Abstract
A measurement of the production cross section of top quark pairs in association with two b jets (tt‾bb‾) is presented using data collected in proton-proton collisions at s=13 TeV by the CMS detector at the LHC corresponding to an integrated luminosity of 35.9 fb−1. The cross section is measured in the all-jet decay channel of the top quark pair by selecting events containing at least eight jets, of which at least two are identified as originating from the hadronization of b quarks. A combination of multivariate analysis techniques is used to reduce the large background from multijet events not containing a top quark pair, and to help discriminate between jets originating from top quark decays and other additional jets. The cross section is determined for the total phase space to be 5.5±0.3(stat)−1.3+1.6(syst)pb and also measured for two fiducial tt‾bb‾ definitions. The measured cross sections are found to be larger than theoretical predictions by a factor of 1.5–2.4, corresponding to 1–2 standard deviations.
Highlights
At the CERN LHC, top quark pairs are produced with copious amounts of additional jets, including those resulting from the hadronization of b quarks (b jets)
Variables used for constructing the classification without labels (CWoLa) boosted decision tree (BDT) are kinematic quantities similar to those used in the permutation BDT, the output value of the permutation BDT, and the b tagging discriminator scores of the two jets identified by the permutation BDT as the b jets originating from the top quark decays
The predicted cross sections are not rescaled by any next-to-leading order (NLO) to NNLO K-factor, which for inclusive tt production amounts to 1.1–1.15 [40]
Summary
At the CERN LHC, top quark pairs are produced with copious amounts of additional jets, including those resulting from the hadronization of b quarks (b jets). Calculations of the production cross section of tt in association with jets have been performed at next-to-leading order (NLO) in QCD and matched with parton showers for up to two additional. The ttbb cross section at NLO, matched with parton showers, has been calculated for massless b quarks (five-flavour scheme, 5FS) [16], and has recently become available for massive b quarks (four-flavour scheme, 4FS) [17,18,19]. This Letter focuses on the all-jet final state of the tt system, where each top quark decays into three jets, leading to a signature of four b jets and four light-quark jets for the ttbb system. The all-jet channel suffers from a large background from multijet production, as well as from the difficulty of identifying jets that originate from decaying top quarks.
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