Addressing the LHC flavour anomalies with horizontal gauge symmetries

TL;DR

This paper proposes a model with a new flavor-dependent gauge symmetry that explains several LHC flavor anomalies without needing vector-like fermions, and makes testable predictions for rare tau decays.

Contribution

It introduces a minimal $U(1)'$ gauge symmetry model with scalar doublets that naturally accounts for observed flavor anomalies and predicts new phenomena like $ au o 3\mu$ decays.

Findings

Successfully explains $B o K^* \mu^+\mu^-$ and $R(K)$ anomalies.

Predicts $ au o 3\mu$ decay rate within experimental reach.

Fits fermion mixing matrices without vector-like fermions.

Abstract

We study the impact of an additional $U(1)'$ gauge symmetry with flavour-dependent charges for quarks and leptons on the LHC flavour anomalies observed in $B \to K^* \mu^+\mu^-$, $R(K) = B \to K \mu^+\mu^-/B \to K e^+e^-$, and $h \to \mu\tau$. In its minimal version with two scalar doublets, the resulting model naturally explains the deviations from the Standard Model observed in $B \to K^* \mu^+\mu^-$ and $R(K)$. The CMS access in $h \to \mu\tau$ can be explained by introducing a third scalar doublet, which gives rise to a prediction for $\tau \to 3\mu$. We investigate constraints from flavour observables and direct LHC searches for $pp \to Z' \to \mu^+\mu^-$. Our model successfully generates the measured fermion-mixing matrices and does not require vector-like fermions, unlike previous attempts to explain these anomalies.