Scalar Leptoquarks Research Articles

Search for new phenomena in pp collisions in final states with tau leptons, b -jets, and missing transverse momentum with the ATLAS detector

A search for new phenomena in final states with hadronically decaying tau leptons, $b$-jets, and missing transverse momentum is presented. The analyzed dataset comprises $pp$~collision data at a center-of-mass energy of $\sqrt s = 13$ TeV with an integrated luminosity of 139/fb, delivered by the Large Hadron Collider and recorded with the ATLAS detector from 2015 to 2018. The observed data are compatible with the expected Standard Model background. The results are interpreted in simplified models for two different scenarios. The first model is based on supersymmetry and considers pair production of top squarks, each of which decays into a $b$-quark, a neutrino and a tau slepton. Each tau slepton in turn decays into a tau lepton and a nearly massless gravitino. Within this model, top-squark masses up to 1.4 TeV can be excluded at the 95% confidence level over a wide range of tau-slepton masses. The second model considers pair production of leptoquarks with decays into third-generation leptons and quarks. Depending on the branching fraction into charged leptons, leptoquarks with masses up to around 1.25 TeV can be excluded at the 95% confidence level for the case of scalar leptoquarks and up to 1.8 TeV (1.5 TeV) for vector leptoquarks in a Yang--Mills (minimal-coupling) scenario. In addition, model-independent upper limits are set on the cross section of processes beyond the Standard Model.

Lepton flavor universality and CP violation in an S3 leptoquark model

We introduce the $CP$ violating scalar leptoquark ${S}_{3}$ to explain the measured values of the lepton universality ratios ${R}_{{K}^{(*)}}$. We derive constraints on the $CP$-even and $CP$-odd components of the leptoquark Yukawa couplings stemming from effects in $b\ensuremath{\rightarrow}s\ensuremath{\mu}\ensuremath{\mu}$ and ${B}_{s}$ mixing. For the $b\ensuremath{\rightarrow}s\ensuremath{\mu}\ensuremath{\mu}$ processes we impose ${R}_{{K}^{(*)}}$, $\mathcal{B}({B}_{s}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}})$, as well as $CP$-sensitive angular asymmetries ${A}_{7,8,9}$, whereas in the ${B}_{s}$ mixing sector $\mathrm{\ensuremath{\Delta}}{M}_{s}$ and ${S}_{\ensuremath{\psi}\ensuremath{\phi}}$ are considered. Combining the constraints within the ${S}_{3}$ model reveals that a large $CP$ phase with a definite sign is perfectly viable for a leptoquark of mass below a few TeV. For larger mass of the ${S}_{3}$ leptoquark the $CP$ phase is suppressed due to the observables pertaining to the ${B}_{s}$ system. We provide predictions of direct and mixing-induced $CP$ asymmetries in $B\ensuremath{\rightarrow}K\ensuremath{\mu}\ensuremath{\mu}$ that could reveal the presence of the novel $CP$ phase.

Neutrino mass models by the 1000s!

After putting the problem of neutrino mass generation in the context of physics beyond the standard model as a whole, I specialise to models of radiative Majorana neutrino masses and mixings. I spend most of the time summarising the results of a systematic classification and model-construction scheme based on opening up gauge-invariant ΔL = 2 effective operators constructed out of standard model fields. The focus is on tree-level openings of all operators up to mass dimension 11 using only exotic fields for the virtual states in the UV. This process produces renormalisable models that furnish mostly loop-level - that is to say radiative - Majorana neutrino mass models. I reference a searchable database authored by Gargalionis, and describe some of the general features of this large family of about 11,000 models. We will see that exotics such as scalar leptoquarks abound, and often lead to models of neutrino mass that can also address some or all of the current B-decay anomalies such R K(*) and R D (*) and the (g – 2) e,μ discrepancies.

Universal scalar leptoquark action for matching

In this study we present a universal effective action for one-loop matching of all scalar leptoquarks. We use both the Universal One-Loop Effective Action (UOLEA) and covariant diagrams to evaluate the Wilson coefficients directly in the Green basis for up to dimension-6 operators. On the technical side, we use the newly developed method of evaluating supertraces, to further validate the results stemming from the use of covariant diagrams. As an application, we perform a fully functional matching onto Standard Model Effective Field Theory (SMEFT) of a model with two scalar leptoquark fields: a weak isospin singlet and a doublet. We demonstrate its use by calculating several observables, such as lepton magnetic and electric dipole moments, neutrino masses, proton decay rate, while we comment upon fine tuning issues in this model. Apart from its phenomenological interest, this model generates the majority of dimension-6 operators and provides an EFT benchmark towards future matching automation.

