We present a comprehensive analysis of low-energy signals of hypothetical scalar leptoquark interactions in lepton and kaon transitions. We derive the most general effective four-fermion Lagrangian induced by tree-level scalar leptoquark exchange and identify the Wilson coefficients predicted by the five possible types of scalar leptoquarks. The current constraints on the leptoquark Yukawa couplings arising from lepton and kaon processes are worked out, including also loop-induced transitions with only leptons (or quarks) as external states. In the presence of scalar leptoquark interactions, we also derive the differential distributions for flavour-changing neutral-current transitions in semileptonic kaon modes, including all known effects within the Standard Model. Their interference with the new physics contributions could play a significant role in future improvements of those constraints that are currently hampered by poorly-determined non-perturbative parameters.
We entertain the possibility that scalar leptoquarks (LQs) generate consequential effects on the strangeness-changing decays of hyperons and kaons involving missing energy carried away by a pair of invisible fermions. Although such processes have suppressed rates in the standard model (SM), they could get significant enhancement in the presence of the LQs. In order to respect the available data on the kaon modes $K\toπν\barν$ and increase the rates of the hyperon decays substantially at the same time, two different scalar LQs are needed. If the LQs have Yukawa couplings solely to SM fermions, …
Recently, several anomalies in flavor physics have been observed, and it was noticed that leptoquarks might account for the deviations from the Standard Model. In this work, we examine the effects of new physics originating from a scalar leptoquark model on the kaon sector. The leptoquark we consider is a TeV-scale particle and within the reach of the LHC. We use the existing experimental data on the several kaon processes including $K^{0}-\bar{K}^{0}$ mixing, rare decays $K^{+} \rightarrow π^{+} ν\barν$, $ K_L \rightarrow π^0 ν\barν$, short-distance part of $K_L\rightarrow μ^{+}μ^{-}$, and le…
Leptoquarks are ubiquitous in several extensions of the Standard Model and seem to be able to accommodate the universality-violation-driven $B$-meson-decay anomalies and the $(g-2)_μ$ discrepancy interpreted as deviations from the Standard Model predictions. In addition, the search for lepton-flavour violation in the charged sector is, at present, a major research program that could also be facilitated by the dynamics generated by leptoquarks. In this article, we consider a rather wide framework of both scalar and vector leptoquarks as the generators of lepton-flavour violation in processes in…
We point out unique relations between the decay widths for leptoquarks and Higgs bosons that can be used to test the unification of quarks and leptons at the TeV scale. We discuss the main predictions of the minimal theory for quark-lepton unification and show how the different decays for leptoquarks and Higgses are related by the symmetry of the theory.
In this work we study possible connections between $B$-meson anomalies and Kaon physics observables in the context of combined solutions with the singlet and triplet scalar leptoquarks $S_1$ and $S_3$. By assuming a flavor structure for the leptoquark couplings dictated by a minimally broken $U(2)^5$ flavor symmetry we can make a sharp connection between these two classes of observables. We find that the bound on $\mathcal{B}(K^+ \rightarrow π^+ νν)$ from NA62 puts already some tension in the model, while the present limits on $\mathcal{B}(K_L \rightarrow μ^+ μ^-)$ and $μ\to e$ conversion in n…
Leptoquarks with masses at the TeV scale have been proposed as possible solutions to flavor anomalies reported in the b-flavor sector. Based on data taken in proton-proton collisions at $\sqrt{s} = 13\,\mathrm{TeV}$ at the LHC, different leptoquark flavors can be probed in various final states. In this article, the results of direct searches for single and pair production of leptoquarks conducted by the ATLAS and CMS Collaborations are summarized.
