Gianfranco DeCastro: SEARCH FOR SUEPS PRODUCED IN ASSOCIATION WITH A VECTOR BOSON AT THE LHC WITH CMS RUN 2 DATA

The Standard Model (SM) of particle physics is the most complete description of the Universe's fundamental particles and the forces governing their interactions. However, it fails to account for the evidence of dark matter (DM), which has puzzled scientists for decades. The Large Hadron Collider (LHC) at CERN in Geneva, Switzerland, hosts the largest international scientific collaboration in history, with one of its primary goals being to reconcile the SM with the existence of DM. This thesis documents a search for a beyond-the-Standard-Model (BSM) signature that could provide such a connection: soft unclustered energy patterns (SUEPs). At sufficiently high energies, a SM-like Higgs boson (H) mediator may hypothetically couple to a dark quantum chromodynamics (QCD) sector populated by DM candidates. These dark-sector particles may subsequently decay back into the SM, producing a soft, isotropic spray of charged particles, referred to as SUEPs. A search for SUEPs produced in association with a \PW or \PZ boson is presented. The analysis is based on 13 TeV proton–proton collision data corresponding to an integrated luminosity of 137.8 1/fb collected between 2016 and 2018 during Run 2 of the LHC with the Compact Muon Solenoid (CMS) detector. Final states with a high multiplicity of low-momentum tracks and either two leptons compatible with a Z boson decay or one lepton and missing transverse momentum (MET) compatible with a W boson decay are explored for the first time. Background contributions are estimated using a data-driven extended ABCD method, and systematic uncertainties are constrained through control regions (CRs) defined orthogonally to a signal-enriched region (SR). A simultaneous fit is performed across decay modes, CRs, and SRs. The results show no significant excess over the SM expectation, and upper limits are set on the branching fraction of H boson decays to SUEPs.