A unified leptoquark model confronted with lepton non-universality in B-meson decays

T Faber,M Hudec,M Malinský,P Meinzinger,W Porod,F Staub

doi:10.1016/j.physletb.2018.10.051

T Faber, M Hudec + Show 4 more

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https://doi.org/10.1016/j.physletb.2018.10.051

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Abstract

The anomalies in the B-meson sector, in particular RK(⁎) and RD(⁎), are often interpreted as hints for physics beyond the Standard Model. To this end, leptoquarks or a heavy Z′ represent the most popular SM extensions which can explain the observations. However, adding these fields by hand is not very satisfactory as it does not address the big questions like a possible embedding into a unified gauge theory. On the other hand, light leptoquarks within a unified framework are challenging due to additional constraints such as lepton flavor violation. The existing accounts typically deal with this issue by providing estimates on the relevant couplings. In this letter we consider a complete model based on the SU(4)C⊗SU(2)L⊗U(1)R gauge symmetry, a subgroup of SO(10), featuring both scalar and vector leptoquarks. We demonstrate that this setup has, in principle, all the potential to accommodate RK(⁎) and RD(⁎) while respecting bounds from other sectors usually checked in this context. However, it turns out that KL→e±μ∓ severely constraints not only the vector but also the scalar leptoquarks and, consequently, also the room for any sizeable deviations of RK(⁎) from 1. We briefly comment on the options for extending the model in order to conform this constraint. Moreover, we present a simple criterion for all-orders proton stability within this class of models.

Highlights

In recent years a few anomalies in the B-meson sector have been observed by different experiments
RDSM = 0.300 ± 0.010, RDSM∗ = 0.252 ± 0.005 . (2). This was first reported by BaBar [1, 2] consistent with measurements by Belle [3,4,5]
In Refs. [14, 15] it has been shown that the deviations in RD and RK can be explained by an effective model adding one generation of scalar leptoquarks (LQs) with the quantum numbers of the right-handed d-quark and an additional scalar gauge singlet which couples to the LQs

Summary

INTRODUCTION

In recent years a few anomalies in the B-meson sector have been observed by different experiments. [14, 15] it has been shown that the deviations in RD and RK can be explained by an effective model adding one generation of scalar leptoquarks (LQs) with the quantum numbers of the right-handed d-quark and an additional scalar gauge singlet which couples to the LQs. it has been shown that this leads to a too large rate for b → sνν [16]. [17] another model with two different LQs, one with gauge quantum numbers of the right-handed d-quark and one with charge 4/3, has been presented which explains neutrino masses at the 2loop level. Beside the above mentioned violations of lepton-universality this model is compatible with the neutrino data Another possibility is that the required leptoquarks are bound states of strongly interacting fermions [27]. In Appendix A we demonstrate that in this class of models proton remains stable to all orders in perturbation theory

MODEL DESCRIPTION

N μN 2

LOW ENERGY OBSERVABLES

DISCUSSION AND CONCLUSIONS