Effects of Higgs sector CP violation in top-quark pair production at the LHC

TL;DR

The paper investigates CP violation in the Higgs sector by examining heavy neutral Higgs resonances with mixed CP in top-quark pair production at the LHC. Using a spin-density-matrix framework, it identifies CP-odd spin-momentum correlations that survive interference with the irreducible $t\bar t$ background and constructs dilepton and lepton+jets observables whose expectation values reveal CP violation when the $t\bar t$ invariant mass is binned around the Higgs mass. It shows that the dispersive CP-odd observable $Q_2$ is particularly sensitive to the CP-violating Yukawa coupling product $\gamma_{CP}$, and that with $L=100\ \mathrm{fb}^{-1}$, signals corresponding to $|\gamma_{CP}|\gtrsim 0.1$ can be statistically significant for heavy Higgs masses in the range $320$--$500$ GeV and moderate $g_{VV}$. The work provides explicit formulae for cross sections, density matrices, and observables, discusses experimental considerations and the role of $M_{t\bar t}$ binning, and argues that the LHC offers a unique opportunity to test Higgs-sector CP violation in the heavy-mass regime.

Abstract

A striking manifestation of CP violation in the electroweak symmetry breaking sector would be the existence of neutral Higgs boson(s) with undefined CP parity. We analyse signatures of such a boson, with a mass of about 300 GeV or larger, produced in high energy proton-proton collisions at LHC energies in its top-quark antitop-quark decay channel. The large irreducible $t\bar t$ background is taken into account. We propose, both for the dilepton and the lepton + jets decay channels of $t\bar t$, several correlations and asymmetries with which (Higgs sector) CP violation can be traced. We show that for judiciously chosen cuts on the $t\bar t$ invariant mass these CP observables yield, for an LHC integrated luminosity of 100 $\rm{fb}^{-1}$, statistically significant signals for a range of Higgs boson masses and Yukawa couplings.