Phenomenological anatomy of top-quark FCNCs induced by a light scalar singlet

TL;DR

This paper explores how a light scalar singlet can induce flavor-changing neutral currents in the top quark sector, affecting low-energy observables and LHC signals, with implications for CP violation and experimental constraints.

Contribution

It provides a systematic analysis of top-quark FCNCs induced by a light scalar singlet, including low-energy constraints, CP-violating effects, and collider sensitivities, highlighting the interplay of different experimental probes.

Findings

Scalar singlet induces large FCNC effects in $B_s \to \mu^+ \mu^-$ decay.

Constraints on $tcS$ couplings are stronger than some LHC sensitivities.

CP phases can be probed via $B_s$ decay observables and neutron EDM.

Abstract

Scalar singlets under the Standard Model gauge group appear naturally in many well-motivated New Physics scenarios, such as the composite Higgs models. Unlike the Higgs boson in the Standard Model, they can induce large flavour-changing neutral currents (FCNCs) in the top sector. We investigate systematically the effects of a light scalar singlet $S$ with top-quark FCNC couplings, by including the low-energy constraints from the $B_s \to \mu^+ \mu^-$ decay, the muon anomalous magnetic moment $(g-2)_\mu$ and the neutron Electric Dipole Moment (EDM). We also perform a detailed Monte-Carlo simulation of the channel $pp \to t S +j$ with $S \to \mu^+ \mu^-$ and $S \to b \bar b$, and investigate the LHC sensitivity to the $tcS$ couplings. It is found that the scalar singlet $S$ can induce scalar-type contributions to the $B_s \to \mu^+ \mu^-$ decay, which do not suffer from the helicity suppression and contain a large CKM factor $V_{cs}^*V_{tb}$. As a result, constraints on the $tcS$ couplings from the measured branching ratio $\mathcal{B}(B_s \to \mu^+ \mu^-)$ are quite stringent, being even stronger than the expected LHC sensitivity in some parameter spaces. Besides the CP-conserving $tcS$ couplings, we have also considered the case of CP-violating $tcS$ couplings, with $y_{R,L}^{ct}=|y_{R,L}^{ct}|e^{i\theta_{R,L}}$. It is found that the CP observables $\mathcal{A}_{\Delta\Gamma_s}^{\mu\mu}$ and $\mathcal{S}_{\mu\mu}$ of the $B_s \to \mu^+ \mu^-$ decay are sensitive to the phase $\theta_R$, while the neutron EDM can provide bounds on the phase difference $\theta_L-\theta_R$. Therefore, they are complementary to each other in probing the CP phases of the $tcS$ couplings.