Camille Normand – Phd’s defense – oct. 13, 2023

The current searches for signals of physics beyond the Standard Model of particle physics (SM) have brought together cosmologists, astrophysicists, and particle physicists, combining their experimental and theoretical efforts. These searches have been ongoing since the discovery of the SM Higgs boson by the ATLAS and CMS collaborations at the LHC, marking the completion of the SM. New hope for signals beyond the SM came recently from the “b anomalies”, a collection of experimental results that deviate from the predictions of the SM. These anomalies concern observables related to a flavor-changing neutral current involving a b quark and an s quark, and to a flavor-changing quark current from a b quark to a c quark. The discrepancies were first observed in 2013 in measurements of the B→K*µµ decay channel, particularly in the P5′ observable that describes the decay amplitude’s angular properties. A few years later, the LHCb experiment made a surprising measurement known as the Lepton Flavor Universality (LFU) test ratios RK and RK*, which showed a significant deviation from the expected LFU, potentially indicating a major discovery. However, an update of this measurement in December 2022 changed the situation, and it now aligns with the predictions of the SM. Similarly, updated measurements of the branching ratio of the decay of a Bs meson into two muons by LHCb and CMS have also shown compatibility with the SM — in another channel considered crucial for its sensitivity to new physics (NP). In this context, a reappraisal of the b anomalies is crucial, especially because the SM predictions for the remaining discrepant data are not as precise as the two aforementioned channels, due to long-distance effects. This reevaluation can take two main approaches. 

First, it involves improving the accuracy of theoretical predictions and experimental measurements for known observables such as the B→K*µµ channel. Second, it involves identifying new decay channels that are sensitive to the same NP signals, whereas they do not suffer from the same long-distance effects. This work focuses on the latter approach by conducting a comprehensive study, both experimental and theoretical, of the decay of a Bs meson into two muons and a photon in a specific region of the phase space. This decay process potentially provides substantial information about the nature of the observed discrepancies. A novel approach to the treatment of data from Quantum Chromodynamics (QCD) on the lattice is presented, enabling the assessment of the sensitivity to potential NP and the study of the effective lifetime of Bs → μμγ, an alternative observable that is sensitive to CP-violating NP. However, measuring this decay channel in the particular kinematic range of interest poses a challenge due to the softness of the photon. Reconstructing such soft photons is quite difficult, particularly at hadronic colliders. 

Hence, the method employed in this study relies on a partial reconstruction of the final state, utilizing only the information from the di-muon system, which is known to have a clean signal at hadronic colliders. The search for this decay is presented as a partially reconstructed signal at the LHCb experiment of the Large Hadron Collider. The study highlights the various techniques employed for background reduction and estimation, which are crucial for accessing a decay process with an extremely low branching fraction.