deep neural network algorithms for the efficient reconstruction of the gamma-interaction using the full signal shape and/or the pre-processed data (aka features engineering). The reconstruction performances of these algorithm will be assessed on real test data acquired with the available ClearMind prototype. Special attention will be made on the development of compact, efficient and fast networks together with a robust uncertainty estimation of the reconstructed parameters in the context of trustworthy AI. The possibility of embedding these algorithms in FPGAs for fast on-line reconstruction will be studied.

Why is the observable Universe today made of matter, without any significant amount of antimatter? Neutrinos shed light on this cosmic mystery.

In 2020, the T2K collaboration in Japan published in the journal Nature new results leading to the best constraint to date on the degree of asymmetry between matter and antimatter. The T2K results exclude for the first time nearly half of the possible values at 99.7% of confidence level and the value most compatible with the data correspond to a maximum asymmetry between matter and antimatter (notably between neutrinos and antineutrinos). T2K has the best world sensitivity for this crucial measurement and is going to collect new data from 2023 with an upgraded detector to search for a possible discovery of symmetry violation.

T2K is a neutrino experiment designed to study the transition of neutrinos and antineutrinos from one flavor to another as they travel (neutrino oscillations). An intense beam of muon neutrinos is generated at the J-PARC site on the East coast of Japan and directed to the Super-Kamiokande neutrino detector in the mountains of western Japan. The beam is measured once before leaving the J-PARC site, using the ND280 near-field detector, and again at Super-Kamiokande: the evolution of the measured intensity and the composition of the beam are used to determine the properties of the neutrinos.

The thesis work will be twofold, including the analysis of the new T2K data for the measurement of the (anti)neutrino oscillations and the commissioning and scientific exploitation of the High-Angle Time Projection Chamber (High-Angle TPC). The objective of this new detector is to improve the performance of the ND280 near detector, to measure the neutrino production and interaction rate so that the uncertainty on the number of events predicted neutrinos at Super-Kamiokande is reduced to about 4%. The student will use cosmic data to align the TPC modules. Then, he will exploit the first data to calibrate the TPC and evaluate its performance.



The student will perform the analysis of the new data which will be collected by T2K to measure the matter-antimatter symmetry violation in neutrino oscillations. The upgrade of the near detector will require to put in place a new analysis strategy. For the first time, the measurement of low momentum protons and neutrons produced by neutrino interactions will be exploited. Another important part of the analysis which must be updated to cope with increased statistics, is the modeling of the flux of neutrinos produced by the accelerator beamline.



A new generation of experiments is expected to multiply by a factor 20 the data production in the next decades. In Japan, the Hyper-K experiment, and in the USA, the DUNE experiment, will be operational around 2027-2028. This thesis work will explore new analysis strategies crucial also for such next-generation experiments. If their new data confirm the preliminary results of T2K, neutrinos could bring before ten years the key to understand the mystery of the disappearance of antimatter in our Universe.

The neutrinoless double beta decay (0nbb) is a very rare nuclear transition that plays a key role in

(astro)particle physics for the study of the nature of neutrinos and lepton number violation. CUPID is

a proposed next-generation experiment to study the 0nbb. The analysis and control of the radiogenic

background is a major challenge for the experiment. CUPID uses scintillating bolometers operating at

temperatures of a few mK. In this thesis work, prototypes of CUPID will be developed and analysed in

surface and underground laboratories (Gran Sasso Laboratory in Italy and Canfranc in Spain). The

radiogenic background model of CUPID, aiming at evaluating the sensitivity of the experiment to new

physics, will be refined by simulations based on the experimental performance of the prototypes,

using the GEANT-4 package and a boosted-decision-tree analysis. The overall objective of the thesis is

the definition of the final configuration of CUPID, based on both the optimization of the CUPID

modules and the improvement of the radiation background model.

The GBAR experiment at CERN aims at measuring the gravitational acceleration of antimatter on Earth using ultra-cold antihydrogen atoms. In order to obtain these ultra-cold anti-atoms, the key is to first produce positive antihydrogen ions (two positrons and one antiproton, equivalent to H–), using positronium (bound state of an electron and a positron) for that purpose.

The PhD topic is dedicated to the study of the charge exchange reaction between an antihydrogen atom and a positronium atom, producing a positive antihydrogen ion. The first objective is to measure the cross sections for this reaction, for which only theoretical values exist, using hydrogen instead of antihydrogen and producing H–. The second objective is to observe the production of antihydrogen ions and optimise it. An experimental measurement of the cross sections will provide a test for several low-energy atomic collision models that currently provide disagreeing theoretical values. The first ever detection of an antihydrogen ion will be a major milestone for GBAR and will also open new opportunities for future antimatter experiments. Finally, an application of the measured cross sections to the positron annihilation in the interstellar medium will be explored.

From 2023 to 2025, GBAR will receive beams of antiprotons and H– and the experimental program of this thesis will be carried out during this period at CERN. 2026 will mainly be dedicated to finalising the data analysis and PhD dissertation writing.

Ten years ago, the ATLAS and CMS experiments at LHC at CERN discovered a new boson, with a dataset of proton-proton collisions of about 10 fb-1 at the centre of mass energy of 7 to 8 TeV [1,2]. Since then, the properties of this particle have been tested by both experiments and are compatible with the Higgs boson properties predicted by the Standard Model of particle physics (SM) within the uncertainties. In absence of direct probes of New Physics, increasing the accuracy of the measurements of the properties of the Higgs boson (its spin, its parity and its couplings to other particles) remains one of the most promising path to pursue.

