Motivated by the disagreement between the experimental data and lattice calculations on the decay constant of the $D_{s}$ meson, we investigate leptoquark (LQ) contributions to the purely leptonic decays of a pseudoscalar (P). We concentrate on the LQs which only couple to the second-generation quarks before the electroweak symmetry breaking and we discuss in detail how flavor symmetry breaking effects are brought into the extension of the standard model after the spontaneous symmetry breaking. We find that the hermiticity of fermion mass matrices can not only reproduce the correct Cabibbo-Kobayashi-Maskawa and Maki-Nakagawa-Sakata matrices, but also reduce the number of independent flavor mixing matrices and lead to $V^{R}_{f}=V^{L}_{f}$ with $L(R)$ denoting the chirality of the f-type fermion. Accordingly, it is found that the decays $D_{s,d}\to \ell^{+} ν$, $B^{+}\to τ^{+} ν$ and $B_c\to \ell^{+} ν$ have a strong correlation in parameters. We predict that the decay constant of the $B_c$ meson calculated by the lattice could be less than the experimental data by 23%. Intriguingly, the resultant upper limits of branching ratios for $D\to μ^{+} μ^{-}$ and $τ\to μ(π^0, η, η', ρ, ω)$ are found to be around $ 5.1\times 10^{-7}$ and $(2.6, 1.5, 0.6, 7.4, 4.8)\times 10^{-8}$, which are below and close to the current experimental upper bounds, respectively.
Leptoquarks (LQ) with couplings to the third generation of Standard Model (SM) quarks have been proposed as possible explanations of the flavour anomalies indicating the violation of lepton flavour universality (LFV). The CMS collaboration has initiated an extensive search programme for these new states in the LHC run-2 data recorded at $\sqrt{s}=13$ TeV. In this article, the CMS search results in the LQ pair-production final states $τb\,τb$, $νt\,νt$, $νb\,νb$, $τt\,τt$, and $μt\,μt$, as well as in the LQ single-production final state $τ\,τb$ are discussed. No significant deviation from the S…
In this work we analyse a non-minimal Lorentz-violating extension of the electroweak theory in the fermionic sector. Firstly we analyse the relation between the CKM rotation in the quark sector and possible contributions of this new coupling to flavour changing neutral currents (FCNC) processes. In sequel we look for non-diagonal terms through possible leptonic flavour violation (LFV) decays. Strong bounds are presented to the Lorentz violating parameters of both the quark and the leptonic sectors.
Some interpretations of $R_{D^{(*)}}$ anomaly in $B$ meson decay using leptoquark (LQ) models can also generate top quark decays through Flavor Changing Neutral Current (FCNC). In this work we focus on two LQs, i.e. scalar $S_1$ and vector $U_1$ which are both singlet under the $SU(2)_L$ gauge group in the Standard Model (SM). We investigate their implications on the 3-body top FCNC decays $t\to c \ell_i \ell_j$ at tree level and the 2-body $t\to c V$ at one-loop level, with $\ell$ being the SM leptons and $V=γ, Z, g$ being the SM gauge bosons. We utilize the $2σ$ parameter fitting ranges of t…
Realistic predictions are made for lepton flavour violating and flavour changing neutral current decays in the large tan beta regime of a SUSY-seesaw model. Lepton flavour violating neutral Higgs decays are discussed within this framework along with the highly constraining charged lepton decays l_i --> l_j + gamma. In the b-s system the important constraint from b --> s + gamma is studied in conjunction with the rare FCNC Bs --> mu mu (tau mu) decays and the recently measured Delta Ms.
The observed flavor-changing neutral-current (FCNC) processes in the standard model (SM) arise from the loop diagrams involving the weak charged currents mediated by the $W$-gauge boson. Nevertheless, the top-quark FCNCs and lepton-flavor violating processes resulting from the same mechanism are highly suppressed. We investigate possible new physics effects that can enhance the suppressed FCNC processes, such as $t\to q(h,V)$ with $V=γ,Z,g$, $h\to \ell \ell'$, and $\ell\to \ell' γ$. To achieve the assumption that the induced-FCNCs are all from quantum loops, we consider the scotogenic mechanis…
Recently it was noticed that among many scenarios of new physics leptoquarks might compensate for the disagreement between the lattice and experimental results for the charmed strange meson decay constant. The leptoquarks might modify also the flavor changing neutral current charm decays. In this study we investigate impact of the scalar leptoquark with electric charge -1/3 on the dilepton invariant mass distribution in the D^+ --> pi^+ mu^+ mu^- decay and on the branching ratios of the D^0 --> mu^+ mu^- using the existing experimental results.
