Abstract
A measurement of the top quark–antiquark pair production cross section sigma _{mathrm {t}overline{mathrm {t}}} in proton–proton collisions at a centre-of-mass energy of 13,text {Te}text {V} is presented. The data correspond to an integrated luminosity of 35.9{,text {fb}^{-1}} , recorded by the CMS experiment at the CERN LHC in 2016. Dilepton events (mathrm {e}^{pm }mathrm {mu }^{{mp }}, mathrm {mu ^+}mathrm {mu ^-}, mathrm {e}^+mathrm {e}^-) are selected and the cross section is measured from a likelihood fit. For a top quark mass parameter in the simulation of m_mathrm {mathrm {t}} ^{mathrm {MC}} = 172.5 ,text {Ge}text {V} the fit yields a measured cross section sigma _{mathrm {t}overline{mathrm {t}}} = 803 pm 2 ,text {(stat)} pm 25 ,text {(syst)} pm 20 ,text {(lumi)} ,text {pb} , in agreement with the expectation from the standard model calculation at next-to-next-to-leading order. A simultaneous fit of the cross section and the top quark mass parameter in the powheg simulation is performed. The measured value of m_mathrm {mathrm {t}} ^{mathrm {MC}} = 172.33 pm 0.14 ,text {(stat)} ,^{+0.66}_{-0.72} ,text {(syst)} ,text {Ge}text {V} is in good agreement with previous measurements. The resulting cross section is used, together with the theoretical prediction, to determine the top quark mass and to extract a value of the strong coupling constant with different sets of parton distribution functions.
Highlights
Measurements of the top quark–antiquark pair cross section σtt in proton–proton collisions provide important tests of the standard model (SM)
The extrapolation uncertainty is determined for each relevant model systematic source j as described in the following: all nuisance parameters except the one under study are fixed to their post-fit values; the nuisance parameter λ j is set to values +1 and − 1, and the variations of A are recorded
The fit is performed by varying mt(mt) in a 5GeV range around the central value used in each parton distribution functions (PDFs)
Summary
Measurements of the top quark–antiquark pair cross section σtt in proton–proton (pp) collisions provide important tests of the standard model (SM). It is a key input to electroweak precision fits [18] and, together with the value of the Higgs boson mass and αS, it has direct implications on the SM predictions for the stability of the electroweak vacuum [19]. A theoretically well-defined mass can be determined by comparing the measured tt cross section to the fixed-order theoretical predictions [1,4,10,11,12]. The cross section σtt is first measured for a fixed value of mMt C = 172.5 GeV, and determined simultaneously with mMt C. Input distributions sensitive to the top quark mass are introduced in order to constrain mMt C. The measured value of σtt at the experimentally constrained value of mMt C is used to extract αS(mZ) and mt in the MS scheme, using different PDF sets.
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