Measuring the CP structure of the top Yukawa coupling in $t\bar{t}H$ events at the LHC

Abstract

The ATLAS and CMS collaborations recently announced the observation of the associated production of the Higgs boson with a top quark pair ($t\bar tH$) at the LHC. This process depends directly on the the top quark Yukawa coupling and provides access to its properties. In particular, a CP-odd component is allowed in models beyond the Standard Model with extended Higgs sectors. Studies of the feasibility of such a measurement at the 13 TeV LHC were carried out, in the $H\rightarrow b\bar b$ decay channel and with the $t\bar t$ system decaying through the semileptonic and dileptonic channels. Fast detector simulation and kinematic fits were applied to samples of SM backgrounds and of signal scenarios with different CP-mixing angles of the coupling. Ratios of projections of momenta and angular distributions using boosted reference frames were found to be sensitive to the CP-mixing angle. Those are expected to be robust relatively to modeling uncertainties and are thus presented as good candidates for experimental use. Expected confidence levels for the exclusion of scenarios with a CP-odd component in the top quark Yukawa coupling were obtained, using different observables as discriminants, and are presented for integrated luminosities up to 3 ab$^{-1}$, as expected after the full HL-LHC program.