Measurement of the inclusive and differential $t$-channel single top quark production cross section at 13 TeV with the CMS experiment

Abstract

A measurement of the inclusive and differential $t$-channel single top quark production cross section is performed in this thesis. The measurement uses 137 fb$^{-1}$ of data recorded at the CMS experiment at the LHC with a center-of-mass energy of 13 TeV. Events are selected with exactly one muon or electron and two or three jets, of which at least one is identified as originating from a bottom quark. In the analysis an improved technique for reconstructing the top quark has been developed that makes use of a neural network in order to achieve a better description of the top quark\u27s kinematic variables. A multiclassification BDT is used to classify events into different process categories. The cross sections are extracted from a fit to the output distribution of the multiclassification BDT. The inclusive cross section of $t$-channel single top quark production was measured to be $\sigma_{\mathrm{t}} = 130 \pm 20 \, \mathrm{pb}$ and the cross section of top antiquark production to be $\sigma_{\mathrm{\bar{t}}} = 80 \pm 15 \, \mathrm{pb}$. The differential cross section measurement is performed via unfolding. The measured differential cross sections as a function of the top quark transverse momentum and rapidity agree with the predictions of the SM. Three angular variables, $\cos x, \cos y$, and $\cos z$, are defined in the top quark rest frame between the charged lepton from the top quark decay and three axes, which are defined based on the direction of the spectator quark and the beamline axis. The asymmetries in these distributions are measured to be: $A_{x}(\mathrm{t}+\mathrm{\bar{t}})=-0.07\pm0.09$, $A_{y}(\mathrm{t}+\mathrm{\bar{t}})=0.00\pm0.05$, and $A_{z}(t+\bar{t})=0.42\pm0.08$. The measured asymmetries are used to constraint the magnitude of possible right handed couplings between the top quark and the W boson