Search for long lived particles in the CMS muon system

This thesis presents two searches for long-lived particles (LLP) with long lifetimes, leading to flight distances in the order of meters. The searches are performed with data from proton-proton collisions at a center-of-mass energy of 13 TeV from the Large Hadron Collider (LHC) corresponding to an integrated luminosity of L = 137 fb${}^{-1}$ recorded with the Compact Muon Solenoid (CMS) detector. Both searches target long-lived particles decaying hadronically inside the CMS muon system, which is uniquely suited for such a search. The Drift Tubes (DT) and Cathode Strip Chambers (CSC) are placed within the iron return yoke, which allows the usage of the muon system as a sampling calorimeter for hadronic showers. These Muon Detector Showers (MDS) are reconstructed with a density based clustering algorithm (DBSCAN), clustering the detector level hits together. This newly created object was developed for the analyses presented in this thesis. The first analysis considers a Twin Higgs benchmark model, with long-lived particle masses between 7 and 55 GeV and lifetimes between 0.1 and 100m, with decays into a pair of b quarks, d quarks or 𝜏 leptons being considered. The LLPs will create an isolated hadronic shower in the muon system, a large amount of missing transverse momentum pointing in the same direction and an initial state radiation jet on the other side of the detector. The main background consists of Standard Model particles from pileup with large displacements and is estimated using a data-driven ABCD method. The analysis finds no evidence for long-lived particles, but improves the current best upper limits at 95 % confidence level on the possible branching ratio of the long-lived particle by a factor between 2 to 6, depending on the mass and lifetime of the LLP. The second analysis targets long-lived heavy neutral leptons (HNL), with HNL masses between 1 and 3.5 GeV and lifetimes between 1mm and 10m. The HNL can either be Majorana or Dirac type, each of which decays into a lepton and a hadronically decaying W boson, forming a shower in the muon system. The signal signature consists of an isolated hadronic shower being aligned with the p${}_T^{miss}$ and a prompt lepton on the other side of the detector. The background consists of pileup and Z → 𝜇𝜇 events, where one muon emits bremsstrahlung, and is estimated using a data driven ABCD method. No evidence for HNLs is found but an upper limit at 95 % confidence level on the mixing angle is set for the different HNL masses and flavors. The current best limit is improved for HNL masses between 1.9 and 3.3 GeV depending on the flavor of the HNL.