Journal Article

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

Yuan, Xing-Bo, Hou, Biao-Feng, Yang, Ya-Dong, Li, Xin-Qiang, Zhang, Ming-Wang

2025

Overview

A bstract 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 ¯ , 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 . As a result, constraints on the tcS couplings from the measured branching ratio 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 . It is found that the CP observables A Δ Γ s μμ and S μμ 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.

• Publisher: Springer Berlin Heidelberg,Springer Nature B.V,SpringerOpen
• Subject: Classical and Quantum Gravitation / Constraints / Couplings / CP Violation / Decay / Dipole moments / Electric Dipole Moments / Electric dipoles / Elementary Particles / Flavor (particle physics) / Helicity / Higgs bosons / Magnetic moments / Monte Carlo simulation / Neutral currents / Parameter sensitivity / Physics / Physics and Astronomy / Quantum Field Theories / Quantum Field Theory / Quantum Physics / Quarks / Rare Decays / Regular Article - Theoretical Physics / Relativity Theory / Standard model (particle physics) / String Theory / Top FCNC