Search for new particles in events with energetic jets and large missing transverse momentum in proton-proton collisions at sqrt{s} = 13 TeV

A search is presented for new particles produced at the LHC in proton-proton collisions at sqrt{s} = 13 TeV, using events with energetic jets and large missing transverse momentum. The analysis is based on a data sample corresponding to an integrated luminosity of 101 fb−1, collected in 2017–2018 with the CMS detector. Machine learning techniques are used to define separate categories for events with narrow jets from initial-state radiation and events with large-radius jets consistent with a hadronic decay of a W or Z boson. A statistical combination is made with an earlier search based on a data sample of 36 fb−1, collected in 2016. No significant excess of events is observed with respect to the standard model background expectation determined from control samples in data. The results are interpreted in terms of limits on the branching fraction of an invisible decay of the Higgs boson, as well as constraints on simplified models of dark matter, on first-generation scalar leptoquarks decaying to quarks and neutrinos, and on models with large extra dimensions. Several of the new limits, specifically for spin-1 dark matter mediators, pseudoscalar mediators, colored mediators, and leptoquarks, are the most restrictive to date.

Improving third-generation leptoquark searches with combined signals and boosted top quarks

Search strategies for the third-generation leptoquarks (LQs) are distinct from other LQ searches, especially when they decay to a top quark and a $\tau$ lepton. We investigate the cases of all TeV-scale scalar and vector LQs that decay to either a top-tau pair (charge-$1/3$ and $5/3$ LQs) or a top-neutrino pair (charge-$2/3$ LQs). One can then use the boosted top (which can be tagged efficiently using jet-substructure techniques) and high-$p_{\rm T}$ $\tau$ leptons to search for these LQs. We consider two search channels with either one or two taus along with at least one hadronically decaying boosted top quark. We estimate the high luminosity LHC (HL-LHC) search prospects of these LQs by considering both symmetric and asymmetric pair and single production processes. Our selection criteria are optimised to retain events from both pair and single production processes. The combined signal has better prospects than the traditional searches. We include new three-body single production processes to enhance the single production contributions to the combined signal. We identify the interference effect that appears in the dominant single production channel of charge-$1/3$ scalar LQ ($S^{1/3}$). This interference is constructive if $S^{1/3}$ is weak-triplet and destructive, for a singlet one. As a result, their LHC prospects differ appreciably.

Complete Lagrangian and set of Feynman rules for scalar leptoquarks

Leptoquarks (LQs) have attracted increasing attention within recent years, mainly since they can explain the flavor anomalies found in R(D(⁎)), b→sℓ+ℓ− transitions and the anomalous magnetic moment of the muon. In this article, we lay the groundwork for further automated analyses by presenting the complete Lagrangian and the corresponding set of Feynman rules for scalar leptoquarks. This means we consider the five representations Φ1,Φ1˜,Φ2,Φ2˜ and Φ3 and include the triple and quartic self-interactions, as well as couplings to the Standard Model (SM) fermions, gauge bosons and the Higgs. The calculations are performed using FeynRules and all model files are publicly available online at https://gitlab.com/lucschnell/SLQrules. Program summaryProgram Title: SLQrulesCPC Library link to program files:https://doi.org/10.17632/sf8s89rhmv.1Developer's repository link:https://gitlab.com/lucschnell/SLQrulesLicensing provisions: CC By 4.0Programming language: Mathematica, FeynRulesNature of problem: In order to explain the deviations from SM predictions in R(D(⁎)), b→ℓ+ℓ− transitions and the muon AMM jointly, models involving multiple LQ representations are necessary. This significantly increases the number of possible interactions and creates the need for computational tools that allow for studying the phenomenology of these LQ models in an automated manner. While model files exist for each LQ representation individually [1], we are not aware of any publicly available model files that combine all scalar LQ representations as well as their self-interactions.Solution method: We implemented the complete scalar LQ Lagrangian in a FeynRules [2] model file and provide the corresponding MOD and UFO model files. These can be imported directly in FeynArts [3] and MadGraph [4] to obtain a versatile toolbox for the study of scalar LQs.