The measurement of the ttH production allows the direct access to the top quark Yukawa coupling, fundamental parameter of the SM. ttH production is a rare process, two orders of magnitude smaller than the dominant Higgs boson production by gluon fusion. This production mode has been observed for the first time in 2018 [3, 4] separately by the CMS and ATLAS experiments, by combining several decay channels. More recently, with the full Run 2 dataset (data recorded between 2016 and 2018, with a total of 138 fb-1 at 13 TeV), this production mode was observed also using solely the diphoton decay channel, and a first measurement of its CP properties was provided again by both experiments, with the exclusion of a pure CP odd state at 3s [5, 6]. The associated production with a single top quark is about 5 times smaller than the ttH production and has never been observed. Thanks to the searches in the diphoton and multilepton channel, very loose constraints on this production modes were set for the first time recently (see Ref. [7]). This production mode is very sensitive to the H-tt coupling CP properties, since in case of CP-odd coupling, its production rate is largely increased. We propose in this thesis to study jointly the two production modes (ttH and tH) and the H-tt coupling CP properties with Run 3 data (data being recorded now and until 2026, with potentially about 250 fb-1 at 13.6 TeV) in the diphoton decay channel. If there was some CP violation in the Higgs sector, excluding small pseudo-scalar contributions will require more data. Pursuing these studies with Run 3 and beyond may allow to pinpoint small deviations not yet at reach. We propose to bring several improvements to the Run 2 analysis strategy and to use novel reconstruction and analysis techniques based on deep-learning, developped in the CEA-Saclay group by our current PhD students but not yet used in physics analyses, in order to make the most of the available dataset.

[1] ATLAS Collaboration, “Observation of a new particle in the search for the Standard Model Higgs boson with the ATLAS detector at the LHC,” Phys. Lett. B 716 (2012) 1.

[2] CMS Collaboration, “Observation of a new boson at a mass of 125 GeV with the CMS experiment at the LHC,” Phys. Lett. B 716 (2012) 30.

[3] ATLAS Collaboration, “Observation of Higgs boson production in association with a top quark pair at the LHC with the ATLAS detector”, Phys. Lett. B 784 (2018) 173.

[4] CMS Collaboration, “Observation of ttH Production”, Phys. Rev. Lett. 120 (2018) 231801.

[5] CMS Collaboration, “Measurements of ttH Production and the CP Structure of the Yukawa Inter- action between the Higgs Boson and Top Quark in the Diphoton Decay Channel”, Phys. Rev. Lett. 125, 061801.

[6] ATLAS Collaboration, “CP Properties of Higgs Boson Interactions with Top Quarks in the ttH and tH Processes Using H ? ?? with the ATLAS Detector” , Phys. Rev. Lett. 125 (2020) 061802.

[7] CMS Collaboration, “A portrait of the Higgs boson by the CMS experiment ten years after the discovery”, Nature 607 (2022) 60.

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.



Over LHC run-3, that started during summer 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 has been significantly modified and upgraded. The upgraded trigger system is based on real time analysis 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, see figure 1), i.e. the board that digitizes the detector analog signals and transmits them to the back-end system.

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.These preliminary studies have been done with standard deconvolution algorithms, based on an a priori knowledge of the signal pulseshape. A very promising improvement would be to use a neural net.



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 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.

The thesis proposes to measure for the first time in a coherent way the different rare processes of production of top quarks in association with bosons, with three charged leptons in the final state at the Large Hadron Collider. The thesis will be based on the analysis of the large set of data collected and being acquired by the ATLAS experiment. The joint analysis of the ttW, ttZ, ttH and 4top processes where one signal is the background of the other will allow for the first time to have complete and unbiased measurements of the final state with three leptons.



These rare processes, recently accessible at the LHC, can probe the models explaining the current anomalies observed in flavor physics. These anomalies could be the first signs of new physics beyond the Standard Model of particle physics. The ttH process also makes possible the direct study of the coupling between the top quark and the Higgs boson, which could provide new sources of matter-antimatter asymmetry. Discovering signs of new physics that go beyond the limitations of the Standard Model and in particular new sources of matter-antimatter asymmetry is a fundamental question in particle physics today.

The Higgs boson discovered at the LHC in 2012 is the cornerstone of the Standard Model (SM). Its properties, such as its mass or spin, are now better and better known. Nevertheless, the total width of the Higgs boson remains a fundamental parameter that is very difficult to measure at the LHC without the support of theoretical assumptions.

In this PhD thesis, we propose to pursue an original approach to measure this parameter, approach only possible in the diphoton decay channel of the Higgs boson. Indeed, in this channel the position of the mass peak depends on the interference between the Higgs boson signal and the background noise. The resulting shift depends on the natural width of the Higgs boson. This is a very small effect in the SM but could be larger when considering Higgs bosons produced at high transverse momentum.

This type of analysis requires a thorough mastery of the various uncertainties related to the experimental apparatus, in particular to the electromagnetic calorimeter (ECAL), and to the reconstruction of the electromagnetic objects. In order to improve the latter, the student will develop a new approach to electromagnetic-object reconstruction based on a technique initiated at CEA-Irfu by the CMS group and using state-of-the-art methods in artificial intelligence (Convolutional NN and Graph NN).

These two aspects will be addressed in parallel during the thesis. The student will be supervised by the CMS group of Irfu whose expertise in the ECAL and in the two-photon Higgs boson decay channel is internationally recognised.