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

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 $pp\to tS+j$ 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 $V_{cs}^{\ast }{V}_{tb}$. 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.

44 pages, 20 figures, comments welcome;