<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>t</mml:mi><mml:mover accent="true"><mml:mi>t</mml:mi><mml:mo stretchy="false">¯</mml:mo></mml:mover><mml:mi>b</mml:mi><mml:mover accent="true"><mml:mi>b</mml:mi><mml:mo stretchy="false">¯</mml:mo></mml:mover></mml:math> at the LHC: On the size of off-shell effects and prompt <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>b</mml:mi></mml:math>-jet identification

Abstract

We investigate full off-shell effects in $t\overline{t}b\overline{b}$ production in the dilepton channel at the LHC with the center-of-mass energy $\sqrt{s}=13\text{ }\text{ }\mathrm{TeV}$. Specifically, we compute next-to-leading-order (NLO) QCD corrections to the $pp\ensuremath{\rightarrow}{e}^{+}{\ensuremath{\nu}}_{e}{\ensuremath{\mu}}^{\ensuremath{-}}{\overline{\ensuremath{\nu}}}_{\ensuremath{\mu}}b\overline{b}b\overline{b}+X$ 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 $pp\ensuremath{\rightarrow}t\overline{t}H(H\ensuremath{\rightarrow}b\overline{b})$, $t\overline{t}$ production in association with two prompt $b$ jets is a primary source of uncertainty in the measurement of $t\overline{t}H(H\ensuremath{\rightarrow}b\overline{b})$. 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 nonresonant 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 labeling prescription, the prompt $b$ jets and the $b$ jets from top-quark decays can be successfully disentangled.

Topics & Concepts