A search for new heavy particles in events with highly ionising, short tracks at the CMS experiment

The main focus of the CMS experiment at the Large Hadron Collider (LHC) is to search for physics beyond the Standard Model and to measure Standard Model parameters. For both purposes it is essential to determine important performance parameters of the CMS detector. The here presented thesis contributes in a twofold way to the physics program of CMS. In the first part of this thesis, a search for physics beyond the Standard Model is presented. It is motivated by supersymmetric models with nearly mass-degenerate lightest neutralinos and lightest charginos. The small mass gap between chargino and neutralino can lead to long lifetimes of the chargino due to phase space suppression. Thus, the chargino can reach the tracking system before its decay. The here presented search targets chargino lifetimes of $\text{c}\tau \approx 1 - 30\,\text{cm}$ where most of the charginos decay in the first layers of the tracker. This search aims at increasing the search sensitivity of existing searches with respect to these models in a twofold way: first, the inclusion of tracks down to three measurements in the tracking system, and second, the discrimination against Standard Model background by the energy loss per path length. The search is performed on $19.7\,\text{fb}^{-1}$ of data recorded at the CMS experiment at a centre-of-mass energy of $8\,\text{TeV}$. No excess above the Standard Model expectation is found and the supersymmetric parameter space is constrained. The search can exclude supersymmetric models with chargino masses of $100\,\text{GeV}$ down to lifetimes of $\text{c}\tau=2\,\text{cm}$ and models with masses of $500\,\text{GeV}$ down to lifetimes of $\text{c}\tau=70\,\text{cm}$. Current limits are confirmed and improvements of the order of $10-40\,\text{GeV}$ in chargino mass are achieved. In the second part of the thesis, a measurement of the jet transverse-momentum resolution at $8\,\text{TeV}$ at the CMS experiment is presented. In order to exploit the good energy resolution of the electromagnetic calorimeter of the CMS detector, the measurement is performed using $\gamma + \text{jet}$ events. Due to the transverse momentum balance in the absence of further jet activity, the photon momentum can be used as a measure for the true jet transverse momentum. The applied method is based on earlier measurements but is further developed within this thesis in order to consistently account for the influence of additional jet activity on the jet transverse-momentum response. Relative differences of the resolution in simulation and in data are measured between 7% and 20%.