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

TL;DR

This paper proposes a unified gauge theory model with scalar and vector leptoquarks to explain B-meson decay anomalies, addressing constraints from flavor violation and proton stability.

Contribution

It introduces a complete $SU(4)_C \otimes SU(2)_L \otimes U(1)_R$ model embedding leptoquarks, analyzing its potential to explain anomalies while satisfying experimental bounds.

Findings

Model can accommodate $R_{K^{(*)}}$ and $R_{D^{(*)}}$ anomalies.

K_L to eμ constraints limit leptoquark deviations.

Proton stability criterion established for the model.

Abstract

The anomalies in the $B$-meson sector, in particular $R_{K^{(*)}}$ and $R_{D^{(*)}}$, 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 $SU(4)_{\rm C}\otimes SU(2)_{\rm L}\otimes U(1)_{\rm R}$ 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 $R_{K^{(*)}}$ and $R_{D^{(*)}}$ while respecting bounds from other sectors usually checked in this context. However, it turns out that $K_L \to e^{\pm} \mu^{\mp}$ severely constraints not only the vector but also the scalar leptoquarks and, consequently, also the room for any sizeable deviations of $R_{K^{(*)}}$ 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.