scalar leptoquarks
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Abstract 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.

Abstract 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 $$ \mathcal{L} $$ L ∼ 2 ab−1.

Abstract We propose a leptoquark model with two scalar leptoquarks $$ {S}_1\left(\overline{3},1,\frac{1}{3}\right) $$ S 1 3 ¯ 1 1 3 and $$ {\tilde{R}}_2\left(3,2,\frac{1}{6}\right) $$ R ˜ 2 3 2 1 6 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 $$ {\tilde{R}}_2 $$ 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.

Abstract A search for pair production of third-generation scalar leptoquarks decaying into a top quark and a τ-lepton is presented. The search is based on a dataset of pp collisions at $$ \sqrt{s} $$ s = 13 TeV recorded with the ATLAS detector during Run 2 of the Large Hadron Collider, corresponding to an integrated luminosity of 139 fb−1. Events are selected if they have one light lepton (electron or muon) and at least one hadronically decaying τ -lepton, or at least two light leptons. In addition, two or more jets, at least one of which must be identified as containing b-hadrons, are required. Six final states, defined by the multiplicity and flavour of lepton candidates, are considered in the analysis. Each of them is split into multiple event categories to simultaneously search for the signal and constrain several leading backgrounds. The signal-rich event categories require at least one hadronically decaying τ-lepton candidate and exploit the presence of energetic final-state objects, which is characteristic of signal events. No significant excess above the Standard Model expectation is observed in any of the considered event categories, and 95% CL upper limits are set on the production cross section as a function of the leptoquark mass, for different assumptions about the branching fractions into tτ and bν. Scalar leptoquarks decaying exclusively into tτ are excluded up to masses of 1.43 TeV while, for a branching fraction of 50% into tτ, the lower mass limit is 1.22 TeV.

Abstract Evidence for electron-muon universality violation that has been revealed in b → sℓℓ transitions in the observables $$ {R}_{KK^{\ast }} $$ R KK ∗ by the LHCb Collaboration can be explained with spin-1 leptoquarks in SU(2)L singlet V1 or triplet V3 representations in the $$ \mathcal{O} $$ 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 $$ {R}_{KK^{\ast }} $$ R KK ∗ -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 $$ {M}_{V_1} $$ M V 1 > 1.9 TeV, $$ {M}_{V_1} $$ M V 1 > 1.9 TeV, and $$ {M}_{V_1} $$ 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 $$ \sqrt{s} $$ 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 $$ {R}_{KK^{\ast }} $$ R KK ∗ -anomalies are observed at the LHC, to fully cover the parameter space pp-collisions beyond the LHC-energies are needed.

AbstractA search for pair production of scalar leptoquarks, each decaying into either an electron or a muon and a top quark, is presented. This is the first leptoquark search using ATLAS data to investigate top-philic cross-generational couplings that could provide explanations for recently observed anomalies in B meson decays. This analysis targets high leptoquark masses which cause the decay products of each resultant top quark to be contained within a single high-$$p_{\mathrm {T}}$$ p T large-radius jet. The full Run 2 dataset is exploited, consisting of $$139~\hbox {fb}^{-1}$$ 139 fb - 1 of data collected from proton–proton collisions at $$\sqrt{s}=13~\mathrm {TeV}$$ s = 13 TeV from 2015 to 2018 with the ATLAS detector at the CERN Large Hadron Collider. In the absence of any significant deviation from the background expectation, lower limits on the leptoquark masses are set at $$1480~\mathrm {GeV}$$ 1480 GeV and $$1470~\mathrm {GeV}$$ 1470 GeV for the electron and muon channel, respectively.

Abstract We investigate the viability of extending the Standard Model with S1 and S3 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 → μγ, $$ {\overline{B}}_s $$ B ¯ s → τ±μ∓ and B+→ K+τ+μ− decays, whose branching ratios are close to the current experimental limits.

Abstract 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 flavor anomalies, i.e. the deviations between the Standard Model predictions and measurements in b → sℓ+ℓ− and b → 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 → ℓ+ℓ−, Z →$$ v\overline{v} $$ v v ¯ , W → ℓν and h → ℓ+ℓ− and can even generate non-zero rates for lepton flavor violating processes such as ℓ → ℓ′γ, Z → ℓ+ℓ′−, h → ℓ+ℓ′− and ℓ → 3ℓ′. In this article we calculate these processes for all five representations of scalar Leptoquarks. We include their most general interaction terms with the Standard Model Higgs boson, which leads to Leptoquark mixing after the former acquires a vacuum expectation value. In our phenomenological analysis we investigate the effects in modified lepton couplings to electroweak gauge bosons, we study the correlations of the anomalous magnetic moment of the muon with h → μ+μ− and Z → μ+μ− as well as the interplay between different lepton flavor violating decays.