Single leptoquark solutions to the B -physics anomalies

We revisit the possibilities of accommodating the experimental indications of the lepton flavor universality violation in $b$-hadron decays in the minimal scenarios in which the Standard Model is extended by the presence of a single $\mathcal{O}(1\,\mathrm{TeV})$ leptoquark state. To do so we combine the most recent low energy flavor physics constraints, including $R_{K^{(\ast)}}^\mathrm{exp}$ and $R_{D^{(\ast)}}^\mathrm{exp}$, and combine them with the bounds on the leptoquark masses and their couplings to quarks and leptons as inferred from the direct searches at the LHC and the studies of the large $p_T$ tails of the $pp\to \ell\ell$ differential cross section. We find that none of the scalar leptoquarks of $m_\mathrm{LQ} \simeq 1\div 2$ TeV can accommodate the $B$-anomalies alone. Only the vector leptoquark, known as $U_1$, can provide a viable solution which, in the minimal setup, provides an interesting prediction, i.e. a lower bound to the lepton flavor violating $b\to s\mu^\pm\tau^\mp$ decay modes, such as $\mathcal{B}(B\to K\mu\tau) \gtrsim 0.7\times 10^{-7}$.

One colorful resolution to the neutrino mass generation, three lepton flavor universality anomalies, and the Cabibbo angle anomaly

We propose a simple model to simultaneously explain four observed flavor anomalies while generating the neutrino mass at the one-loop level. Specifically, we address the measured anomalous magnetic dipole moments of the muon, ∆aμ, and electron, ∆ae, the observed anomaly of b → sl+l− in the B-meson decays, and the Cabibbo-angle anomaly. The model consists of four colorful new degrees of freedom: three scalar leptoquarks with the Standard Model quantum numbers (3, 3, −1/3), (3, 2, 1/6), and (3, 1, 2/3), and one pair of down-quark-like vector fermion in (3, 1, −1/3). The baryon number is assumed to be conserved for simplicity.Phenomenologically viable solutions with the minimal number of real parameters can be found to accommodate all the above-mentioned anomalies and produce the approximate, close to 1σ, neutrino oscillation pattern at the same time. From general consideration, the model robustly predicts: (1) neutrino mass is of the normal hierarchy type, and (2) {mathcal{M}}_{ee}^{nu } ≲ 3 × 10−4 MeV.The possible UV origin to explain the flavor pattern of the found viable parameter space is briefly discussed. The parameter space can be well reproduced within a simple split fermion toy model.

Distinguishing Leptoquarks at the LHC/FCC

In this article, we deal with how to distinguish the signatures of different Leptoquarks at the LHC/FCC if all of them lie within similar mass and coupling range and can be produced at present and future colliders. It has been found that hard scattering cross-sections and angular distributions can be used to differentiate scalar and vector Leptoquarks. On the other hand, final state topology and determination of jet charge can separate Leptoquarks with same spin even from same SU(2)L multiplet. We performed a PYTHIA8 based analysis considering all the dominant Standard Model (SM) backgrounds at the LHC/FCC with centre of mass energies of 14, 27 and 100 TeV for scalar (S1) and vector (U˜1μ) Leptoquarks. We see that confirming evidence of scalar Leptoquark at 14 TeV requires 1000 fb−1 of integrated luminosity, whereas the vector Leptoquark can be probed with very early data. But, at 100 TeV with 1000 fb−1 of integrated luminosity, scalar Leptoquark of mass 3.5 TeV and vector Leptoquark of mass more than 5 TeV can be probed easily.

Scalar leptoquarks, baryon number violation, and Pati-Salam symmetry

One or more scalar leptoquarks with masses around a few TeV may provide a solution to some of the flavor anomalies that have been observed. We discuss the impact of such new degrees on baryon number violation when the theory is embedded in a Pati-Salam model. The Pati-Salam embedding can suppress renormalizable and dimension-five baryon number violation in some cases. Our work extends the results of Assad, Grinstein, and Fornal who considered the same issue for vector leptoquarks.

Enhancement of the double Higgs production via leptoquarks at the LHC

Measurements of single Higgs production and its decays are in good agreement with the Standard Model. There is still room for large modifications in double Higgs production at LHC, though these effects may be correlated with large corrections to other observables, in particular single Higgs production. In this work we address the issue of enhancing double Higgs production in the presence of scalar leptoquarks while satisfying all experimental constraints. We show at leading order that including more than one species of leptoquarks, large cubic interactions with the Higgs can lead to sizable enhancement of di-Higgs production cross section at LHC, while at the same time keeping other Higgs observables and precision measurements under control. For masses above 800 GeV these corrections are in general below 30%, whereas in a viable scenario in which one of the leptoquarks can be light, specifically in the mass range 400 − 600 GeV, we show that it is possible to roughly double the SM cross section for di-Higgs production, implying that possible first hints of it may be probed at the high luminosity LHC at mathcal{L} ∼ 2 ab−1.

