The fate of $V_1$ vector leptoquarks: the impact of future flavour data
C. Hati, J. Kriewald, J. Orloff, A. M. Teixeira
备注：
42 pages, 7 figures, v3: takes into account updated $b\to s\ell\ell$ measurements, several improvements in the numerical analysis; conclusions unchanged, matches version published in EPJC
Motivated by the recent experimental progress on the $B$-meson decay anomalies (in particular the angular observables in $B\to K^\astμμ$), we rely on a simplified-model approach to study the prospects of vector leptoquarks in what concerns numerous flavour observables, identifying several promising decay modes which would allow to (indirectly) probe such an extension. Our findings suggest that the confirmation of the $B$-meson decay anomalies, in parallel with positive signals (at Belle II or LHCb) for $τ\to ϕμ$, $B_{(s)}$-meson decays to $τ^+ τ^-$ and $τ^+ μ^-$ ($τ^+ e^-$) final states, as well as an observation of certain charged lepton flavour violation decays (at COMET or Mu2e), would contribute to strengthen the case for this scenario. We also illustrate how the evolution of the experimental determination of $R_{D^{(*)}}$ could be instrumental in falsifying an explanation of the anomalous $B$-meson decay data via a vector $V_1$ leptoquark.
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 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…
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 comprehensively examine precision predictions for scalar leptoquark pair production at the LHC. In particular, we investigate the impact of lepton $t$-channel exchange diagrams that are potentially relevant in the context of leptoquark scenarios providing an explanation for the flavour anomalies. We also evaluate the corresponding total rates at the next-to-leading order in QCD. Moreover, we complement this calculation with the resummation of soft-gluon radiation at the next-to-next-to-leading logarithmic accuracy, hence providing the most precise predictions for leptoquark pair production …
In view of the emerging hints for the violation of lepton flavour universality in several $B$-meson decays, we conduct a model-independent study (effective field theory approach) of several well-motivated new physics scenarios. Taking into account the most recent LHCb data, we provide updates to Wilson coefficient fits for numerous popular new physics hypotheses. We also consider a promising model of vector leptoquarks, which in addition to explaining the $B$-meson decay anomalies ($R_{K^{(*)}}$ and $R_{D^{(*)}}$) would have an extensive impact for numerous flavour observables. We identify pro…
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 …
Given the hints of lepton-flavour non-universality in semi-leptonic $B$ decays, leptoquark (LQ) models with sizeable couplings to heavy-quark flavours are enjoying a renaissance. While such models are subject to stringent constraints from low-energy experiments also bounds from non-resonant dilepton searches at the Large Hadron Collider (LHC) turn out to be phenomenologically relevant. Based on the latest LHC dilepton analyses corresponding to an integrated luminosity of around $140 \, {\rm fb}^{-1}$ of proton-proton collisions at $\sqrt{s} = 13 \, {\rm TeV}$, we present improved limits on the…
We explore a novel mechanism to restrict TeV-scale leptoquark interactions and render the proton exactly stable to all orders in the effective field theory expansion. A scalar condensate breaks a lepton-flavoured $\mathrm{U}(1)_X$ gauge symmetry in the ultraviolet and generates neutrino masses, leaving a discrete $\mathbb{Z}_9$ or $\mathbb{Z}_{18}$ gauge symmetry in the infrared, forbidding $ΔB = 1$ processes. This provides an elegant framework to address the flavour anomalies and can be adapted to many other new-physics models. The $ \mathrm{U}(1)_X $ can emerge from a gauge-flavour unified $…
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 vector leptoquark representation, $U_μ= (3,1,2/3)$, was recently identified as an exceptional single mediator model to address experimental hints on lepton flavour universality violation in semileptonic $B$-meson decays, both in neutral ($b \to s μμ$) and charged ($b \to c τν$) current processes. Nonetheless, it is well-known that massive vectors crave an ultraviolet (UV) completion. We present the first full-fledged UV complete and calculable gauge model which incorporates this scenario while remaining in agreement with all other indirect flavour and electroweak precision measurements, as…
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 …
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 construct an extension of the Standard Model with a scalar leptoquark $ϕ\sim (3,1,-\tfrac13)$ and the discrete flavour symmetry $G_f=D_{17}\times Z_{17}$ to explain anomalies observed in charged-current semi-leptonic $B$ meson decays and in the muon anomalous magnetic moment, together with the charged fermion masses and quark mixing. The symmetry $Z_{17}^{\rm diag}$, contained in $G_f$, remains preserved by the leptoquark couplings, at leading order, and efficiently suppresses couplings of the leptoquark to the first generation of quarks and/or electrons, thus avoiding many stringent experi…
In this talk, we discuss a simplified version of a recently proposed twin Pati-Salam (PS) theory of flavour broken to the $G_{4321}$ gauge group at high energies, then to the Standard Model at low energies, yielding a TeV scale vector leptoquark $U^μ_1(3,1,2/3)$ which may be used to address the lepton universality anomalies $R_{K^{(*)}}$ and $R_{D^{(*)}}$ in $B$ decays. The model also accounts for quark and lepton mass hierarchies, providing a link between the flavour anomalies and a theory of flavour.
