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

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 → $$ b\overline{b} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>b</mml:mi> <mml:mover> <mml:mi>b</mml:mi> <mml:mo>¯</mml:mo> </mml:mover> </mml:math> , 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 $$ {V}_{cs}^{\ast }{V}_{tb} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup> <mml:mi>V</mml:mi> <mml:mi>cs</mml:mi> <mml:mo>∗</mml:mo> </mml:msubsup> <mml:msub> <mml:mi>V</mml:mi> <mml:mi>tb</mml:mi> </mml:msub> </mml:math> . As a result, constraints on the tcS couplings from the measured branching ratio $$ \mathcal{B} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>B</mml:mi> </mml:math> ( 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 $$ {y}_{R,L}^{ct}=\left|{y}_{R,L}^{ct}\right|{e}^{i{\theta}_{R,L}} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup> <mml:mi>y</mml:mi> <mml:mrow> <mml:mi>R</mml:mi> <mml:mo>,</mml:mo> <mml:mi>L</mml:mi> </mml:mrow> <mml:mi>ct</mml:mi> </mml:msubsup> <mml:mo>=</mml:mo> <mml:mfenced> <mml:msubsup> <mml:mi>y</mml:mi> <mml:mrow> <mml:mi>R</mml:mi> <mml:mo>,</mml:mo> <mml:mi>L</mml:mi> </mml:mrow> <mml:mi>ct</mml:mi> </mml:msubsup> </mml:mfenced> <mml:msup> <mml:mi>e</mml:mi> <mml:mrow> <mml:mi>i</mml:mi> <mml:msub> <mml:mi>θ</mml:mi> <mml:mrow> <mml:mi>R</mml:mi> <mml:mo>,</mml:mo> <mml:mi>L</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:msup> </mml:math> . It is found that the CP observables $$ {\mathcal{A}}_{\Delta {\varGamma}_s}^{\mu \mu} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup> <mml:mi>A</mml:mi> <mml:mrow> <mml:mi>Δ</mml:mi> <mml:msub> <mml:mi>Γ</mml:mi> <mml:mi>s</mml:mi> </mml:msub> </mml:mrow> <mml:mi>μμ</mml:mi> </mml:msubsup> </mml:math> and $$ {\mathcal{S}}_{\mu \mu} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>S</mml:mi> <mml:mi>μμ</mml:mi> </mml:msub> </mml:math> 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.