MEASUREMENT OF THE MAGNETIC AND ELECTRIC DIPOLE MOMENTS OF THE $\LAMBDA$ BARYON AT LHCB

One of the most profound and unresolved questions in modern physics is why the Universe is composed predominantly of matter, with negligible antimatter. This imbalance suggests the existence of processes that violate the fundamental symmetry between matter and antimatter, the so-called CP symmetry. However, the amount of CP violation predicted by the Standard Model of particle physics (SM), via the Cabibbo-Kobayashi-Maskawa (CKM) quark-mixing mechanism, cannot explain the large observed asymmetry, thus new sources of CP violation, beyond the SM, are expected. A possible way to probe such sources is to search for electric dipole moments (EDMs) of fundamental particles. This thesis explores that possibility through the study of the \Lz baryon: an EDM observation with the current sensitivities would constitute evidence of CP violation beyond the SM. Moreover, the magnetic dipole moment (MDM) of the \Lz baryon and the $\bar{\Lz}$ antibaryon will be measured simultaneously, allowing an unprecedented test of the CPT symmetry in this particle-antiparticle system. Notably, the MDM of the $\bar{\Lz}$ antibaryon has never been measured to date. All these measurements of the \Lz baryon electric and magnetic dipole moments are performed for the first time at a collider experiment. The analysis is conducted using data collected by the LHCb experiment during the \mbox{Run 1} and \mbox{Run 2} of the LHC, corresponding to an integrated luminosity of about 9 fb$^{-1}$. The LHCb detector was designed to investigate CP-violating phenomena of beauty and charm hadrons. This thesis describes how its capabilities have been extended to enable investigations of dipole moments of strange baryons. The measurement relies on the spin precession of longitudinally polarized \Lz baryons in the magnetic field of the LHCb tracking system. The polarization is inferred via an angular analysis of the \Lz decay products. A novel aspect of this work is the use of particles that decay downstream of the LHCb magnet, reconstructed solely with hits in the final tracking stations. These so-called T tracks present significant challenges in the reconstruction due to their limited momentum and vertex resolutions. Their exploitation in physics analyses is being pioneered in the context of this study. To overcome the performance limitations, a dedicated study to improve the reconstruction of long-lived \Lz and \KS decays using T tracks has been conducted in the first part of the thesis, developing custom techniques with significant impact on the reconstruction efficiency and resolutions. In the second part, the measurement of the electric and magnetic dipole moments is performed using \LbToJpsiLz decays. The analysis is validated using \BdToJpsiKS decays as control sample, in which the long-lived $K_S^0$ meson, being a pseudoscalar particle, does not exhibit any polarization nor dipole moments. The measurement of dipole moments is performed on the control sample as a null test to validate the method before applying it to the signal decays. The LHCb detector has undergone a major upgrade for the data-taking during Run 3 of the LHC. In this context, the new detector is briefly presented, with a particular focus on the Upstream Tracker. The author of this thesis contributed to this subsystem during its assembly, installation, and commissioning phases.