Abstract
Normalised multi-differential cross sections for top quark pair (hbox {t}{bar{hbox {t}}}) production are measured in proton-proton collisions at a centre-of-mass energy of 13,{text {TeV}} using events containing two oppositely charged leptons. The analysed data were recorded with the CMS detector in 2016 and correspond to an integrated luminosity of 35.9{,{text {fb}}^{-1}} . The double-differential hbox {t}{bar{hbox {t}}} cross section is measured as a function of the kinematic properties of the top quark and of the hbox {t}{bar{hbox {t}}} system at parton level in the full phase space. A triple-differential measurement is performed as a function of the invariant mass and rapidity of the hbox {t}{bar{hbox {t}}} system and the multiplicity of additional jets at particle level. The data are compared to predictions of Monte Carlo event generators that complement next-to-leading-order (NLO) quantum chromodynamics (QCD) calculations with parton showers. Together with a fixed-order NLO QCD calculation, the triple-differential measurement is used to extract values of the strong coupling strength alpha _{S} and the top quark pole mass (m_{{text {t}}}^{{text {pole}}}) using several sets of parton distribution functions (PDFs). The measurement of m_{{text {t}}}^{{text {pole}}} exploits the sensitivity of the hbox {t}{bar{hbox {t}}} invariant mass distribution to m_{{text {t}}}^{{text {pole}}} near the production threshold. Furthermore, a simultaneous fit of the PDFs, alpha _{S}, and m_{{text {t}}}^{{text {pole}}} is performed at NLO, demonstrating that the new data have significant impact on the gluon PDF, and at the same time allow an accurate determination of alpha _{S} and m_{{text {t}}}^{{text {pole}}}. The values alpha _{S}(m_{{text {Z}}}) = 0.1135{}^{+0.0021}_{-0.0017} and m_{{text {t}}}^{{text {pole}}} = 170.5 pm 0.8 ,{text {GeV}} are extracted, which account for experimental and theoretical uncertainties, the latter being estimated from NLO scale variations. Possible effects from Coulomb and soft-gluon resummation near the hbox {t}{bar{hbox {t}}} production threshold are neglected in these parameter extractions. A rough estimate of these effects indicates an expected correction of m_{{text {t}}}^{{text {pole}}} of the order of +1 ,{text {GeV}} , which can be regarded as additional theoretical uncertainty in the current m_{{text {t}}}^{{text {pole}}} extraction.
Highlights
Measurements of top quark pair production are important for checking the validity of the standard model (SM)and searching for new phenomena
The tt simulation is normalised to a cross section of 832 +−2209(scale)±35(PDF+αS) pb calculated with the Top++ program [59] at NNLO including resummation of next-to-next-to-leading-logarithm (NNLL) soft-gluon terms assuming mpt ole = 172.5 GeV and the proton structure described by the CT14 NNLO parton distribution functions (PDFs) set [60]
The normalised differential cross sections of tt production are measured in the full phase space at parton level for top quarks and at particle level for additional jets in the events, for the following variables: 1. double-differential cross sections as a function of pair of variables:
Summary
Measurements of top quark pair (tt) production are important for checking the validity of the standard model (SM). The large data set delivered by the CERN LHC allows precise measurements of the tt production cross section as a function of tt kinematic observables. These can be used to check the most recent predictions of perturbative quantum chromodynamics (QCD) and to constrain input parameters, some of which are fundamental to the SM. For the first time at the LHC, the triple-differential cross section is measured as a function of M(tt), y(tt), and Njet, where Njet is the number of extra jets not arising from the decay of the tt system For this purpose, the kinematic reconstruction algorithm is optimised to determine the invariant mass of the tt system in an unbiased way.
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