We propose a leptoquark model with two scalar leptoquarks $S^{}_1 \left( \bar{3},1,\frac{1}{3} \right)$ and $\widetilde{R}^{}_2 \left(3,2,\frac{1}{6} \right)$ to give a combined explanation of neutrino masses, lepton flavor mixing and the anomaly of muon $g-2$, satisfying the constraints from the radiative decays of charged leptons. The neutrino masses are generated via one-loop corrections resulting from a mixing between $S^{}_1$ and $\widetilde{R}^{}_2$. With a set of specific textures for the leptoquark Yukawa coupling matrices, the neutrino mass matrix possesses an approximate $μ$-$τ$ refl…
The LHC search strategies for leptoquarks that couple dominantly to a top quark are different than for the ones that couple mostly to the light quarks. We consider charge $1/3$ ($ϕ_1$) and $5/3$ ($ϕ_5$) scalar leptoquarks that can decay to a top quark and a charged lepton ($t\ell$) giving rise to a resonance system of a boosted top and a high-$p_{\rm T}$ lepton. We introduce simple phenomenological models suitable for bottom-up studies and explicitly map them to all possible scalar leptoquark models within the Buchmüller-Rückl-Wyler classifications that can have the desired decays. We study pa…
Leptoquarks are hypothetical new particles, which couple quarks directly to leptons. They experienced a renaissance in recent years as they are prime candidates to explain the so-called \textit{flavor anomalies}, i.e. the deviations between the Standard Model predictions and measurements in $b\to s\ell^{+}\ell^{-}$ and $b\to cτν$ processes and in the anomalous magnetic moment of the muon. At the one-loop level these particles unavoidably generate effects in the purely leptonic processes like $Z\to\ell^{+}\ell^{-}$, $Z\toν\barν$, $W\to\ellν$ and $h\to\ell^{+}\ell^{-}$ and can even generate non-…
The long-standing anomaly in the anomalous magnetic moment of the muon indicates the presence of chirality violating new physics contributions. A possible solution involves scalar leptoquarks with left- and right-handed couplings to the top quark. Two such representations of scalar leptoquarks exist for which the contribution to $(g-2)_μ$ can possess an $m_t/m_μ$ enhancement compared to the Standard Model. The leptoquarks also induce loop corrections to $Z$ couplings to muons which probe as well new physics contributions which possess sources of $SU(2)$ symmetry breaking and we find that this …
We examine the utility of employing polarized lepton (electron and muon) beams to perform indirect searches for scalar leptoquarks. We find that polarization can extend the reach in excluding leptoquark masses for both $e^+e^-$ and $μ^+μ^-$ machines. Polarization can also provide a diagnostic tool for determining leptoquark couplings.
When a TeV-scale leptoquark has a sizeable Yukawa coupling, its dominant production mechanism at hadron colliders is the partonic-level lepton-quark fusion. Even though the parton distribution functions for leptons inside the proton are minuscule, they get compensated by the resonant enhancement. We present the first computation of higher order radiative corrections to the resonant leptoquark production cross section at the Large Hadron Collider (LHC). Next-to-leading (NLO) QCD and QED corrections are similar in size but come with the opposite sign. We compute NLO $K$-factors for a wide range …
We propose a quark-lepton model via leptoquarks and modular $A_4$ symmetry. Since the neutrino mass is induced at one-loop level mediated by down quarks as well as leptoquarks, we have to explain lepton and quark masses and mixings with a single modulus $τ$. Here, we find predictions for lepton and quark sectors with unified modulus $τ$, and show several constraints originating from leptoquarks.
