Abstract
A measurement is presented of the associated production of a single top quark and a Z boson. The study uses data from proton–proton collisions at s=13TeV recorded by the CMS experiment, corresponding to an integrated luminosity of 35.9 fb−1. Using final states with three leptons (electrons or muons), the tZq production cross section is measured to be σ(pp→tZq→Wbℓ+ℓ−q)=123−31+33(stat)−23+29(syst)fb, where ℓ stands for electrons, muons, or τ leptons, with observed and expected significances of 3.7 and 3.1 standard deviations, respectively.
Highlights
At the CERN LHC, single top quark production proceeds through three electroweak interaction processes: t-channel, s-channel, and associated tW production
Cross sections for single top quark production have been reported by the CDF and D0 Collaborations [1, 2], as well as by the ATLAS [3–7] and CMS [8–11] Collaborations
The energy of the photons is directly obtained from the electromagnetic calorimeter (ECAL) measurement, corrected for zerosuppression effects, while that of the electrons is determined from a combination of the electron momentum at the primary interaction vertex as determined by the tracker, the energy of the corresponding ECAL cluster, and the total energy of all bremsstrahlung photons spatially compatible with originating from the electron track
Summary
At the CERN LHC, single top quark production proceeds through three electroweak interaction processes: t-channel, s-channel, and associated tW production. The high centre-of-mass proton–proton (pp) collision energy of 13 TeV at the LHC, together with large integrated luminosities, allows the study of processes with very small cross sections that were not accessible at lower energies. One example of such a process is the rare associated production of a single top quark with a Z boson. The next-to-leading-order (NLO) cross section for tZq → Wb + −q, considering only the leptonic decays of Z bosons (to electrons, muons, or τ leptons, generically denoted by ), is calculated for pp collisions at a centre-of-mass energy of 13 TeV, using the Monte Carlo (MC) generator MadGraph5_amc@nlo 2.2.2 [15]. Several control regions are defined to better constrain the backgrounds, each containing different contributions from signal and background processes
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