Leptoquarks and real singlets: A richer scalar sector behind the origin of dark matter

We investigate scenarios with $\mathcal{O}(1\mathrm{\,TeV})$ scalar leptoquarks that act as portals between the Standard Model and Dark Matter. We assume that Dark Matter is a scalar singlet $S$ which couples to a scalar leptoquark $\Delta$ and the Higgs boson via the terms in the scalar potential. In addition, the leptoquark is endowed with Yukawa couplings to quarks and leptons that may address the anomalies in $B$ meson decays. We consider the $SS$ annihilation cross sections to estimate the Dark Matter relic abundance and explore the interplay between astrophysical, collider and flavour physics bounds on such models. In the heavy Dark Matter window, $m_S > m_\Delta$, the leptoquark portal becomes the dominant mechanism to explain the Dark Matter abundance. We find that the leptoquark Yukawa couplings, relevant for quark and lepton flavour physics, are decoupled from the dark matter phenomenology. By focussing on a scenario with a single leptoquark state, we find that relic density can only be explained when both $\Delta$ and $S$ masses are lighter than $\mathcal{O}(10\mathrm{\,TeV})$.

A model of muon anomalies

The Standard Model (SM) is augmented with a U(1)B−3Lμ gauge symmetry spontaneously broken above the TeV scale when an SM-singlet scalar condenses. Scalar leptoquarks S1(3)=(3‾,1(3),13) charged under U(1)B−3Lμ mediate the intriguing effects observed in muon (g−2), RK(⁎), and b→sμ+μ− angular distributions, while generically evading all other phenomenological constraints. The fermionic sector is minimally extended with three right-handed neutrinos, and a successful type-I seesaw mechanism is realized. Charged lepton flavor violation is effectively suppressed, and proton decay—a common prediction of leptoquarks—is postponed to the dimension-6 effective Lagrangian. Unavoidable radiative corrections in the Higgs mass and muon Yukawa favor leptoquark masses interesting for collider searches. The parameters of the model are radiatively stable and can be evolved by the renormalization group to the Planck scale without inconsistencies. Alternative lepton-flavored gauge extensions of the SM, under which leptoquarks become muoquarks, are proposed for comparison.

Leptoquark option for B -meson anomalies and leptonic signatures

We entertain the option of scalar leptoquarks to explain the anomalies in the semileptonic decays of $B$ mesons and the discrepancies in the lepton $(g\ensuremath{-}2{)}_{l}$'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 $B\ensuremath{\rightarrow}K\ensuremath{\nu}\overline{\ensuremath{\nu}}$. We discuss the correlation between the leptonic signatures from leptoquarks such as $\ensuremath{\mu}\ensuremath{\rightarrow}e\ensuremath{\gamma}$ and the electric dipole moment of the electron and show that desirable neutrino masses can be generated dominantly by top-quark loops in the extension of the model with several doublet leptoquarks.

Radiative neutrino masses, lepton flavor mixing and muon g \u2212 2 in a leptoquark model

We propose a leptoquark model with two scalar leptoquarks {S}_1left(overline{3},1,frac{1}{3}right) and {tilde{R}}_2left(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 S1 and {tilde{R}}_2 . With a set of specific textures for the leptoquark Yukawa coupling matrices, the neutrino mass matrix possesses an approximate μ-τ reflection symmetry with (Mν)ee = 0 only in favor of the normal neutrino mass ordering. We show that this model can successfully explain the anomaly of muon g − 2 and current experimental neutrino oscillation data under the constraints from the radiative decays of charged leptons.

Exploring the role of new physics in $b \to u \tau \bar \nu$ decays

Abstract The recent measurements on $R_D$, $R_{D^*}$, and $R_{J/\psi}$ by three pioneering experiments, BaBar, Belle, and LHCb, indicate that the notion of lepton flavor universality is violated in weak charged-current processes, mediated through $b \to c \ell \bar \nu_\ell$ transitions. These intriguing results, which delineate a tension with their Standard Model predictions at the level of (2–3)$\sigma$, have triggered many new physics propositions in recent times, and are generally attributed to the possible implication of new physics in $ b \to c \tau \bar \nu$ transitions. This, in turn, opens up another avenue, i.e., $ b \to u \tau \bar \nu$ processes, to look for new physics. Since these processes are doubly Cabibbo suppressed, the impact of new physics could be significant enough, leading to sizable effects in some of the observables. In this work, we investigate in detail the role of new physics in $B \to (\pi,\rho,\omega)\tau \bar \nu$ and $B_s \to (K,K^*) \tau \bar \nu$ processes considering a model-independent approach. In particular, we focus on standard observables like branching fraction, the lepton flavor non-universality (LNU) parameter, forward–backward asymmetry, and polarization asymmetries. We find significant deviations in some of these observables, which can be explored by the currently running experiments, LHCb and Belle-II. We also briefly comment on the impact of the scalar leptoquark $R_2(3,2,7/6)$ and vector leptoquark $U_1(3,1,2/3)$ on these decay modes.

