Z Boson Radiative Decay $Z\to \mu^+ \mu^- \gamma$ at the LHC

TL;DR

This paper analyzes the $Z o \, ext{mu}^+ \, ext{mu}^- \, ext{gamma}$ decay at the LHC, demonstrating its potential for precise Standard Model tests and probing new physics like ALPs and anomalous gauge bosons.

Contribution

It provides a detailed SM analysis and explores the sensitivity of this decay mode to leptophilic new physics, including projections for future collider runs.

Findings

Measured the branching ratio from Run-1 data as (3.34 ± 0.016)×10^{-4}.

Showed that the decay can be measured with sub-percent statistical precision at HL-LHC.

Demonstrated the decay's sensitivity to ALPs and $U(1)_X$ gauge bosons, extending collider reach.

Abstract

We study the radiative decay of the $Z$ boson, $Z \to \mu^+\mu^-\gamma$, at the LHC, providing both Standard Model (SM) precision analysis and new physics projections. With detailed analysis of Run-2 and future HL-LHC performances, we demonstrate that this decay mode can be measured with a statistical precision at the sub-percentage level. From existing Run-1 data, we extract $\text{Br}^\text{fid}(Z \to \mu\mu\gamma) = (3.34 \pm 0.016)\times 10^{-4}$. We further explore the sensitivity of this channel to axion-like particles (ALPs) and to an anomalous $U(1)_X$ gauge boson coupled to the muon. Both scenarios feature resonant structures in the dimuon invariant mass spectrum within the $Z \to a/X + \gamma \to \mu^+\mu^-\gamma$ final state. Our results show that the radiative $Z$ decay provides a clean and statistically powerful probe of such leptophilic new physics, extending the current collider reach for ALPs and anomalous gauge forces down to $g_X \sim \mathcal{O}(10^{-3})$. This study highlights the potential of rare electroweak gauge boson decays as precision tests of the SM and sensitive probes of new interactions at the LHC.