$B$-physics data and flavor symmetries suggest that leptoquarks can have masses as low as few ${\cal{O}}(\mbox{TeV})$, predominantly decay to third generation quarks, and highlight $pp \to b μμ$ signatures from single production and $pp \to b b μμ$ from pair production. Abandoning flavor symmetries could allow for inverted quark hierarchies, and cause sizable $pp \to j μμ$ and $jj μμ$ cross sections, induced by second generation couplings. Final states with leptons other than muons including lepton flavor violation (LFV) ones can also arise. The corresponding couplings can also be probed by pr…
We show that FCNC processes are unavoidable for leptoquarks that couple to left handed quarks, and derive new FCNC bounds from neutral meson mixing. Despite being induced only at one loop, these processes lead to significant bounds since the leptoquark contributions do not suffer from GIM cancellations. Studying the implications of these bounds we find that (i) The D-Dbar mixing bound is the first significant FCNC bound from the up sector. Combining it with FCNC bounds from the down sector we arrive at a bound on the first generation couplings. (ii) The K-Kbar and D-Dbar mixings bound g^2/M wh…
We study the three body lepton flavor violating (LFV) decays $μ^- \to e^- e^+ e^-$, $τ^- \to l_i^- l_j^+ l_j^-$ and the semileptonic decay $τ\to μϕ$ in the flavor changing neutral current (FCNC) mediated $Z$ boson model. We also calculate the branching ratios for LFV leptonic B decays, $B_{d,s} \to μe$, $B_{d,s} \to τe$, $B_{d,s} \to τμ$ and the conversion of muon to electron in Ti nucleus. The new physics parameter space is constrained by using the experimental limits on $μ^- \to e^- e^+ e^-$ and $τ^- \to μ^- μ^+ μ^-$. We find that the branching ratios for $τ\to eee $ and $τ\to μϕ$ processes …
Significant deviations from Standard Model (SM) predictions have been observed in $ b \to s μ^+ μ^-$ decays and in the muon $g-2$. Scalar leptoquark extensions of the SM are known to be able to address these anomalies, but generically give rise to lepton flavor violation (LFV) or even proton decay. We propose new muon flavored gauge symmetries as a guiding principle for leptoquark models that preserve the global symmetries of the SM and explain the non-observation of LFV. A minimal model is shown to easily accommodate the anomalies without encountering other experimental constraints. This talk…
Large long-distance standard model effects in FCNC semileptonic D decays can make observable these processes in future measurements. Eventual disagreements in this sector and/or the observation of lepton family violating (LFV) D decays would require an explanatio beyond the SM framework. In this paper we confront present experimental data on leptonic and semileptonic FCNC and LFV D meson decays with a version of the two-Higgs doublet model that allows these effects to occur at tree- level. The stringent bounds on the parameters of the model are obtained from D^0 --> l^+l'^- and D --> pi l^+l'^…
The accurate prediction of neutrino beam produced in muon decays and the absence of opposite helicity contamination for a particular neutrino flavour makes a future neutrino factory (NF) based on a muon storage ring (MSR), the ideal place to look for the lepton flavour violating (LFV) effects. In this paper, we address the contribution of mediating LFV leptoquarks (LQ) in $ν(\barν)-{N}$ interactions leading to production of $τ$'s and wrong sign $μ$'s at MSR and investigate the region where LQ interactions are significant in the near-site and short baseline experiments.
LFV decays in a 3-3-1 model with singlet leptoquarks
Van Lang University（万浪大学）；Can Tho University（芹苴大学）；Nam Can Tho University（南芹苴大学）；An Giang University（安江大学）；Vietnam National University（越南国立大学）；Hanoi Pedagogical University 2（河内第二师范大學）
Motivated by the recent muon $g-2$ data, we study the lepton flavor violating $l \to l' γ$ and $Z \to l \bar l'$ ($l, l' = e, μ, τ$) decays with $l \neq l'$ in a scalar leptoquark model. Leptoquarks can produce sizable LFV $l\to l'γ$ decay rates that can be easily reached by present or near future experiments. % Leptoquark masses and couplings are constrained by the muon $g-2$ data and the current $l \to l' γ$ bounds. We predict $Br(Z \to τ^\mp e^\pm)$ reaching the present limit ($10^{-5}$) and $Br(Z \to μ^\mp τ^\pm)$ reaching $2\times 10^{-8}$, which will be accessible by future linear collid…
The contributions of scalar leptoquarks in lepton flavor violating (LFV) processes of type $l_i\to l_j γ$ are investigated in frame of the minimal model with the four color symmetry and Higgs mechanism of quark and lepton mass generation. It is shown that experimental data on the decays $μ\to e γ$, $τ\to μγ$, $τ\to e γ$ allow the existence of light scalar leptoquarks of type under consideration, with masses of order 1 TeV or below.
