Description du sujet de thèse

Measurements of $b$-hadron decays provide powerful tests of the Standard Model, and can help shed light on some of the great unanswered questions in particle physics, such as the matter-antimatter asymmetry in the universe and the underlying nature of the fermion families, their number and their mass hierarchy.

In search of answers to these questions, the LHCb experiment has produced leading measurements in rare decays and $CP$ Violation in the heavy quark sector using the first two runs of its operation. A particularly interesting set of channels are $B_{(s)}\to hh$ decays, which can provide powerful probes of QCD, as well as potential New Physics. Recent measurements of these decays from the first data-taking period of LHCb and Belle show tantalising patterns of deviations from the Standard Model flavor SU(3) symmetry of the strong interactions. Understanding the origin of these deviations requires the analysis of additional data, which LHCb is in the process of collecting. In fact, the experiment has recently removed its hardware trigger and is relying on a purely software-based trigger system, in order to maximise its efficiency in signatures of interest. First data from 2024 confirm the impressive increase in trigger yields of key physics channels. The candidate will analyse this brand new data-set, investigating $B_{(s)}\to hh$ signatures that are expected to particularly benefit from the new trigger system.

The second part of the thesis subject concerns physics sensitivity studies to place detector requirements for experiments at the Future Circular Collider FCC-ee. This projected facility at CERN features a 90 km long tunnel hosting an $e^+e^-$ collider with a centre-of-mass energy spanning from the $Z$ pole (91 GeV) to the top quark pair production (365 GeV). The exquisite design luminosity makes it simultaneously an electroweak gauge bosons, Higgs and top factory. The operation at the $Z$ pole provides about $10^{13}$ $Z$ decays, resulting in abundant samples of $b$, $c$ and $\tau$ particles, making a case for the continuation of the Flavour physics program and inducing specific detector requirements. Among the channels to be studied within this thesis framework, the decays of heavy-flavoured particles involving neutral pions (useful for $CP$-symmetry breaking measurements) will likely place stringent requirements on the energy resolution of the electromagnetic calorimeter. The canonical mode we'd like to explore in this Ph.D. program is the decay $B^0 \to \pi^0 \pi^0 $ featuring a subsequent Dalitz decay $\pi^0 \to e^+e^- \gamma$ of one of the two $\pi^0$. The presence of the electron pair in the final state will allow simultaneously calorimetric electron reconstruction to be established and a time-dependent study of the decay to be performed.

Contexte de travail

This Ph.D. program is embodied in a research team, comprising researchers from Clermont and Orsay universities, with longstanding collaborative researches.

The Irène Joliot-Curie Laboratory of Physics of the Two Infinities is a physics laboratory specializing in the two infinities under the supervision of the CNRS, Paris-Saclay University, and the University of Paris. It was created in 2020 from the merger of five joint research units located on the Orsay university campus: the Center for Nuclear Sciences and Material Sciences (CSNSM), the Laboratory for Imaging and Modeling in Neurobiology and Cancer Research (IMNC), the Orsay Institute of Nuclear Physics (IPNO), the Linear Accelerator Laboratory (LAL), and the Laboratory of Theoretical Physics (LPT).

The laboratory's research topics include nuclear physics, high-energy physics, astroparticles and cosmology, theoretical physics, particle accelerators and detectors, as well as technical research and development and related applications for energy, health, and the environment.

The facility has significant technical capabilities (approximately 280 engineers and technicians) in all the major fields required to design, develop, and implement the experimental devices needed for its scientific activities: mechanics, electronics, computer science, instrumentation, acceleration techniques, and biology techniques. These technical strengths are a major asset for the design, development, and use of the necessary instruments (accelerators and detectors). The presence of research infrastructures and technological platforms on the laboratory site is also a major asset. Finally, approximately 90 administrative and support staff work alongside scientists and engineers.

IJCLAB bases its recruitment policy on the promotion of equality, diversity, and inclusion. These core values enable the professional development of employees, who are key players in collective success, as well as the development of the laboratory itself.

Contraintes et risques

The successful candidate will spend a year and a half in each institute with regular meetings with the supervisors at distance. Regular stays at CERN are foreseen in the course of this 3-years program.