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arXiv · 2604.13156

Z Boson Radiative Decay $Z\to μ^+ μ^- γ$ at the LHC

Yifan Fei·Peiran Li·Zhen Liu·Maxim Pospelov
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Abstract

We study the radiative decay of the $Z$ boson, $Z \to μ^+μ^-γ$, 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 $\text{Br}^\text{fid}(Z \to μμγ) = (3.34 \pm 0.016)\times 10^{-4}$. 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 $Z \to a/X + γ\to μ^+μ^-γ$ 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.

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Yifan Fei, Peiran Li, Zhen Liu, Maxim Pospelov. 2026-08-21. Z Boson Radiative Decay $Z\to μ^+ μ^- γ$ at the LHC. https://arxiv.org/abs/2604.13156

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