Model for the singlet-triplet leptoquarks

The deviations of $B$-meson decays measured in ${R}_{{D}^{(*)}}^{\ensuremath{\tau}\ensuremath{\ell}}$ and ${R}_{{K}^{(*)}}^{\ensuremath{\mu}e}$ can be explained by the presence of two scalar leptoquarks, a singlet ${S}_{1}$ and a triplet ${S}_{3}$, mostly coupled to the third generation. We consider a theory of resonances, as an effective description of a strongly interacting theory, that generates the leptoquarks and the Higgs as Nambu-Goldstone bosons, with the rest of the resonances at a scale of order 10--30 TeV. We assume anarchic partial compositeness for the flavor of the SM fermions. Under this hypothesis, we study whether it is possible to reproduce the deviations in the $B$ decays without being in conflict with flavor and electroweak bounds. We find a tension between ${R}_{{D}^{(*)}}^{\ensuremath{\tau}\ensuremath{\ell}}$ and some flavor observables, dominated by flavor violating $\ensuremath{\tau}$ decays and $\mathrm{\ensuremath{\Delta}}{m}_{{B}_{s}}$, that require a tuning of order 10%--25%. We also compute the potential of the scalars showing that leptoquarks with masses $O(2--3)\text{ }\text{ }\mathrm{TeV}$ can be naturally expected in the model. We discuss briefly the phenomenology of the other resonances.

Interplay of New Physics effects in (g \u2212 2)\u2113 and h \u2192 \u2113+\u2113\u2212 \u2014 lessons from SMEFT

We explore the interplay of New Physics (NP) effects in (g− 2)ℓ and h→ℓ+ℓ− within the Standard Model Effective Field Theory (SMEFT) framework, including one-loop Renormalization Group (RG) evolution of the Wilson coefficients as well as matching to the observables below the electroweak symmetry breaking scale. We include both the leading dimension six chirality flipping operators including a Higgs and SU(2)L gauge bosons as well as four-fermion scalar and tensor operators, forming a closed operator set under the SMEFT RG equations. We compare present and future experimental sensitivity to different representative benchmark scenarios. We also consider two simple UV completions, a Two Higgs Doublet Model and a single scalar LeptoQuark extension of the SM, and show how tree level matching to SMEFT followed by the one-loop RG evolution down to the electroweak scale can reproduce with high accuracy the (g−2)ℓ and h→ℓ+ℓ− contributions obtained by the complete one- and even two-loop calculations in the full models.

Flavorful leptoquarks at the LHC and beyond: spin 1

Evidence for electron-muon universality violation that has been revealed in b → sℓℓ transitions in the observables {R}_{KK^{ast }} by the LHCb Collaboration can be explained with spin-1 leptoquarks in SU(2)L singlet V1 or triplet V3 representations in the mathcal{O} (1 − 10) TeV range. We explore the sensitivity of the high luminosity LHC (HL-LHC) and future proton-proton colliders to V1 and V3 in the parameter space connected to {R}_{KK^{ast }} -data. We consider pair production and single production in association with muons in different flavor benchmarks. Reinterpreting a recent ATLAS search for scalar leptoquarks decaying to bμ and jμ, we extract improved limits for the leptoquark masses: for gauge boson-type leptoquarks (κ = 1) we obtain {M}_{V_1} > 1.9 TeV, {M}_{V_1} > 1.9 TeV, and {M}_{V_1} > 1.7 TeV for leptoquarks decaying predominantly according to hierarchical, flipped and democratic quark flavor structure, respectively. Future sensitivity projections based on extrapolations of existing ATLAS and CMS searches are worked out. We find that for κ = 1 the mass reach for pair (single) production of V1 can be up to 3 TeV (2.1 TeV) at the HL-LHC and up to 15 TeV (19.9 TeV) at the FCC-hh with sqrt{s} = 100 TeV and 20 ab−1. The mass limits and reach for the triplet V3 are similar or higher, depending on flavor. While there is the exciting possibility that leptoquarks addressing the {R}_{KK^{ast }} -anomalies are observed at the LHC, to fully cover the parameter space pp-collisions beyond the LHC-energies are needed.