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Register a new userLow-energy phenomenology of scalar leptoquarks at one-loop accuracy
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We perform a complete study of the low-energy phenomenology of $S_1$ and $S_3$ lepto-quarks, aimed at addressing the observed deviations in $B$-meson decays and the muon magnetic dipole moment. Leptoquark contributions to observables are computed at one-loop accuracy in an effective field theory approach, using the recently published complete one-loop matching of these leptoquarks to the Standard Model effective field theory. We present several scenarios, discussing in each case the preferred parameter space and the most relevant observables.
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In this paper we present the complete one-loop matching conditions, up to dimension-six operators of the Standard Model effective field theory, resulting by integrating out the two scalar leptoquarks $S_{1}$ and $S_{3}$. This allows a phenomenological study of low-energy constraints on this model at one-loop accuracy, which will be the focus of a subsequent work. Furthermore, it provides a rich comparison for functional and computational methods for one-loop matching, that are being developed. As a corollary result, we derive a complete set of dimension-six operators independent under integration by parts, but not under equations of motions, called Greens basis, as well as the complete reduction formulae from this set to the Warsaw basis.
We examine new aspects of leptoquark (LQ) phenomenology using effective field theory (EFT). We construct a complete set of leading effective operators involving SU(2) singlets scalar LQ and the SM fields up to dimension six. We show that, while the renormalizable LQ-lepton-quark interaction Lagrangian can address the persistent hints for physics beyond the Standard Model in the B-decays $bar B to D^{(*)} tau bar u$, $bar B to bar K ell^+ ell^-$ and in the measured anomalous magnetic moment of the muon, the LQ higher dimensional effective operators may lead to new interesting effects associated with lepton number violation. These include the generation of one-loop sub-eV Majorana neutrino masses, mediation of neutrinoless double-$beta$ decay and novel LQ collider signals. For the latter, we focus on 3rd generation LQ ($phi_3$) in a framework with an approximate $Z_3$ generation symmetry, and show that one class of the dimension five LQ operators may give rise to a striking asymmetric same-charge $phi_3 phi_3$ pair-production signal, which leads to low background same-sign leptons signals at the LHC. For example, with $M_{phi_3} sim 1$ TeV and a new physics scale of $Lambda sim 5$ TeV, we expect about $5000$ positively charged $tau^+ tau^+$ events via $pp to phi_3 phi_3 to tau^+ tau^+ + 2 cdot j_b$ ($j_b$=b-jet) at the 13 TeV LHC with an integrated luminosity of 300 fb$^{-1}$. It is interesting to note that, in the LQ EFT framework, the expected same-sign lepton signals have a rate which is several times larger than the QCD LQ-mediated opposite-sign leptons signals, $gg,q bar q to phi_3 phi_3^* to ell^+ ell^- +X$. We also consider the same-sign charged lepton signals in the LQ EFT framework at higher energy hadron colliders such as a 27 TeV HE-LHC and a 100 TeV FCC-hh.
We study the phenomenology of electric dipole moments (EDMs) induced in various scalar leptoquark models. We consider generic leptoquark couplings to quarks and leptons and match to Standard Model effective field theory. After evolving the resulting operators to low energies, we connect to EDM experiments by using up-to-date hadronic, nuclear, and atomic matrix elements. We show that current experimental limits set strong constraints on the possible CP-violating phases in leptoquark models. Depending on the quarks and leptons involved in the interaction, the existing searches for EDMs of leptons, nucleons, atoms, and molecules all play a role in constraining the CP-violating couplings. We discuss the impact of hadronic and nuclear uncertainties as well as the sensitivities that can be achieved with future EDM experiments. Finally, we study the impact of EDM constraints on a specific leptoquark model that can explain the recent $B$-physics anomalies.
Leptoquarks (LQs) have attracted increasing attention within recent years, mainly since they can explain the flavor anomalies found in $R(D^{(*)})$, $b rightarrow s ell^+ ell^-$ 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 $Phi_1, Phi_{tilde1}, Phi_2, Phi_{tilde2}$ and $Phi_3$ 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.
In recent years, intriguing hints for the violation of lepton flavour universality have accumulated. In particular, deviations from the Standard-Model (SM) predictions in $Bto D^{(*)}tau u/Bto D^{(*)}ell u$, in the anomalous magnetic moment of the muon and {in} $bto sell^+ell^-$ data were observed with a significance of $!>3,sigma$, $>!4,sigma$ and $>!5,sigma$, respectively. Furthermore, in a recent re-analysis of 2018 Belle data, it was found that the forward-backward asymmetry of $bar B to D^{*}mubar u$ vs $bar Bto D^{*}ebar u$ disagrees with the SM prediction by $approx!!4,sigma$ which would be an additional sign of lepton flavour universality violation. Since one naturally expects muon-related new effects to also emerge at some point in $b to cmu u$ decays, the above putative deviation might share a common origin with the other flavour anomalies. We show that a tensor operator is necessary to significantly improve the global fit w.r.t. the SM, which can only be induced (at tree-level in a renormalizable model) by a scalar leptoquark. Interestingly, among the two possible representations, the $SU(2)_L$-singlet $S_1$ and the doublet $S_2$, which can both also account for the anomalous magnetic moment of the muon, only $S_1$ can provide a good fit as it naturally gives rise to the scenario $C_{VL}, C_{SL}=-4 C_T$. While the constraints from (differences of) other angular observables prefer a smaller value $Delta A_{rm FB}$, this scenario is significantly preferred ($approx 3 sigma$) over the SM hypothesis and compatible with constraints such as $Bto K^* u u$ and electroweak precision bounds.
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