We investigate the consequence of vector leptoquarks on the rare semileptonic lepton flavour violating decays of $B$ meson which are more promising and effective channels to probe the new physics signal. We constrain the resulting new leptoquark parameter space by using the branching ratios of $B_{s, d} \to l^+ l^-$, $K_L \to l^+ l^-$ and $τ^- \to l^- γ$ processes. We estimate the branching ratios of rare lepton flavour violating $B \to K(π)l_i^- l_j^+$ processes using the constrained leptoquark couplings. We also compute the forward-backward asymmetries and the lepton non-universality paramet…
Motivated by the rencent LHC data on the lepton-flavor violating (LFV) decays $h\to \ell_1 \ell_2$ and $B_{s,d}\to \ell_1 \ell_2$, we study the Higgs-mediated flavor-changing neutral current (FCNC) interactions in the effective field theory (EFT) approach without and with the minimal flavor violation (MFV) hypothesis, and concentrate on the later. After considering the $B$ and $K$ physics data, the various LFV processes, and the LHC Higgs data, severe constraints on the Higgs FCNC couplings are derived, which are dominated by the LHC Higgs data, the $B_s - \bar B_s$ mixing, and the $μ\to e γ$ …
We study the effect of scalar leptoquarks in the lepton flavour violating $B$ meson decays induced by the flavour changing transitions $b \to q l_i^+ l_j^-$ with $q=s,d$. In the standard model these transitions are extremely rare as they are either two-loop suppressed or proceed via box diagrams with tiny neutrino masses in the loop. However, in the leptoquark model they can occur at tree level and are expected to have significantly large branching ratios. The leptoquark parameter space is constrained using the experimental limits on the branching ratios of $B_q \rightarrow l^+ l^-$ processes.…
We show that the scalar leptoquark Yukawa couplings generate a significant lepton flavour violation. We compute the light scalar leptoquark contributions to the branching ratios (Br) of the lepton flavour violating (LFV) decays $\ell \to \ell_i \ell_j \bar{\ell}_j$ and $\ell \rightarrow \ell^{\prime} γ$ with (i,j = e,$μ$). We discuss the role of the relevant input parameters to these decay rates which are the Yukawa couplings ($h_{a\ell}$) with ($a=u,c,t$), the light scalar mass $M_{S_{1}}$ and the mixing angle $\sin2θ_{LQ}$. We investigate the experimental limits from $(g-2)_μ$, $μ-e$ convers…
We consider the feasibility of interpreting the $R_{K^{(*)}}$ anomaly in the colored Zee-Babu model. The model generates neutrino masses at two loops with the help of a scalar leptoquark $S\sim(3,3,-\frac{1}{3})$ and a scalar diquark $ω\sim(6,1,-\frac{2}{3})$, and contributes to the transition $b\to s\ell\ell$ via the exchange of a leptoquark $S$ at tree level. Under constraints from lepton flavor violating (LFV) and flavor changing neutral current (FCNC) processes, and direct collider searches for heavy particles, we acquire certain parameter space that can accommodate the $R_{K^{(*)}}$ anoma…
In order to satisfy current FCNC and CP violation bounds, SUSY flavour structures cannot be generic. An interesting solution to these SUSY Flavour and CP Problems lies on the use of an SU(3) family symmetry which spontaneously breaks CP symmetry. Typical observables of such a model are electric dipole moments and LFV processes. In addition, these models can give contributions to CP violation in neutral meson mixing. We show how the latter can be used to restrict the allowed SUSY parameter space, and induce correlations between LFV and EDM predictions.
