The role of the $S_3$ GUT leptoquark in flavor universality and collider searches

Abstract

We investigate the ability of the $S_3$ scalar leptoquark to address the recent hints of lepton universality violation in $B$ meson decays. The $S_3$ leptoquark with quantum numbers $(\overline{3},3,1/3)$ naturally emerges in the context of an $SU(5)$ GUT model without any conflict with the stringent limits from observed nucleon stability. Scalar leptoquark $S_3$ with left-handed couplings to 2nd and 3rd generations of charged leptons and down-type quarks seems well-suited to address both $R_{K^{(\ast )}}$ and $R_{D^{(\ast )}}$. We quantify this suitability with numerical fits to a plethora of relevant flavor observables. The proposed $SU(5)$ model calls for a second leptoquark state, i.e., ${\tilde{R}}_2$ with quantum numbers $(3,2,1/6)$, if one is to generate gauge coupling unification and neutrino mass. We accordingly include it in our study to investigate ${\tilde{R}}_2$'s ability to offset adverse effects of $S_3$ and thus improve a quality of numerical fits. A global fit of the leptoquark Yukawa couplings shows that large couplings of light $S_3$ to $\tau$ leptons are preferred. We furthermore identify $B\to K^{(\ast )}\bar{\nu }\nu$ as the most sensitive channel to probe the preferred region of parameter space. Large couplings of $S_3$ to $\tau$ leptons are finally confronted with the experimental searches for $\tau$ final states at the Large Hadron Collider. These searches comprise a study of decay products of the leptoquark pair production, as well as, and more importantly, an analysis of the high-mass $\tau \tau$ final states.

Published version. 32 pages, 6 figures

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