Flavor changing neutral current decays $$t\rightarrow c X$$ ($$X=\gamma ,\,g,\, Z,\, H$$) and $$t\rightarrow c{{\bar{\ell }}}\ell $$ ($$\ell =\mu ,\,\tau $$) via scalar leptoquarks

Abstract

The flavor changing neutral current decays t → cX (X = γ, g, Z , H) and t → c¯ (= μ, τ) are studied in a renormalizable scalar leptoquark (LQ) model with no proton decay, where a scalar SU (2) doublet with hypercharge Y = 7/6 is added to the standard model, yielding a non-chiral LQ Ω 5/3. Analytical results for the one-loop (tree-level) contributions of a scalar LQ to the f i → f j X (f i → f jfm f l) decays, with f a = q a , a , are presented. We consider the scenario where Ω 5/3 couples to the fermions of the second and third families, with its right-and left-handed couplings obeying λ u i R /λ u i L = O(), where parametrizes the relative size between these couplings. The allowed parameter space is then found via the current constraints on the muon (g − 2), the τ → μγ decay, the LHC Higgs boson data, and the direct LQ searches at the LHC. For m Ω 5/3 = 1 TeV and = 10 −3 , we find that the t → cX branching ratios are of similar size and can be as large as 10 −8 in a tiny area of the parameter space, whereas Br(t → cτ τ) [Br(t → cμμ)] can be up to 10 −6 (10 −7).

Lepton Flavor Violating $h\to\tau\mu$ decay induced by leptoquarks

“…[47,55] and more recently in Ref. [49] where constraints on the couplings to a lepton-quark pair were obtained through the analysis of the µAMDM and the LFV tau decay τ → µγ. We are interested in the contribution of the non-chiral leptoquark since it gives rise to a chirality-flippin term which is proportional to the internal quark mass that can enhance the Higgs boson decay h → τ µ.…”

“…We are interested in the contribution of the non-chiral leptoquark since it gives rise to a chirality-flippin term which is proportional to the internal quark mass that can enhance the Higgs boson decay h → τ µ. Although the complete expressions for the non-chiral LQ contribution to h → τ µ were obtained in [49], the aim of this work is the analysis of the signals events in the present and future colliders. For our calculations, we require the LQ couplings to fermions, the gauge bosons, and the Higgs boson, which can be obtained from the effective Lagrangians presented below.…”

“…The leptoquark doublet R 2 can give large contributions to the oblique parameters unless the mass eigenstates Ω 5/3 and Ω 2/3 have a small mass splitting [59], then we will assume the bound m Ω 5/3 ≥ 1 TeV in our analysis. On the other hand, the scalars LQs can considerably modify other loop-induced Higgs boson processes such as H → γγ and the production cross-section of the Higgs boson via the gluon fusion mechanism gg → H. In particular, for the model, we are interested in the leptoquarks Ω 5/3 and Ω 2/3 that contribute to the loop functions and the corresponding LQ coupling to the Higgs boson λ Ω = λ Ω 5/3 = λ Ω 2/3 can be constrained by the so-called Higgs boson coupling modifiers Such analysis was carried out in [49], where the coupling λ Ω can take values in the interval (−5, 5) for an LQ mass m Ω 5/3 = 1 TeV. For larger values of the LQ mass, the allowed area slightly increases, being of order O(10); how-ever, we will consider the conservative value: λ Ω = 4, which agrees with the perturbative limit.…”

“…The LQs may or may not have well-define baryon (B) and lepton (L) number; however, those with violating interactions can mediate the proton decay unless an extra symmetry is invoked to forbid the diquark couplings, otherwise the leptoquark masses are expected to be at the Planck scale to ensure the stability of the proton [46]. The low-energy LQ phenomenology has received considerable attention, and possible LQ manifestation in various processes has been extensively investigated [47][48][49]. The LQs with left and righthanded couplings to fermions are interesting candidates to explain the discrepancy of the muon anomalous magnetic moment (µAMDM) since they induce an enhancement by a factor m t /m µ ∼ O (10 3 ) or m b /m µ ∼ O(10) compared to the SM.…”

Single top production via FCNC couplings at an $e^{-}e^{+}$ collider with center-of-mass energy $\sqrt{s}=240$ GeV

“…The top quark FCNC decays include the modes: t → cX, with X = h, Z, γ, g, which were calculated first in [4,5], and later on they were studied for other models [6][7][8][9][10][11][12][13][14] including its search at the LHC [15][16][17]. One can also consider the FCNC decays of heavy particles (H , Z ) that can decay into t c [18][19][20].…”