Probing flavoured Axions in the Tail of $B_q \to \mu^+\mu^-$

TL;DR

This paper explores how LHCb di-muon data can be used to detect or constrain flavoured axions through rare B-meson decays, providing new calculations of form factors and sensitivity estimates for future experiments.

Contribution

It introduces the first QCD sum rule calculations of off-shell B to gamma form factors relevant for axion searches and analyzes LHCb data sensitivity to flavoured axions.

Findings

LHCb data can probe axion-coupling scales up to 10^6 GeV.

Computed B_q to gamma form factors using QCD sum rules.

Future LHCb runs will significantly improve axion constraints.

Abstract

We discuss how LHC di-muon data collected to study $B_q \to \mu\mu$ can be used to constrain light particles with flavour-violating couplings to $b$-quarks. Focussing on the case of a flavoured QCD axion, $a$, we compute the decay rates for $B_q \to \mu \mu a$ and the SM background process $B_q \to \mu \mu \gamma$ near the kinematic endpoint. These rates depend on non-perturbative $B_q \to \gamma^{(*)}$ form factors with on- or off-shell photons. The off-shell form factors -- relevant for generic searches for beyond-the-SM particles -- are discussed in full generality and computed with QCD sum rules for the first time. With these results, we analyse available LHCb data to obtain the sensitivity on $B_q \to \mu \mu a$ at present and future runs. We find that the full LHCb dataset alone will allow to probe axion-coupling scales of the order of $10^6$ GeV for both $b\to d$ and $b \to s$ transitions.