Lepton Number Violation at the LHC in Radiative Neutrino Mass Models with Leptoquarks
Oklahoma State University（俄克拉荷马州立大学）；The University of Tokyo（东京大学）；Kavli Institute for the Physics and Mathematics of the Universe (WPI)（Kavli宇宙物理与数学研究所（WPI））
We study implications of large lepton-quark-leptoquark couplings for direct leptoquark searches at Large Hadron Collider. We present all existing flavor constraints on the strength of these couplings assuming that leptoquarks under consideration interact exclusively with charged leptons and quarks of the same generation. We find that these leptoquarks can have sizeable couplings to the Standard Model fermions. This insures a self consistency of our study. We discuss the leptoquark production mechanisms at LHC and demonstrate the importance of inclusion of a t-channel pair production and, in pa…
We discuss the minimal theory for quark-lepton unification at the low scale. In this context, the quarks and leptons are unified in the same representations and neutrino masses are generated through the inverse seesaw mechanism. The properties of the leptoquarks predicted in this theory are discussed in detail and we investigate the predictions for the leptonic and semi-leptonic decays of mesons. We study the possibility to explain the current value of $\mathcal{R}_K$ reported by the LHCb collaboration and the value of the muon anomalous magnetic moment reported by the Muon $g-2$ experiment at…
Recent hints for lepton-flavor non-universality in $B$-meson decays can be interpreted as hints for the existence of leptoquarks. We show that scalar leptoquarks unavoidably arise in grand unified theories, using the well-known Pati-Salam model as an example. These GUT-motivated leptoquarks can have a number of appealing features including automatic absence of proton decay, purely chiral couplings, and relations between the various leptoquark couplings. We show that $R(K^{(*)})$ can be connected to the neutrino mass matrix that arises via type-II seesaw, resulting in testable lepton flavor vio…
We propose that three recent anomalies in $B$ meson decays, $R_{D^{(*)}}$, $R_K$, and $P_5'$, might be explained by a single vector leptoquark weak triplet state. The constraints on the parameter space are obtained by considering $t \to b τ^+ ν$ data, lepton flavor universality tests in the kaon sector, bounds on $B \to K^{(*)} \bar νν$, bound on the lepton flavor violating decay $B \to K μτ$, and measurements of $b \to c μ^- \barν$ decays. The presence of such vector leptoquark could be exposed in precise measurements of $t \to b τν$ and $B \to K^{(*)} \bar νν$ decays. The model also predicts…
The vertex corrections to the leptonic partial widths of the $Z$ induced by leptoquarks that couple leptons to the top quark are considered. We obtain stringent bounds on the parameter space of the masses and Yukawa couplings of these leptoquarks, using the latest information on the $Z \rightarrow l^+l^-$ decay widths measured at LEP. Leptoquarks coupling with electroweak strength to top quarks are constrained to be heavier than several hundred GeV, at 95$\%$ C.L. As a consequence, such leptoquarks cannot make a significant contribution to lepton asymmetries, $τ$ polarisation asymmetries or $A…
We investigate a model in which tiny neutrino masses are generated at the two-loop level by using scalar leptoquark and diquark multiplets. The diquark can be singly produced at the LHC, and it can decay into a pair of leptoquarks through the lepton number violating interaction. Subsequent decays of the two leptoquarks can provide a clear signature of the lepton number violation, namely two QCD jets and a pair of same-signed charged leptons without missing energy. We show that the signal process is not suppressed while neutrino masses are appropriately suppressed.
We study the phenomenological constraints and consequences in the flavor sector, of introducing a new fourth generation $\mathcal{Z}_{2}$ odd vector-like lepton doublet along with a Standard Model (SM) singlet scalar and an $SU(2)_{L}$ singlet scalar leptoquark carrying electromagnetic charge of $+2/3$, both odd under a $\mathcal{Z}_{2}$. We show that with little fine tuning among the various Yukawa couplings in the new physics (NP) Lagrangian along with the CKM parameters, the model is able to push the theoretical value of $R(D^{*})^{th}$ from $0.252 \pm 0.003$ to $0.263 \pm 0.051$ and $R(D)^…
We entertain the option of scalar leptoquarks to explain the anomalies in the semi-leptonic decays of $B$-mesons and the discrepancies in the lepton $(g-2)_l$'s including the recent results at Fermilab E989. The $R_{K^{(*)}}$ and $R_{D^{(*)}}$ anomalies can be accommodated by the specific couplings for triplet and singlet leptoquarks, respectively, subject to the bounds from $B\to Kν{\barν}$. We discuss the correlation between the leptonic signatures from leptoquarks such as $μ\to eγ$ and the electric dipole moment of electron and show that desirable neutrino masses can be generated dominantly…
Motivated by the measurements of $b\to s\ell^+\ell^-$ transitions, including $R(K)$ and $R(K^*)$, we examine lepton flavour (universality) violation in $B$ decays and its connections to $μ\to eγ$ in generic leptoquark models. Considering all 10 representations of scalar and vector leptoquarks under the Standard Model gauge group we compute the tree-level matching for semileptonic $b$-quark operators as well as their loop effects in $\ell\to\ell^\primeγ$. In our phenomenological analysis we correlate $R(K)$, $R(K^*)$ and the other $b\to sμ^+μ^-$ data to $μ\to eγ$ and $b\to sμe$ transitions for …
A search for second and third generation scalar and vector leptoquarks produced in ep collisions via the lepton flavour violating processes ep to mu-X and ep to tau-X is performed by the H1 Collaboration at HERA. The full H1 ep data sample taken at a centre-of-mass energy of 319 GeV is used for the analysis, corresponding to an integrated luminosity of 411 pb^-1. No evidence for the production of such leptoquarks is observed in the H1 data. Leptoquarks produced in ep collisions with a coupling strength of lambda=0.3 and decaying with the same coupling strength to a muon-quark pair or a tau-qua…
The contributions into radiative correction parameters S, T, U from the scalar leptoquarks are calculated in the minimal gauge model with the four-color quark-lepton symmetry. It is shown that the contributions into T and U from the scalar leptoquark doublets are not positive definite. Using the current experimental data on S, T, U the bounds on the scalar leptoquark masses are obtained. In particular, the existence of the relatively light scalar leptoquark with electric charge 2/3 is shown to be compatible with the current experimental data on S, T, U.
