Search for New Physics in All-Hadronic Final States with the CMS Experiment

The most accredited description of our universe is given by the Standard Model (SM) theory, which explains a wide variety of natural phenomena and whose correctness was confirmed by several experimental observations. Despite the huge success of the SM of particle physics, there are some unsolved phenomena, such as the hierarchy problem and the nature of Dark Matter (DM). The stabilization of the Higgs boson mass, whose value differs from its radiative corrections, appears to be unnatural and suggests that underlying theories may exist. Also, even if many astrophysical observations confirmed the existence of DM, information about its nature or non-gravitational interactions is not yet available. The Large Hadron Collider (LHC) at the European Organization for Nuclear Research (CERN) has been built to seek answers to these questions and explain the shortcomings of the SM. Two searches for beyond the Standard Model (BSM) physics are performed using proton-proton collision data recorded by the CMS detector at a center-of-mass energy of 13 TeV. Both searches target fully hadronic final state events, which are particularly challenging because of large background contributions from multijet events from Quantum Chromodynamics (QCD) processes. The first search presented aims at seeking new heavy fermionic resonances called vector-like quarks (VLQs) which if exist, could solve the hierarchy problem. In particular, I focus on the search for a single produced VLQ B decaying in a bottom quark and a Higgs boson, that sub-sequentially decays to a b pair. This analysis uses 35.9 fb$^{−1}$ of integrated luminosity collected in 2016 by the CMS experiment. The fully hadronic final state and the large contamination from multijet background processes make this signature particularly challenging. The results of this search show that this is a promising signature that could play a crucial role in future searches for new physics. Moreover, this search is the first exploration at CMS of non-narrow resonance widths of VLQs. No significant deviation from the SM expectation is observed and 95% CL upper limits on the VLQ B production cross-section times the branching ratio are set in both the narrow width and the finite width regimes. Expected and observed limits at 95% confidence level vary from 1.20 to 0.07 pb and from 1.28 to 0.07 pb, respectively, for VLQ B quark masses in the mass range considered, which extends from 700 to 1800 GeV. The second part of this thesis focuses on the search for DM in the context of Hidden Valley models. These theories predict the existence of dark quarks, that manifest as semi-visible jets in the CMS detector. In particular, the model investigated in this thesis predicts the existence of a leptophobic heavy resonance, a Z' gauge boson, connecting the SM and DM sectors, and giving rise to two dark quarks that shower and hadronize leading to the formation of two semi-visible jets. This search uses 35.9 fb$^{−1}$ of data collected in 2016, 2017 and 2018 by the CMS experiment. It is the first analysis investigating the semi-visible jets hypothesis at particle colliders. It plays a pivotal role in the context of DM searches and in general in the query of new physics hiding in hadronic final states with missing momentum signatures. No excess of events over the expected SM background is observed, therefore upper limits are placed on the production cross-section as a function of the mediator mass. The specific Hidden Valley model probed in this thesis work is excluded for mediator bosons with mass up to 3.9 TeV.