Measurement of the $t$-channel single top quark cross section in proton-proton collisions at $\sqrt{s}$ = 5.02 TeV

Abstract

The single top quark $t$-channel production cross section is measured in proton-proton collisions at the CERN LHC at $\sqrt{s}$ = 5.02 TeV, using data recorded with the CMS detector in 2017, corresponding to an integrated luminosity of 302 pb$^{-1}$, and resulting in the first CMS measurement of the process at that energy. Events with one electron or muon and two or more jets, among which at least one is identified as originating from a b quark fragmentation, are analyzed. The combined cross section of single top quark (tq) and single top antiquark ($\mathrm{\bar{t}q}$) production is $σ_{\mathrm{tq+\bar{t}q}}$ = 25.4$^{+3.6}_{-3.5}$ (stat) $^{+4.2}_{-3.9}$ (syst) $\pm$ 0.5 (lumi) pb. The individual cross sections are measured to be $σ_{\mathrm{tq}}$ = 17.6$^{+2.8}_{-2.7}$ (stat) $^{+2.6}_{-2.4}$ (syst) $\pm$ 0.3 (lumi) pb and $σ_{\mathrm{\bar{t}q}}$ = 6.6$^{+2.4}_{-1.6}$ (stat) $^{+2.1}_{-2.5}$ (syst) $\pm$ 0.1 (lumi) pb. Their ratio is measured to be $\mathcal{R}_{\mathrm{t-ch}}$ = 2.7$^{+1.5}_{-0.8}$ (stat) $^{+1.3}_{-0.3}$(syst). The absolute value of the Cabibbo$-$Kobayashi$-$Maskawa matrix element is found to be $\lvert f_{\mathrm{LV}}V_\mathrm{tb}\rvert$ = 0.92 $\pm$ 0.09 (exp) $\pm$ 0.01 (thy). The measurements are in good agreement with the standard model predictions at next-to-next-to-leading order accuracy in quantum chromodynamics.

Figures (10)

• Figure 1: Representative leading-order Feynman diagrams for single top quark (left) and antiquark (right) production via the $t$ channel.
• Figure 2: Observed and predicted number of events in each of the twelve categories considered in the analysis, before the ML fit. The vertical error bars represent the statistical uncertainty associated to the data, and the hatched band the uncertainty in the prediction. All uncertainties considered in the analysis are included in the uncertainty band. The lower panels show the data-to-prediction ratio. The normalizations of the processes are those of the SM predictions except for QCD, which is estimated from data. The ${ \mathup{{{W}}{} _{ {}} ^{ {}}} }\xspace\text{+jets}$ background is split as explained in Section \ref{['sec:Background']}.
• Figure 3: Observed and predicted distributions of the leading jet $p_{\mathrm{T}}$ (left) and $\eta$ (right), in the $2 \mathup{{{j}}{} _{ {}} ^{ {}}} \xspace1{ \mathup{{{b}}{} _{ {}} ^{ {}}} }\xspace$ category, before the ML fit. The vertical error bars represent the statistical uncertainty associated to the data, and the hatched band the uncertainty in the prediction. The lower panels show the data-to-prediction ratio. The first and last bins in each distribution include underflow and overflow events, respectively. The normalizations of the processes are those of the SM predictions except for QCD, which is estimated from data. The ${ \mathup{{{W}}{} _{ {}} ^{ {}}} }\xspace\text{+jets}$ background is split as explained in Section \ref{['sec:Background']}.
• Figure 4: Observed and predicted distributions of $\lvert \eta_{{ \mathup{{{u}}{} _{ {}} ^{ {}}} }\xspace_0} \rvert$ in the $3 \mathup{{{j}}{} _{ {}} ^{ {}}} \xspace1{ \mathup{{{b}}{} _{ {}} ^{ {}}} }\xspace$ (left) and $3 \mathup{{{j}}{} _{ {}} ^{ {}}} \xspace2{ \mathup{{{b}}{} _{ {}} ^{ {}}} }\xspace$ (right) categories, before the ML fit. The vertical error bars represent the statistical uncertainty associated to the data, and the hatched band the uncertainty in the prediction. The lower panels show the data-to-prediction ratio. The normalizations of the processes are those of the SM predictions except for QCD, which is estimated from data. The ${ \mathup{{{W}}{} _{ {}} ^{ {}}} }\xspace\text{+jets}$ background is split as explained in Section \ref{['sec:Background']}.
• Figure 5: Observed and predicted distributions of the most discriminating input variable used in the random forest training, $\lvert \eta_{{ \mathup{{{u}}{} _{ {}} ^{ {}}} }\xspace_0} \rvert$ (left), and the output MVA discriminator (right), in the $2 \mathup{{{j}}{} _{ {}} ^{ {}}} \xspace1{ \mathup{{{b}}{} _{ {}} ^{ {}}} }\xspace$ category, before the ML fit. The vertical error bars represent the statistical uncertainty associated to the data, and the hatched band the uncertainty in the prediction. The lower panels show the data-to-prediction ratio. The first and last bins in the MVA output distribution include underflow and overflow events, respectively. The normalizations of the processes are those of the SM predictions except for QCD, which is estimated from data. The ${ \mathup{{{W}}{} _{ {}} ^{ {}}} }\xspace\text{+jets}$ background is split as explained in Section \ref{['sec:Background']}.