Constraining ALP-Top Interaction from the Chromoelectric Dipole Moment of the Top Quark

TL;DR

This paper investigates how axion-like particles (ALPs) with CP violation influence the top quark's chromoelectric dipole moment (CEDM), deriving constraints on ALP-top couplings from experimental EDM bounds.

Contribution

It provides a detailed calculation of the ALP-induced top quark CEDM at one- and two-loop levels, considering off-shell gluons and evaluating at the top mass.

Findings

Strongest constraints on ALP-top coupling from neutron EDM bounds

Analytical expressions for ALP-induced top CEDM at relevant energy scales

Quantitative limits on ALP parameters based on current EDM measurements

Abstract

The couplings of axion-like particles (ALPs) to Standard Model fermions are proportional to the fermion masses, making the interaction with the top quark particularly significant. In this study, we consider an ALP that is a mixture of CP-even and CP-odd components, thereby introducing CP violation. This CP violation, in turn, gives rise to electric dipole moments (EDMs) of quarks and leptons, as well as chromoelectric dipole moments (CEDMs) of quarks. We compute the one-loop and two-loop contributions to the top quark CEDM induced by the ALP. In our calculation, we treat the external gluon as off-shell with momentum $q^2 \neq 0$, derive the analytical results, and finally evaluate the top quark CEDM at $q^2 = m_t^2$, corresponding to the top quark pole mass. This value is relevant for subsequent calculations of the EDMs of the neutron and mercury. By applying current experimental bounds on EDMs and CEDMs, we derive constraints on the ALP-top quark coupling, with the strongest limit coming from the neutron EDM.