While the LHC has not directly observed any new particle so far, experimental results from LHCb, BELLE and BABAR point towards the violation of lepton flavour universality in $b\to s\ell^{+}\ell^{-}$ and $b\to c\ellν$. In this context, also the discrepancy in the anomalous magnetic moment of the muon can be interpreted as a sign of lepton flavour universality violation. Here we discuss how these hints for new physics can also be explained by introducing leptoquarks as an extension of the Standard Model. Indeed, leptoquarks are good candidates to explain the anomaly in the anomalous magnetic mo…
We study the implications of single leptoquark extensions of the Standard Model (SM) under the assumption that their enhanced Yukawa sectors are invariant under global Abelian flavour symmetries already present in SM mass terms. Such symmetries, assumed to be the `residual' subgroups of an ultra-violet flavour theory, have previously been considered in order to predict fermionic mixing angles. Here we focus instead on their effect on the novel flavour structures sourced by the leptoquark representations that address the present ${R}_{K^{(\star)}}$ anomalies in semileptonic rare $B$-decays. Com…
We investigate the viability of extending the Standard Model with $S_1$ and $S_3$ scalar leptoquarks when the flavour structure is parametrized in terms of Froggatt-Nielsen charges. In contrast to a similar analysis with a vector leptoquark, we find essentially two solutions for the charges that fit the experimental constraints, which are dominated by the current tensions in $B$ decays. These two scenarios differ in their estimate of the anomalous magnetic moment of the muon, $(g-2)$, but they both predict sizeable contributions to $τ\toμγ$, $\bar B_s\toτ^\pmμ^\mp$ and $B^+\to K^+τ^+μ^-$ decay…
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 propose a twin Pati-Salam (PS) theory of flavour broken to the $G_{4321}$ gauge group at high energies, then to the Standard Model at low energies, yielding a TeV scale vector leptoquark $U^μ_1(3,1,2/3)$ which has been suggested to address the lepton universality anomalies $R_{K^{(*)}}$ and $R_{D^{(*)}}$ in $B$ decays. Quark and lepton masses are mediated by vector-like fermions, with personal Higgs doublets for the second and third families, which may be replaced by a two Higgs doublet model (2HDM). The twin PS theory of flavour successfully accounts for all quark and lepton (including neu…
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 employ a bottom-up and model-independent technique to search for non-Abelian discrete flavour symmetries capable of predicting viable CKM and PMNS matrices alongside of special patterns of leptoquark couplings. In particular, we analyze patterns derived when an ultra-violet flavour theory is assumed to break to global Abelian symmetries in Standard Model fermion masses and new Yukawa-like terms sourced by the leptoquark representation. The phenomenology of different classes of these `simplified models' can be explored without reference to explicit model-building assumptions, e.g. the nature…
The LHCb Collaboration recently gave an update on testing lepton flavour universality with $B^+ \rightarrow K^+ \ell^+ \ell^-$, in which a 3.1 standard deviations from the standard model prediction was observed. The g-2 experiment also reports a 3.3 standard deviations from the standard model on muon anomalous magnetic moment measurement. These deviations could be explained by introducing new particles including leptoquarks. In this paper, we show the possibility to search for heavy spin-1 leptoquarks at a future TeV scale muon collider by performing studies from three channels: 1) same flavou…
