ttbb at the LHC - On the size of off-shell effects and prompt b-jet identification
\(t\bar{t}b\bar{b}\) at the LHC: On the size of off-shell effects and prompt b-jet identification
Authors: Giuseppe Bevilacqua, Huan-Yu Bi, Heribertus Bayu Hartanto, Manfred Kraus, Michele Lupattelli, Malgorzata Worek
Abstract: We investigate full off-shell effects in \(t\bar{t}b\bar{b}\) production in the dilepton channel at the LHC with the center-of-mass energy \(\sqrt{s} = 13\) TeV. Specifically, we compute NLO QCD corrections to the \(pp \to e^+ \nu_e \mu^- \bar{\nu}_\mu b \bar{b} b \bar{b} + X\) process and provide a prescription for \(b\)-jet identification to distinguish prompt $b$ jets from $b$ jets originating from the decay of the top quarks. As an important irreducible background to \(pp \to t\bar{t}H (H\to b\bar{b})\), \(t\bar{t}\) production in association with two prompt $b$ jets is a primary source of uncertainty in the measurement of \(t\bar{t}H (H\to b\bar{b})\). In quantifying full off-shell effects, we perform comparisons between the state-of-the-art full off-shell computation and the calculation in the narrow width approximation. The former includes all double-, single- and non-resonant Feynman diagrams, interferences as well as finite-width effects of the top quarks and $W$ gauge bosons. The latter restricts the unstable top quarks and \(W\) gauge bosons to on-shell states and includes for the first time NLO QCD corrections to both production and decays. We observe that full off-shell effects are subdominant compared to the scale uncertainties for the integrated fiducial cross section and for the majority of differential observables in the phase-space regions that we investigated. However, for a number of observables related to beyond the Standard Model searches, full off-shell effects are significant. Furthermore, with our \(b\)-jet labelling prescription, the prompt \(b\) jets and the $b$ jets from top-quark decays can be successfully disentangled.
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