A Search for the Production of a Single Top Quark in association with a Z Boson at the LHC

A search for the production of a single top quark in association with a Z boson and an additional jet using data from proton-proton collisions at $\sqrt{s}$ = 13 TeV collected by the Compact Muon Solenoid (CMS) experiment at the Large Hadron Collider is presented. This is a rare process that is predicted by the Standard Model. This search focussed on identifying the final state containing two leptons from the Z boson decay, two jets from the decay of the W boson produced by the top quark decay, a b-jet from the top quark decay and a recoil jet. The signal was dominated by backgrounds involving a real Z boson or two promptly produced leptons consistent with a Z boson decay, primarily Z+jet and top quark pair production. As such, a Boosted Decision Tree was used to enhance the separation between the signal and background processes. Using a dataset corresponding to 35.9 $\text{fb}^\text{$-$1}$, signal strengths of $6.21_{-2.67}^{+2.34}$ and $4.73_{-2.02}^{+1.92}$ were measured for this process when the Z boson decays into a pair of electron or muons, respectively, and the W boson decay hadronically. These measurements correspond to an observed (expected) signal significance of $2.72\sigma$ ($0.46\sigma$) and $2.50 \sigma$ ($0.54\sigma$), respectively, when compared to the background-only hypothesis. These measurements are consistent within two standard deviations of the Standard Model prediction. The CMS experiment's new tracking detector at the High-Luminosity Large Hadron Collider will require the ability to reconstruct all charged tracks with transverse momentum greater than 2-3 GeV within 4 $\mu\text{s}$ so that they can be used in the Level-1 trigger decision. One of the proposed track finders is an FPGA-based based solution using a fully time-multiplexed architecture, where track candidates are reconstructed using a projective binning algorithm based on the Hough Transform. Studies into the suitability of a linearised $\chi^{2}$ algorithm for fitting track parameters were undertaken and it was found that its performance was inferior compared to that of a combinatorial Kalman Filter fitter. The impact of reducing the minimum track transverse momentum from 3 GeV to 2 GeV on the proposed system was also evaluated. The resulting degradation of performance was found to be recoverable by improving the handling of multiple scattering in the track finding and fitting algorithms.