Tracing CP violation in the production of top quark pairs by multiple TeV proton-proton collisions

TL;DR

The paper investigates non-standard CP violation in top-quark pair production at multi-TeV hadron colliders using a density-matrix framework for the dominant partonic processes $gg\to t\bar{t}$ and $q\bar{q}\to t\bar{t}$. It specializes to two-Higgs doublet models with CP violation, computing CP-odd contributions to the production density matrices via neutral-Higgs exchange and identifying parton-level observables sensitive to these effects. While parton-level asymmetries can reach a few percent for favorable parameters, the authors construct hadron-level observables from top decay kinematics that yield signal-to-noise ratios around $10^{-3}$, with CP-conserving backgrounds strongly suppressed. The results indicate that, given sufficient statistics and careful detector control, experimental probes of CP violation in tt production are worth pursuing at future high-luminosity colliders.

Abstract

We investigate the possibilities of searching for non-standard CP violation in $pp\to t\bar{t}X$ at multiple TeV collision energies. A general kinematic analysis of the underlying partonic production processes $gg\to t\bar{t}$ and $q\bar{q}\to t\bar{t}$ in terms of their density matrices is given. We evaluate the CP-violating parts of these matrices in two-Higgs doublet extensions of the standard model (SM) and give results for CP asymmetries at the parton level. We show that these asymmetries can be traced by measuring suitable observables constructed from energies and momenta of the decay products of $t$ and $\bar{t} $. We find CP-violating effects to be of the order of $10^{-3}$ and show that possible contaminations induced by SM interactions are savely below the expected signals.