Abstract: The Standard Model is the most successful theory for describing the fundamental laws of nature. Despite its outstanding success, it is known that it is not a complete theory. Consequently, the search of new physics beyond the Standard Model stands as one of the main goals of the particle physics community. An experiment that contributes to this effort is LHCb, one of the four major detectors located at the LHC particle accelerator operated by CERN. Its research focuses on high-precision measurements of particles involving heavy b and c quarks.

Rare b—>sll transitions are strongly suppressed in the Standard Model (SM), making them sensitive to physics beyond the SM. Leptons from the different families have the same coupling to electroweak bosons in the SM, symmetry that is known as Lepton Flavour Universality (LFU). Precise measurements of LFU ratios in b—>sll decays provide, then, a very powerful null test of the SM, since a significant deviation from unity would clearly imply the existence of new physics. This thesis aims to update the LFU test performed with Lb—>pKll using Run 1 and 2016 data, which was the first LFU test involving b-baryons published by LHCb. This analysis includes the full Run 1 and Run 2 dataset (adds 2017-18 data), a full re-optimisation of the selection after the observation of the electronic decay, with particular focus on the MVA and particle identification requirements. Additionally, the measurement splitted in low and central q2 regions will be provided. The preliminary results show a clear improvement of the statistical uncertainty (16% to 9.5%) with a better control of the backgrounds.

In Run 3 the LHCb experiment underwent a major upgrade to operate at higher instantaneous luminosity. In the first years of Run 3 data taking a task-force was carried out to understand and validate the data recorded by the new detector. The first analysis of a decay with electrons in the final state using LHCb Run 3 data is also reported in this thesis.

Tribunal:

President: Dr. Lluís Garrido Beltran

Secretary: Dr. Maria Vieites Diaz

Vocal: Dr. Lesya Shchutska

Suplents:

Dr. Míriam Calvo Gómez

Dr. Martino Borsato

Directors:

Dr. Eugeni Graugés Pous

Dr. Carla Marin Benito

Tutor: Dr. Joan Soto Riera