Chromomagnetic and chromoelectric dipole moments of the top quark in the 4GTHDM

TL;DR

This paper calculates the chromomagnetic and chromoelectric dipole moments of the top quark within a four-generation two-Higgs doublet model, revealing potential large contributions from new scalar bosons and heavy quarks, consistent with current experimental bounds.

Contribution

It provides analytical expressions for the dipole moments in the 4GTHDM and explores their dependence on new scalar and quark parameters, highlighting possible significant effects beyond standard models.

Findings

Top quark CMDM can reach 10^{-4} to 10^{-3} with dominant fourth-generation quark contributions.

Top quark CEDM can be as large as 10^{-19} to 10^{-18} ecm, mainly from the charged scalar and b' quark.

Parameter space regions are identified where predictions are consistent with LHC Higgs data.

Abstract

The chromo magnetic dipole moment (CMDM) and chromo electric dipole moment (CEDM) of the top quark are calculated at the one-loop level in the framework of the two-Higgs doublet model with four fermion generations (4GTHDM), which is still consistent with experimental data and apart from new scalar bosons ($H^0$, $A^0$, and $H^\pm$) and quarks ($b'$ and $t'$) predicts new sources of $CP$ violation via the extended $4\times 4$ CKM matrix. Analytical expressions for the CMDM and CEDM of a quark are presented both in terms of Feynman parameter integrals and Passarino-Veltman scalar functions, with the main contributions arising from loops carrying the scalar bosons accompanied by the third- and fourth-generation quarks. The current bounds on the parameter space of the 4GTHDM are discussed and a region still consistent with the LHC data on the 125 GeV Higgs boson is identified. It is found that the top quark CMDM, which is induced by all the scalar bosons, can reach values of the order of $10^{-4}$-$10^{-3}$, with the dominant contributions arising from the fourth generation quarks, though may also be large cancellations for some parameter values, thereby giving a negligible CMDM. As for the top quark CEDM, it only receives contributions from the charged scalar boson and can reach values of the order of $10^{-19}$-$10^{-18}$ ecm for relatively light $m_{H^\pm}$ and large $m_{b'}$, with the dominant contribution arising from the $b'$ quark. The latter would be the most interesting prediction of this model as can be larger than the value predicted by the usual THDMs by one or two orders of magnitude.