Top-quark charge asymmetry with a jet handle

Abstract

Pairs of top and antitop quarks are produced at the LHC to a large extent in association with a hard jet. We investigate the charge asymmetry in $t\overline{t}+j$ production in quantum chromodynamics (QCD) and with additional massive color-octet vector bosons. The total charge asymmetry at the LHC is suppressed by the large charge-symmetric background from gluon-gluon fusion. We show to what extent the asymmetry can be enhanced by suitable phase space cuts and, in particular, elaborate on the kinematics of the hard jet in the $t\overline{t}+j$ final state. We demonstrate that in QCD, the asymmetry amounts to 1.5% for central jets without an excessive reduction of the cross section. By applying additional kinematical cuts, the asymmetry can be enhanced to 4%, but at the cost of a strong reduction of the cross section. Massive color-octet states can generate sizeable effects in $t\overline{t}+j$ production, both on the charge asymmetry and on the cross section. The charge asymmetry probes both vector and axial-vector couplings to quarks. We show that massive color octets can generate asymmetries up to $\ifmmode\pm\else\textpm\fi{}10%$ for moderate and up to $\ifmmode\pm\else\textpm\fi{}30%$ for strong kinematical cuts to be used in experimental analyses at the LHC. Jet kinematics can be used to obtain further information about the nature of the couplings and thereby to discriminate between different models.