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 phenomenological signatures of Simplified Models of Flavourful Leptoquarks, whose Beyond-the-Standard Model (SM) couplings to fermion generations occur via textures that are well motivated from a broad class of ultraviolet flavour models (which we briefly review). We place particular emphasis on the study of the vector leptoquark $Δ_μ$ with assignments $\left({\bf{3}}, {\bf{1}}, 2/3 \right)$ under the SM's gauge symmetry, $SU(3)_C \times SU(2)_L \times U(1)_Y$, which has the tantalising possibility of explaining both $\mathcal{R}_{K^{(\star)}}$ and $\mathcal{R}_{D^{(\star)}}$ a…
Extending the Standard Model with higher-dimensional operators in an effective-field-theory (EFT) approach provides a systematic framework to study new-physics (NP) effects from a bottom-up perspective, as long as the NP scale is sufficiently large compared to the energies probed in the experimental observables. However, when taking into account the different quark and lepton flavours, the number of free parameters increases dramatically, which makes generic studies of the NP flavour structure infeasible. In this paper, we address this issue in view of the recently observed "flavour anomalies"…
The Leptoquark model has been instrumental in explaining the observed lepton flavour universality violating charged ($b\to c$) and neutral ($b\to s$) current anomalies that have been the cause for substantial excitement in particle physics recently. In this article we have studied the role of one (designated as $V_2^{\frac 43}$) of the components of {\boldmath${V}_2$} Vector Leptoquark doublet with electromagnetic charge $\frac 43$ in explaining the neutral current ($b\to s$) anomalies $R_{K^{(*)}}$ and $B_s\toμ^+μ^-$. Moreover, we have performed a thorough collider search for this $V_2^{\frac…
We analyse scalar leptoquark pair production at the LHC with predictions including $t$-channel lepton exchange contributions up to next-to-leading order (NLO) in QCD. In particular, we calculate NLO-QCD predictions for off-diagonal production channels, {\it i.e.}\ channels that involve two different leptoquark eigenstates and are driven solely by diagrams involving Standard Model leptons in the $t$-channel at leading order, as opposed to diagonal channels where a pair of the same leptoquark eigenstate is produced. We find that reliable theoretical predictions for both channels require NLO accu…
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…
We present a detailed analysis of the collider signatures of TeV-scale massive vector bosons motivated by the hints of lepton flavour non-universality observed in $B$-meson decays. We analyse three representations that necessarily appear together in a large class of ultraviolet-complete models: a colour-singlet ($Z'$), a colour-triplet (the $U_1$ leptoquark), and a colour octet ($G'$). Under general assumptions for the interactions of these exotic states with Standard Model fields, including in particular possible right-handed and flavour off-diagonal couplings for the $U_1$, we derive a serie…
We introduce two scalar leptoquarks, the SU$(2)_L$ isosinglet denoted $ϕ\sim(\mathbf{3}, \mathbf{1}, -1/3)$ and the isotriplet $φ\sim(\mathbf{3}, \mathbf{3}, -1/3)$, to explain observed deviations from the standard model in semi-leptonic $B$-meson decays. We explore the regions of parameter space in which this model accommodates the persistent tensions in the decay observables $R_{D^{(*)}}$, $R_{K^{(*)}}$, and angular observables in $b\to s μμ$ transitions. Additionally, we exploit the role of these exotics in existing models for one-loop neutrino mass generation derived from $ΔL=2$ effective …
Motivated by the recent experimental progress on the $R_{D^{(\ast)}}$ and $R_{K^{(\ast)}}$ anomalies in $B$-decays, we consider an extension of the Standard Model by a single vector leptoquark field. We study how one can achieve the required lepton flavour non-universality, starting from a priori universal gauge couplings. While the unitary coupling flavour structure, induced by the mass misalignment of quarks and leptons after $SU(2)_L$ breaking, does not allow to comply with stringent bounds from lepton flavour violating processes, we find that effectively non-unitary couplings, due to mixin…
