Closing the window on single leptoquark solutions to the $B$-physics anomalies

TL;DR

This paper investigates whether light leptoquarks can explain B-physics anomalies, concluding that only a vector leptoquark called U1 at 1-2 TeV can potentially do so, but with tight experimental constraints.

Contribution

It provides a comprehensive analysis combining low-energy observables and LHC searches, showing scalar leptoquarks cannot alone explain the anomalies and re-evaluating the minimal U1 vector leptoquark scenario.

Findings

Scalar leptoquarks around 1 TeV cannot explain anomalies alone.

The vector leptoquark U1 at 1-2 TeV can potentially explain anomalies.

Future bounds on B decays could challenge the minimal U1 model.

Abstract

We examine various scenarios in which the Standard Model is extended by a light leptoquark state to solve for one or both $B$-physics anomalies, viz. $R_{D^{(\ast)}}^\mathrm{exp}> R_{D^{(\ast)}}^\mathrm{SM}$ or/and $R_{K^{(\ast)}}^\mathrm{exp}< R_{K^{(\ast)}}^\mathrm{SM}$. To do so we combine the constraints arising both from the low-energy observables and from direct searches at the LHC. We find that none of the scalar leptoquarks of mass $m_\mathrm{LQ} \simeq 1$ TeV can alone accommodate the above mentioned anomalies. The only single leptoquark scenario which can provide a viable solution for $m_\mathrm{LQ} \simeq 1\div 2$ TeV is a vector leptoquark, known as $U_1$, which we re-examine in its minimal form (letting only left-handed couplings to have non-zero values). We find that the limits deduced from direct searches are complementary to the low-energy physics constraints. In particular, we find a rather stable lower bound on the lepton flavor violating $b\to s\ell_1^\pm\ell_2^\mp$ modes, such as $\mathcal{B}(B\to K\mu\tau)$. Improving the experimental upper bound on $\mathcal{B}(B\to K\mu\tau)$ by two orders of magnitude could compromise the viability of the minimal $U_1$ model as well.