Interpretation of excess in $H \to Z \gamma$ using a light axion-like particle

Abstract

We interpret the recent excess in a rare decay of the Higgs boson, $H\to Z\gamma$, using a light axion-like particle (ALP) in the massrange $0.05 - 0.1$ GeV.The dominant decay of such a light ALP is into a pair of collimated photons, whose decay is required to happen before reaching the ECAL detector, such that it mimics a single photon in the detector. It can explain the excess with a coupling $C^{\rm eff}_{aZH} / \Lambda \sim 4 \times 10^{-5}\;{\rm GeV}^{-1}$, while the decay of the ALP before reaching the ECAL requires the diphoton coupling $C^{\rm eff}_{\gamma\gamma}/ \Lambda \ge 0.35 \,{\rm TeV}^{-1} (0.1\,{\rm eV}/m_a)^2$. A potential test would be the rare decay of the $Z$ boson $Z \to a H^* \to a (b \bar b)$ at the Tera-$Z$ option of the future FCC and CEPC. However, it has a branching ratio of only $O(10^{-12})$, and thus barely testable. The production cross section for $pp \to Z^* \to a H$ via the same coupling $C^{\rm eff}_{aZH} / \Lambda$ at the LHC is too small for detection.