We study the scalar leptoquark effects on the rare semileptonic decays of $Λ_b$ baryon, governed by the quark level transition $b \to s l^+ l^-$. We estimate the branching ratios, forward-backward asymmetries, lepton polarization parameters and the lepton flavour non-universality effects in these decay channels. We find significant deviations from the corresponding standard model predictions in some of the observables due to leptoquark effects. We also investigate the lepton flavour violating decays $Λ_b \to Λl_i^- l_j^+$, the branching ratios of which are found to be ${\cal O}(10^{-10} - 10^{…
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…
We discuss a theory of flavour in which Higgs Yukawa couplings are related to those of the new scalar triplet leptoquark and/or $Z'$ responsible for $R_{K^{(*)}}$, with all couplings arising effectively from mixing with a vector-like fourth family, whose mass may be anywhere from the Planck scale to the electroweak scale for the leptoquarks explanation, but is pinned down to the TeV scale if the $Z'$ exchange plays a role. However, in this particular model, only leptoquark exchange can contribute significantly to $R_{K^{(*)}}$, since $Z'$ exchange is too constrained from $B_s$ mixing and $τ\ri…
Motivated by the recent measurement of the lepton flavour nonuniversality ratio $R_{K^*}$ by the LHCb Collaboration, we study the implications of vector leptoquarks on the observed anomalies associated with the $\bar B \to \bar K^* l^+ l^-$ decay processes. The leptoquark couplings are constrained from the measured branching ratios of $B_s \to l^+ l^-$, $K_L \to l^+ l^-$ and $B_s \to μ^\mp e^\pm$ processes. Using these constrained couplings, we estimate the branching ratios, forward-backward and lepton polarization asymmetries and also the form factor independent optimized observables ($P_{i}^…
In order to simultaneously account for both $R_{D^{(\ast)}}$ and $R_{K^{(\ast)}}$ anomalies in $B$-decays, we consider an extension of the Standard Model by a single vector leptoquark field, and study how one can achieve the required lepton flavour non-universality, starting from a priori universal gauge couplings. While the unitary quark-lepton mixing induced by $SU(2)_L$ breaking is insufficient, we find that effectively nonunitary mixings hold the key to simultaneously address the $R_{K^{(\ast)}}$ and $R_{D^{(\ast)}}$ anomalies. As an intermediate step towards various UV-complete models, we…
Motivated by an explanation of the $R_{K^{(*)}}$ anomalies, we propose a Standard Model extension via two scalar SU(2)$_L$ triplet leptoquarks and three generations of triplet Majorana fermions. The gauge group is reinforced by a $Z_2$ symmetry, ensuring the stability of the lightest $Z_2$-odd particle, which is a potentially viable dark matter candidate. Neutrino mass generation occurs radiatively (at the three-loop level), and leads to important constraints on the leptoquark couplings to leptons. We consider very generic textures for the flavour structure of the $h_1$ leptoquark Yukawa coupl…
We implement scalar and vector leptoquark (LQ) models in the universal FeynRules output (UFO) format assuming the Standard Model fermion content and conservation of baryon and lepton numbers. Scalar LQ implementations include next-to-leading order (NLO) QCD corrections. We report the NLO QCD inclusive cross sections in proton-proton collisions at 13 TeV, 14 TeV, and 27 TeV for all on-shell LQ production processes. These comprise (i) LQ pair production ($p p \to ΦΦ$) and (ii) single LQ + lepton production ($p p \to Φ\ell$) for all initial quark flavours ($u$, $d$, $s$, $c$, and $b$). Vector LQ …
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.