We calculate the CP-violating electric and weak dipole form factors of the top quark and the tau lepton in models with scalar leptoquarks coupling only to the third generation of quarks and leptons. We obtain numerical values of the real and imaginary parts of these form factors at various energies for different values of leptoquark masses and couplings. The existing limits on the tau electric and weak dipole form factors allows us to put a limit on the masses and couplings of such leptoquarks and therefore on the top electric and weak dipole form factors. We also discuss constraints on the fo…
The properties of light leptoquarks predicted in the context of a simple grand unified theory and their observability at the LHC are investigated. The SU(5) symmetry of the theory implies that the leptoquark couplings to matter are related to the neutrino mass matrix. We study the resulting connection between neutrino masses and mixing parameters and the leptoquark decays, and show that different light neutrino hierarchies imply distinctive leptoquark decay signatures. We also discuss low-energy constraints implied by searches for charged lepton flavour violation, studies of meson decays, and …
The cross section of single vector leptoquark production in direct lepton-gluon interaction is calculated. In a model independent analysis an effective Lagrangian describing the most general $C$ and $P$ conserving coupling of the leptoquarks to gluons is considered. An analytical expression is derived for the cross section in the case of anomalous vector leptoquark couplings to the gluon field. The cross sections of inclusive production of ($eq$)-, ($μq$)- and ($τq$)-type vector leptoquarks in $ep$ and $pp$ collisions due to direct lepton-gluon interaction are evaluated for $q=u, b, t$. Depend…
We propose a mechanism that allows for sizeable flavour violation in quark-lepton currents, while suppressing flavour changing neutral currents in quark-quark and lepton-lepton sectors. The mechanism is applied to the recently proposed 4321 renormalizable model, which can accommodate the current experimental anomalies in $B$-meson decays, both in charged and neutral currents, while remaining consistent with all other indirect flavour and electroweak precision measurements and direct searches at high-$p_T$. To support this claim, we present an exhaustive phenomenological survey of this fully ca…
We study the phenomenology of a particular leptoquark extension of the Standard Model (SM), namely the doublet-singlet scalar leptoquark extension of the SM (DSL-SM). Besides generating Majorana mass for neutrinos, these leptoquarks contribute to muon and electron $(g-2)$ and various lepton flavour violating processes. Collider signatures of the benchmark points (BPs), consistent with the neutrino oscillation data, anomalous muon/electron magnetic moments, experimental bounds on the charged lepton flavour violation observables, etc., are studied at the LHC/FCC with centre-of-mass energies of 1…
We study the sensitivity of the Tevatron and the 7 TeV LHC to a leptoquark S coupling to a top quark and a charged lepton L (= e, mu, or tau). For the Tevatron, we focus on the case m_S < m_t, where the leptoquark pair production cross section is large, and the decay is three-body: S --> W b L^{\pm}. We argue that existing Tevatron observations could exclude m_S \lsim 160 GeV. For m_S > m_t, we show that the LHC experiments with low integrated luminosity could be sensitive to such leptoquarks decaying to tl^{\pm} with l= mu or tau.