In this paper we present general formulae for the calculation of LFV Higgs decays $H_r \to l_a l_b$ at one-loop, with $H_r$ being part of the Higgs spectrum of a generic multi-scalar extension of the Standard Model (SM) with neutrino masses. We develop a method based on a classification of the particles appearing in the loop diagrams (scalars, fermions and vectors), and by identifying the corresponding couplings, we are able to present compact expressions for the form factors involved in the amplitudes. Our results are applicable to models where Flavor Changing Neutral Currents (FCNC) are forb…
In this paper we present a phenomenological analysis of the Partially Aligned Two Higgs Doublet Model (PA-2HDM) by using leptonic decays of mesons and $B^0_{d,s}$-$\bar B^0_{d,s}$ mixing. We focus our attention in a scenario where the leading contribution to FCNC is given by the tree level interaction with the light pseudoscalar $A^0$ ($M_{A^0}\sim 250$ GeV). We show how an underlying flavor symmetry controls FCNC in the quark and lepton couplings with the pseudoscalar, without alignment between Yukawa matrices. Upper bounds on the free parameters are calculated in the context of the leptonic …
In this paper we firstly study various model-independent bounds on lepton flavor violation (LFV) in processes of $J/Ψ$, $Ψ'$ and $Υ$ two-body decays, then calculate their branch ratios % By using the constraints from other ways, we obtain %the indirect bounds of ${\rm Br} (J/Ψ,Ψ',Υ\to ll')$ in models of the leptoquark, $R$ violating MSSM and topcolor assisted technicolor(TC2) models.
The dimension-six CP-conserving $SU_L(2)\times U_Y(1)$ invariant operators involving the tau lepton are studied. It leads to the new physics effects to the lepton flavor violation (LFV) and lepton flavor-changing neutral current(FCNC). With the available experimental data on the decays $Z \to τ^+ + τ^- $, $Z\to μ^+ + μ^- $ and $μ^- \to e^- +γ$, the constraints on parameters of the effective lagrangian can be given. According to such constraints, the branching ratio of the decay $Z\to μ^\pm + τ^\mp $ could reach to $10^{-6}-10^{-7}$. The size of the LFV effects depends crucially on the dynamics…
We investigate the effect of scalar leptoquarks and family non-universal $Z^\prime$ boson on the rare leptonic decays of $B_{s, d}^*$ mesons, mediated by the FCNC transitions $b \to (d, s) l^+ l^-$. They are sensitive to a variety of new physics operators as opposed to the $B_{s,d}$ leptonic modes and hence, can provide an ideal testing ground to look for new physics beyond the standard model. We work out the constraint on new physics parameter space using the measured branching ratios of $B_{d, s} \to μ^+ μ^-$ processes and the $B_q - \bar B_q$ mixing data. Using the constrained parameters we…
We investigate the possibility of searching the lepton-flavor-violating (LFV) $τ\rightarrow μγ$ rare decay at the Super Charm-Tau Factory (CTF). By comparing the kinematic distributions of the LFV signal and the standard model (SM) background, we develop an optimized event selection criteria which can significantly reduce the background events. It is concluded that new $2 σ$ upper limit of about $1.9 \times 10^{-9}$ on $Br(τ\rightarrow μγ)$ can be obtained at the CTF, which is beyond the capability of Super-B factory in searching $τ$ lepton rare decay. Within the framework of the scalar leptoq…
Flavor violating Higgs signals, such as the top FCNC decay $t\to ch^0$ and the LFV Higgs decay $h^0\to τμ$, have been studied at the LHC. These signals can arise within the general Two-Higgs doublet model (THDM), where each Higgs doublet couples to all fermions types through Yukawa matrices $Y^f_1$ and $Y^f_2$. The Yukawa matrices can be assumed to have the same form or they could have different structures. In this paper we study the case when both $Y^f_1$ and $Y^f_2$ have completely different forms, but in such a way that they complement to produce a specific hermitian mass matrix. We find th…
Motivated by the recent LHCb announcement of a $3.1σ$ violation of lepton-flavor universality in the ratio $R_K=Γ(B\to Kμ^+μ^-)/Γ(B\to K e^+ e^-)$, we present an updated, comprehensive analysis of the flavor anomalies seen in both neutral-current ($b\to s\ell^+\ell^-$) and charged-current ($b\to cτ\barν$) decays of $B$ mesons. Our study starts from a model-independent effective field-theory approach and then considers both a simplified model and a UV-complete extension of the Standard Model featuring a vector leptoquark $U_1$ as the main mediator of the anomalies. We show that the new LHCb dat…
Flavor changing neutral current $|ΔB|=|ΔS|=1$ processes are sensitive to possible new physics at the electroweak scale and beyond, providing detailed information about flavor, chirality and Lorentz structure. Recently the LHCb collaboration announced a $2.6 σ$ deviation in the measurement of $R_K={\cal{B}}(\bar B \to \bar K μμ)/{\cal{B}}(\bar B \to \bar K ee)$ from the standard model's prediction of lepton universality. We identify dimension six operators which could explain this deviation and study constraints from other measurements. Vector and axial-vector four-fermion operators with flavor…
