Search for long-lived particles decaying into displaced dilepton vertices in proton-proton collisions at $\mathbf{\sqrt{s} = 13}$ TeV with the CMS detector

Some of the most popular extensions of the Standard Model of Particle Physics contain long-lived particles, which can be framed in a wide variety of theories, covering different regions of the phase space. Long-lived particles manifest in the particle detectors in the form of non-conventional signatures that often require the development and optimization of new analysis techniques, different to those that are commonly used in other searches where the particles have smaller lifetimes. This thesis presents a search for long-lived particles decaying into final states with a displaced dilepton vertex. It analyzes proton-proton collisions at a center-of-mass energy of 13 TeV that were produced by the CERN LHC and collected by the CMS detector in 2016, 2017 and 2018. Leptons in this analysis appear as displaced from the interaction point and need to be reconstructed with special algorithms. The collision events are required to have at least two opposite charge, same flavor leptons fitted to a common displaced vertex in the tracker, which is the main handle of the analysis. The results of the search are interpreted in the context of an extended Higgs-sector model with a non-SM Higgs boson decaying to two spinless long-lived bosons. The search is designed to cover a wide mass range, and be sensitive to small and medium lifetimes from 0.1 cm to 1000 cm. Observed data in the analysis is found to be in good agreement with the Standard Model expectations. Upper limits have been derived for a Higgs model decaying into two scalar long-lived particles decaying into displaced leptons. The limits are estimated separately for the muon and electron channels, and then both combined. These results are competitive with similar results shown by the CMS and ATLAS collaborations and in particular they provided the most stringent limits for the electron channel.