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arXiv · 2501.07341

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

Biao-Feng Hou·Xin-Qiang Li·Ya-Dong Yang·Xing-Bo Yuan·Ming-Wang Zhang
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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 $B_s \to μ^+ μ^-$ 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 $pp \to t S +j$ with $S \to μ^+ μ^-$ 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 $B_s \to μ^+ μ^-$ 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 $\mathcal{B}(B_s \to μ^+ μ^-)$ 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 $y_{R,L}^{ct}=|y_{R,L}^{ct}|e^{iθ_{R,L}}$. It is found that the CP observables $\mathcal{A}_{ΔΓ_s}^{μμ}$ and $\mathcal{S}_{μμ}$ of the $B_s \to μ^+ μ^-$ 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.

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Biao-Feng Hou, Xin-Qiang Li, Ya-Dong Yang, Xing-Bo Yuan, Ming-Wang Zhang. 2025-01-13. Phenomenological anatomy of top-quark FCNCs induced by a light scalar singlet. https://arxiv.org/abs/2501.07341

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