Doctoral Dissertation Open Access

Search for single top quark production in the s channel at 13 TeV with the CMS experiment

Müller, Denise

Thesis supervisor(s)

Müller, Thomas; Husemann, Ulrich

A search for s-channel single top quark production is presented, using 137 fb-1 of data recorded at a center-of-mass energy of 13 TeV by the CMS experiment at the CERN LHC. In the search, leptonically decaying top quarks are analyzed, resulting in a final state of the signal process consisting of one charged muon or electron and its corresponding neutrino, and two jets originating from hadronized bottom quarks. Depending on the jet and b jet multiplicities, signal and control event categories, accounting for different background processes, are defined. All signal and background processes are modeled via event simulation, except for the QCD multijet background contribution, which is modeled with a data-driven approach. A dedicated multivariate method is employed to enhance the separation between the signal process and the background processes, mainly top quark pair production. By performing a simultaneous fit on the multivariate discriminator in all event categories, the significance for observing s-channel single top quark production, as well as the inclusive cross section and the absolute value of the CKM matrix element Vtb, are extracted.

With an observed (expected) significance of 6.0 (4.7) standard deviations, this analysis is the first discovery for s-channel single top quark production using proton-proton collisions of the LHC. The measured cross section of this process is σs-ch. = 14.65+2.48-2.37 (stat. + syst.) pb and Vtb is measured to be |fLV Vtb| = 1.19 ± 0.09 (exp.) ± 0.02 (theo.), with the form factor for potential anomalous left-handed vector boson couplings fLV being 1 in the SM. These measured values are both within two standard deviations of the SM predictions.

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