Scalar leptoquarks, with baryon and lepton number conserving interactions, could have TeV scale masses, and be produced at colliders or contribute to a wide variety of rare decays. In pursuit of some insight as to the most sensitive search channels, We assume that the leptoquark-lepton-quark coupling can be constructed from the known mass matrices. We estimate the rates for selected rare processes in three cases: leptoquarks carrying lepton and quark flavour, leptoquarks with quark flavour only, and unflavoured leptoquarks. We find that leptoquark decay to top quarks is an interesting search c…
We analyse for the first time the CP violating ratio $\varepsilon'/\varepsilon$ in $K\to ππ$ decays in leptoquark (LQ) models. Assuming a mass gap to the electroweak (EW) scale, the main mechanism for LQs to contribute to $\varepsilon'/\varepsilon$ is EW gauge-mixing of semi-leptonic into non-leptonic operators, which we treat in the Standard Model effective theory (SMEFT). We perform also the one-loop decoupling for scalar LQs, finding that in all models with both left-handed and right-handed LQ couplings box-diagrams generate numerically strongly enhanced EW-penguin operators $Q_{8,8'}$ alre…
We examine various scenarios that involve a light ${\cal O}(1 TeV)$ leptoquark state and select those which are compatible with the current experimental values for $\mathcal{B}(B_s\to μμ)$, $\mathcal{B}(B\to Kμμ)_{\mathrm{large}-q^2}$, $R_K=\mathcal{B^\prime}(B\to Kμμ)/\mathcal{B^\prime}(B\to Kee)$, and which lead to predictions consistent with other experimental data. We show that two such scenarios are phenomenologically plausible, namely the one with a doublet of scalar leptoquarks of hypercharge $1/6$, and the one with a triplet of vector leptoquarks of hypercharge $2/3$. We also argue tha…
Leptoquarks provide some of the simplest explanations to the hints of lepton flavor non-universality in $B$ decays. In particular, a new confining gauge group can provide a natural and appealing origin for the leptoquarks. So far, direct collider searches have been based on two body decays, namely to a quark and a lepton. We study how composite dynamics can give rise to additional states resulting in modified collider signatures of leptoquarks, as well as new production modes in cascade decays of heavier states. Instead of the standard signature, each leptoquark can result in as many as four j…
In this talk I summarize the capability of future lepton colliders to discover leptoquarks and to determine their electroweak quantum numbers. This analysis is an updated discussion based on the results presented in the Snowmass 1996 New Phenomena Working Group report as well as some more recent work that has appeared in the literature as a result of the HERA high-$Q^2$ excess.
Leptoquarks can be produced in substantial numbers for masses very close to the collider centre of mass energy in $e^+e^-$, $eγ$, and $μ^+μ^-$ collisions due to the quark content of the photon resulting in equivalently high discovery limits. Using polarization asymmetries in an $eγ$ collider the ten different types of leptoquarks listed by Buchmüller, Rückl and Wyler can be distinquished from one another for leptoquark masses essentially up to the kinematic limit. Thus, if a leptoquark were discovered, an $eγ$ collider could play a crucial role in determining its origins.
Leptoquarks have been suggested to solve a variety of discrepancies between the expected and observed phenomenon. In this paper, we investigate the effect of scalar leptoquarks on the lepton flavor violating B meson rare decays which involve the quark level transition $\bar{b} \to \bar{s} \ell^-_i \ell^+_j(i \neq j)$. The leptoquark parameter space is constrained by using the recently measured upper limits on the $\mathcal{B}(B_s^0 \to \ell^-_i \ell^+_j)$ and $\mathcal{B}(B \to K^{(*)}\ell^-_i \ell^+_j)$. Using such constrained leptoquark parameter space, some relevant physical quantities are …
The low-energy manifestations of a minimal extension of the electroweak standard model based on the quark-lepton symmetry $SU(4)_V \otimes SU(2)_L \otimes G_R$ of the Pati--Salam type are analyzed. Given this symmetry the third type of mixing in the interactions of the $SU(4)_V$ leptoquarks with quarks and leptons is shown to be required. An additional arbitrariness of the mixing parameters could allow, in principle, to decrease noticeably the lower bound on the vector leptoquark mass originated from the low-energy rare processes, strongly suppressed in the standard model.
