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

Abstract

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 → μ + μ − decay, the muon anomalous magnetic moment (g − 2) μ and the neutron Electric Dipole Moment (EDM). We also perform a detailed Monte-Carlo simulation of the channel pp → tS + j with S → μ + μ − and S → b b ¯ $$ b\overline{b} $$ , 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 → μ + μ − decay, which do not suffer from the helicity suppression and contain a large CKM factor V cs ∗ V tb $$ {V}_{cs}^{\ast }{V}_{tb} $$ . As a result, constraints on the tcS couplings from the measured branching ratio B $$ \mathcal{B} $$ (B s → μ + μ − ) 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 θ R , L $$ {y}_{R,L}^{ct}=\left|{y}_{R,L}^{ct}\right|{e}^{i{\theta}_{R,L}} $$ . It is found that the CP observables A Δ Γ s μμ $$ {\mathcal{A}}_{\Delta {\varGamma}_s}^{\mu \mu} $$ and S μμ $$ {\mathcal{S}}_{\mu \mu} $$ of the B s → μ + μ − decay are sensitive to the phase θ R , while the neutron EDM can provide bounds on the phase difference θ L − θ R . Therefore, they are complementary to each other in probing the CP phases of the tcS couplings.

Keywords

• CP Violation
• Top FCNC
• Rare Decays
• Electric Dipole Moments