Phenomenological anatomy of top-quark FCNCs induced by a light scalar singlet
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2025
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| author | Hou, Biao-Feng Li, Xin-Qiang Yang, Ya-Dong Yuan, Xing-Bo Zhang, Ming-Wang |
| author_facet | Hou, Biao-Feng Li, Xin-Qiang Yang, Ya-Dong Yuan, Xing-Bo Zhang, Ming-Wang |
| contents | 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. |
| format | Preprint |
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arxiv_https___arxiv_org_abs_2501_07341 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Phenomenological anatomy of top-quark FCNCs induced by a light scalar singlet Hou, Biao-Feng Li, Xin-Qiang Yang, Ya-Dong Yuan, Xing-Bo Zhang, Ming-Wang High Energy Physics - Phenomenology 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. |
| title | Phenomenological anatomy of top-quark FCNCs induced by a light scalar singlet |
| topic | High Energy Physics - Phenomenology |
| url | https://arxiv.org/abs/2501.07341 |