Probing muon anomaly and lepton flavor violation with scalar leptoquarks in the 331LHN model
Institute for Advanced Study in Technology, Ton Duc Thang University（隆德大学先进技术研究学院）；Faculty of Applied Sciences, Ton Duc Thang University（隆德大学应用科学学院）；Institute of Physics, Vietnam Academy of Science and Technology（越南科学技术研究院物理研究所）；Hanoi Padegogical 2（河内第二师范大学）；Phenikaa Institute for Advanced Study, Phenikaa University（仙川大学先进研究学院）
The weak dipole moments of elementary fermions are calculated at the one-loop level in the framework of a renormalizable scalar leptoquark model that forbids baryon number violating processes and so is free from the strong constraints from experimental data. In this model there are two scalar leptoquarks accommodated in an $SU_L(2)\times U_Y(1)$ doublet: one of such leptoquarks is non-chiral and has electric charge of $5/3e$, whereas the other one is chiral and has electric charge $2/3e$. In particular, a non-chiral leptoquark contributes to the weak properties of an up fermion via a chirality…
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.…
Models where the baryon (B) and lepton (L) numbers are local gauge symmetries that are spontaneously broken at a low scale are revisited. We find new extensions of the Standard Model which predict the existence of fermions that carry both baryon and lepton numbers (i.e., leptoquarks). The local baryonic and leptonic symmetries can be broken at a scale close to the electroweak scale and we do not need to postulate the existence of a large desert to satisfy the experimental constraints on baryon number violating processes like proton decay.
Motivated by the disagreement between experimental and lattice QCD results on the D_s decay constant we systematically reinvestigate role of leptoquarks in charm meson decays. We consider scalar leptoquarks that transform as a weak interaction triplet, doublet, or singlet in a model independent approach, and also argue that in a particular SU(5) GUT model these leptoquark states, contained in the 45-dimensional Higgs representation, could be safe against proton decay bounds. Using the current experimental measurements in tau, kaon and charm sectors, we find that scalar leptoquarks cannot natur…
The anomalies in semileptonic $B$-decays are often attributed to new physics scenarios featuring leptoquark degrees of freedom. Attempts to accommodate all the deviations at once usually result in an elaborate model building way beyond the minimal scenarios containing the desired degrees of freedom. However, as it is far from clear whether all these signals (if any) survives the future experimental scrutiny, in this contribution we decided to take the opposite standpoint and review the actual room available for various $B$-decay anomalies within the very minimal low-energy leptoquark gauge mod…
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 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…
Leptoquarks are prime candidates for explaining the intriguing hints for lepton flavour universality violation. In particular, the $SU(2)_L$ doublet of scalar leptoquarks $S_2$ is capable of providing an explanation for the tensions between the measurements and the Standard Model predictions in $(g-2)_μ$, $b\to s\ell^+ \ell^-$ and $b\to cτν$ processes, as well as in non-resonant di-electron production. However, in the minimal setup with a single leptoquark generation, a common explanation for all these issues is not possible as this would lead to unacceptably large charged lepton flavour viola…
We perform a model independent analysis so as to constrain the leptoquark (LQ) models from negative searches for $μ\to e γ$, $μ\to 3e$ decays (and analogous processes in the $τ$ sector), and coherent $μ-e$ conversion in nuclei. We considerably improve some constraints obtained by analyses known in the literature, analyses which we show have by far underestimated the LQ contributions to the $μ\to 3e$. In particular we find that the coherent $μ-e$ conversion in nuclei mediated by the photon--conversion mechanism and the $μ\to 3e$ decay are golden plates where the flavor changing leptoquark coupl…
Purely leptonic $B$ meson decays provide unique probes for physics Beyond the Standard Model. We study the impact of a scalar leptoquark, $S_1$, on $B \to μ\barν$ decay. We find that, for $m_{S_1}\sim 1$ TeV, the $S_1$ leptoquark can modify the $B \to μ\barν$ rate significantly. Such a leptoquark can in prinicple also alter the $B \to τ\barν$ rate. However, current searches from LHC and low energy physics provide some constraints on the parameter space.