Given the hints of lepton-flavour non-universality in $B$-meson decays, leptoquarks (LQs) are enjoying a renaissance. We propose novel Large Hadron Collider (LHC) searches for such hypothetical states that do not rely on strong production only, but can also receive important contributions from quark-lepton annihilation. For the cases of a resonant signal involving a bottom quark and a tau lepton ($b + τ$), a top quark and missing transverse energy ($E_{T}^{\rm miss}$) and light-flavour jets plus $E_{T}^{\rm miss}$, we develop realistic analysis strategies and provide detailed evaluations of th…
LHCb found hints for physics beyond the Standard Model (SM) in $B\to K^*μ^+μ^-$, $R(K)$ and $B_s\toϕμ^+μ^-$. These intriguing hints for NP have recently been confirmed by the LHCb measurement of $R(K^*)$ giving a combined significance for NP above the $5\,σ$ level. In addition, the BABAR, BELLE and LHCb results for $B\to D^{(*)}τν$ also point towards lepton flavour universality (LFU) violating new physics (NP). Furthermore, there is the long-standing discrepancy between the measurement and the theory prediction of the anomalous magnetic moment of the muon ($a_μ$) at the $3\,σ$ level. Concernin…
If electroweak symmetry breaking arises via strongly-coupled physics, the observed suppression of flavour-changing processes suggests that fermion masses should arise via mixing of elementary fermions with composite fermions of the strong sector. The strong sector then carries colour charge, and may contain composite leptoquark states, arising either as TeV scale resonances, or even as light, pseudo-Nambu-Goldstone bosons. The latter, since they are coupled to colour, get a mass of the order of several hundred GeV, beyond the reach of current searches at the Tevatron. The same generic mechanis…
The $U_1$ leptoquark is known to be a suitable candidate for explaining the semileptonic $B$-decay anomalies. We derive precise limits on its parameter space relevant for the anomalies from the current LHC high-$p_{\rm T}$ dilepton data. We consider an exhaustive list of possible $B$-anomalies-motivated simple scenarios with one or two new couplings that can also be used as templates for obtaining bounds on more complicated scenarios. To obtain precise limits, we systematically consider all possible $U_1$ production processes that can contribute to the dilepton searches, including the resonant…
Leptoquarks provide viable solutions to the flavour anomalies, i.e. they can explain the tensions between the measurements and the Standard Model predictions of the anomalous magnetic moment of the muon as well as $b\to s\ell^+\ell^-$ and $b\to cτν$ processes. However, leptoquarks also contribute to other flavour observables, such as $ΔF=2$ processes, at the loop-level. In particular, $B_s-\bar B_s$ mixing provides a crucial bound in setups addressing $b\to cτν$ data, often excluding a big portion of the parameter space that could otherwise account for it. In this article, we first derive the …
In this manuscript, based on arXiv:2209.00276, I present a model which can simultaneously explain and connect the flavour hierarchies of the Standard Model with flavour anomalies in $B$-physics. I will briefly introduce the model and highlight the main features, including a common origin of Yukawa couplings and vector leptoquark $U_{1}$ couplings to Standard Model fermions. A GIM-like mechanism allows for large leptoquark couplings which can explain the $B$-anomalies, while protecting from the most dangerous FCNCs that could be mediated by a heavy coloron and $Z'$. Finally, I will highlight so…
With the recent experimental hint of new physics from flavor physics anomalies, combined with the evidence from neutrino mass and dark matter, we consider a minimal extension of SM with a scalar leptoquark and a fermion triplet. The scalar leptoquark with couplings to leptons and quarks can explain lepton flavor non-universality observables $R_K$, $R_{K^{(*)}}$, $R_{D^{(*)}}$ and $R_{J/ψ}$. Neutral component of fermion triplet provides current abundance of dark matter in the Universe. The interesting feature of the proposal is that the minimal addition of these phenomenologically rich particle…
We explore a new mechanism for reproducing the Dark Matter (DM) abundance: scatterings of one DM particle on light Standard Model particles. Strong bounds on its decays can be satisfied if DM undergoes freeze-in and has a mass around or below the pion mass. This happens, for example, in theories with a right-handed neutrino interacting with charged fermions through a leptoquark exchange. These leptoquarks can be linked to the ones motivated by the B-physics anomalies if assumptions about the flavour structure are made. DM signals are unusual, with interesting possibilities for direct and indir…
