Abstract
Inclusive and differential cross sections of single top quark production in association with a Z boson are measured in proton-proton collisions at a center-of-mass energy of 13 TeV with a data sample corresponding to an integrated luminosity of 138 fb−1 recorded by the CMS experiment. Events are selected based on the presence of three leptons, electrons or muons, associated with leptonic Z boson and top quark decays. The measurement yields an inclusive cross section of {87.9}_{-7.3}^{+7.5}{left(mathrm{stat}right)}_{-6.0}^{+7.3}left(mathrm{syst}right) fb for a dilepton invariant mass greater than 30 GeV, in agreement with standard model (SM) calculations and represents the most precise determination to date. The ratio between the cross sections for the top quark and the top antiquark production in association with a Z boson is measured as {2.37}_{-0.42}^{+0.56}{left(mathrm{stat}right)}_{-0.13}^{+0.27}left(mathrm{syst}right) . Differential measurements at parton and particle levels are performed for the first time. Several kinematic observables are considered to study the modeling of the process. Results are compared to theoretical predictions with different assumptions on the source of the initial-state b quark and found to be in agreement, within the uncertainties. Additionally, the spin asymmetry, which is sensitive to the top quark polarization, is determined from the differential distribution of the polarization angle at parton level to be 0.54 ± 0.16 (stat) ± 0.06 (syst), in agreement with SM predictions.
Highlights
Background determinationSeveral background contributions to the signal region are studied, divided into two main categories
Inclusive and differential cross sections of single top quark production in association with a Z boson are measured in proton-proton collisions at a center-of-mass energy of 13 TeV with a data sample corresponding to an integrated luminosity of 138 fb−1 recorded by the CMS experiment
Combining the statistical and systematic uncertainties in quadrature, the measured tZq cross section has a precision of 11%, which is an improvement over the previous measurements of this process [1, 2]
Summary
To reconstruct the physics objects described below, the same algorithms are applied to simulated events and data. A number of variables discriminating between prompt and nonprompt leptons use information about the reconstructed jet with the smallest ∆R with respect to the identified lepton, requiring. This jet is used to compute the number of charged particles matched to the jet, the ratio of the jet pT to the lepton pT, the lepton momentum projected on the transverse plane to the reconstructed jet direction, as well as the output discriminator value of the DeepJet b tagging algorithm. Leptons that pass the requirement on the lepton MVA are labeled as “tight” leptons and are selected for further analysis Leptons that fail this requirement are subjected to additional criteria, including requirements on the relative lepton isolation and the DeepJet discriminator value of the jet that is closest to the lepton.
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