Cross section measurement of t-channel single top quark production in pp collisions at sqrt(s) = 13 TeV

TL;DR

CMS measures the t-channel single top quark production cross section in pp collisions at sqrt(s)=13 TeV using 2.2 fb^-1. A neural-network based multivariate discriminator separates signal from dominant backgrounds, and a simultaneous fit yields sigma(t-channel, t) = 154 pb and R_tch = 1.81, from which sigma(t-channel, tbar) and |V_tb| are inferred. The results agree with SM predictions and SM cross sections, and provide a direct determination of |V_tb| with competitive precision. The analysis demonstrates robust background control and represents a high-precision 13 TeV CMS measurement of t-channel single top production.

Abstract

The cross section for the production of single top quarks in the t channel is measured in proton-proton collisions at 13 TeV with the CMS detector at the LHC. The analyzed data correspond to an integrated luminosity of 2.2 inverse femtobarns. The event selection requires one muon and two jets where one of the jets is identified as originating from a bottom quark. Several kinematic variables are then combined into a multivariate discriminator to distinguish signal from background events. A fit to the distribution of the discriminating variable yields a total cross section of 238 +/- 13 (stat) +/- 29 (syst) pb and a ratio of top quark and top antiquark production of R[t-ch.] = 1.81 +/- 0.18 (stat) +/- 0.15 (syst). From the total cross section the absolute value of the CKM matrix element V[tb] is calculated to be 1.05 +/- 0.07 (exp) +/- 0.02 (theo). All results are in agreement with the standard model predictions.

Figures (5)

• Figure 1: Feynman diagrams for single top quark production in the $t$ channel: (top) 2$\to$2 and (bottom) 2$\to$3 processes.
• Figure 2: Fit to the $m_{\mathrm{T}}^{\mathrm{W}}\xspace$ distributions in the 2-jets--0-tag sample (upper row) and the 2-jets--1-tag sample (lower row) for all events (left), for positively charged muons only (middle), and for negatively charged muons only (right). The QCD fit template is derived from a sideband region in data. Only statistical uncertainties are taken into account in the fit.
• Figure 3: Neural network distributions for all (left), positively (middle), and negatively (right) charged muons normalized to the yields obtained from the simultaneous fit in the 2-jets--1-tag (upper), 3-jets--1-tag (middle), and 3-jets--2-tags region (lower). The ratio between data and simulated distributions after the fit is shown at the bottom of each figure. The hatched areas indicate the post-fit uncertainties.
• Figure 4: Comparison of the measured $R_{t\text{-ch.}}$ (dotted line) with the prediction from different PDF sets: CT14 NLO CT14, ABM11 NLO and ABM12 NNLO ABM, MMHT14 NLO MMHT14, HERAPDF2.0 NLO HERAPDF, NNPDF 3.0 NLO NNPDF. The PowHeg 4FS calculation is used. The nominal value for the top quark mass is $172.5\,\text{Ge\spaceV}\xspace$. The error bars for the different PDF sets include the statistical uncertainty, the uncertainty due to the factorization and renormalization scales, derived varying both of them by a factor 0.5 and 2, and the uncertainty in the top quark mass, derived varying the top quark mass between 171.5 and 173.5$\,\text{Ge\spaceV}\xspace$. For the measurement, the inner and outer error bars correspond to the statistical and total uncertainties, respectively.
• Figure 5: The summary of the most precise CMS measurements Chatrchyan:2012epPASSingleTopCrossSection for the total $t$-channel single top quark cross section, in comparison with NLO+NNLL QCD calculations Kidonakis:2011wy. The combination of the Tevatron measurements PhysRevLett.115.152003 is also shown.