Abstract
The first measurement of the inclusive cross section for top quark pairs (tt‾) produced in association with two additional charm jets is presented. The analysis uses the dileptonic final states of tt‾ events produced in proton-proton collisions at a centre-of-mass energy of 13 TeV. The data correspond to an integrated luminosity of 41.5fb−1, recorded by the CMS experiment at the LHC. A new charm jet identification algorithm provides input to a neural network that is trained to distinguish among tt‾ events with two additional charm (tt‾cc‾), bottom (tt‾bb‾), and light-flavour or gluon (tt‾LL) jets. By means of a template fitting procedure, the inclusive tt‾cc‾, tt‾bb‾, and tt‾LL cross sections are simultaneously measured, together with their ratios to the inclusive tt‾ + two jets cross section. This provides measurements of the tt‾cc‾ and tt‾bb‾ cross sections of 10.1±1.2(stat)±1.4(syst)pb and 4.54±0.35(stat)±0.56(syst)pb, respectively, in the full phase space. The results are compared and found to be consistent with predictions from two different matrix element generators with next-to-leading order accuracy in quantum chromodynamics, interfaced with a parton shower simulation.
Highlights
The modelling of top quark pair production in association with jets from the hadronization of bottom (b) or charm (c) quarks, referred to as b jets and c jets, respectively, in proton-proton collisions at the CERN LHC is challenging
The uncertainties arise from the different energy scales in ttbb production, which range from the large scales associated with the top quark mass, to the relatively small scales associated with additional jets resulting mostly from gluon splitting into bb pairs
We present the first measurement of the inclusive ttcc cross section and its ratio to the inclusive tt + two jets cross section
Summary
The modelling of top quark pair (tt) production in association with jets from the hadronization of bottom (b) or charm (c) quarks, referred to as b jets and c jets, respectively, in proton-proton (pp) collisions at the CERN LHC is challenging. Differences are much less pronounced for c jets This explains the challenge of simultaneously separating ttcc and ttbb events from a large background of tt events with additional LF or gluon jets (ttLL). The measurement is performed in the dileptonic decay channel of the tt events using a data sample of pp collisions at a centre-of-mass energy of 13 TeV, collected with the CMS detector in 2017, corresponding to an integrated luminosity of 41.5 fb−1 [15] This data set benefits from the upgrade of the pixel tracking detector [16], which was installed in winter 2016–2017 and which has been shown to significantly improve the performance of heavy-flavour (HF) jet identification [17]. The techniques described here provide a basis for a simultaneous measurement of the ttbb and ttcc processes from data that can be adopted in future ttH analyses to significantly reduce the uncertainties related to these backgrounds
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