We investigate Majorana dark matter in a new variant of $U(1)_{L_μ-L_τ}$ gauge extension of Standard Model, containing three additional neutral fermions $N_{e}, N_μ, N_τ$, along with a $(\bar{3},1,1/3)$ scalar Leptoquark (SLQ) and an inert doublet, to study the phenomenology of dark matter, neutrino mass generation and flavour anomalies on a single platform. The lightest mass eigenstate of the $N_μ, N_τ$ neutral fermions plays the role of dark matter. We compute the WIMP-nucleon cross section in leptoquark portal and the relic density mediated by inert doublet components, leptoquark and the ne…
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 …
We study limits from dilepton searches on leptoquark completions to the Standard Model in the parameter space motivated by anomalies in the $b \rightarrow s$ sector. After a full Run-2 analysis by LHCb, the disparity in lepton flavour violation has disappeared. However, the mismatch in angular distributions as well as in $B_s \rightarrow μ^+ μ^-$ partial width is still unresolved and still implies a possible new physics contribution. We probe three models of leptoquarks -- scalar models $S_3$ and $R_2$ as well as vector leptoquark model $U_1$ using non-resonant dilepton searches to place limit…
We attempt to explain recent anomalies in semileptonic $B$ decays at LHCb via a composite Higgs model, in which both the Higgs and an $SU(2)_L$-triplet leptoquark arise as pseudo-Goldstone bosons of the strong dynamics. Fermion masses are assumed to be generated via the mechanism of partial compositeness, which largely determines the leptoquark couplings and implies non-universal lepton interactions. The latter are needed to accommodate tensions in the $b \to s μμ$ dataset and to be consistent with a discrepancy measured at LHCb in the ratio of $B^+ \to K^+ μ^+ μ^-$ to $B^+ \to K^+ e^+ e^-$ br…
Discrepancies between recent experimental results and their respective Standard Model predictions, known as flavour anomalies, are reported in semileptonic charged and neutral-current $B$-decays, the muon magnetic moment $(g-2)_μ$, and the extraction of the Cabibbo angle. In this proceedings, we review two New Physics models that introduce two scalar mediators at the TeV scale and aim at a combined explanation of the flavour anomalies. The first model features the leptoquarks $S_1$ and $S_3$ and provides tree level solutions to both $B$-anomalies and one-loop level solution to the anomalous $(…
We study some rare decays of $B$ meson involving the quark level transition $b \to q l^+l^- (q=d,s)$ in the scalar leptoquark model. We constrain the leptoquark parameter space using the recently measured branching ratios of $B_{s,d} \to μ^+ μ^-$ processes. Using such parameters, we obtain the branching ratios, direct CP violation parameters and isospin asymmetries in $B \to K μ^+ μ^-$ and $B \to πμ^+ μ^-$ processes. We also obtain the branching ratios for some lepton flavour violating decays $B \to l_i^+ l_j^-$. We find that the various anomalies associated with the isospin asymmetries of $B …
While the $S_3$ scalar leptoquark presents a possible tree-level explanation of the $b \to s \ell \ell$ flavour anomalies, it suffers from two conceptual problems which are often disregarded by model-builders. Firstly, the quantum numbers of the $S_3$ allow for a renormalisable diquark operator that would trigger rapid proton decay unless its coupling were tuned away. Secondly, one expects the leptoquark to have generic couplings to leptons, which require tuning to avoid stringent experimental bounds on lepton flavour violation. By gauging a $U(1)$ current that acts as $L_μ- L_τ$ on the Standa…
We demonstrate that in a recently proposed unified leptoquark model based on the gauge group $SU(4)_C\times SU(2)_L \times U(1)_R$ significant deviations from the Standard Model values of $R_K$ and $R_{K^*}$ can be accommodated without any need of extra heavy fermions. Low energy data, in particular lepton flavour violating $μ$ decays and $K_L\to e μ$, severely constrain the available parameter space. We show that in the allowed part of the parameter space (i) some of the lepton-flavour-violating tau decay branching ratios are predicted to be close to their current experimental limits. (ii) Th…
