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 $C_{aZH}^{\mathrm{eff}}/\Lambda \sim 4\times 10^{-5}{\mathrm{GeV}}^{-1}$, while the decay of the ALP before reaching the ECAL requires the diphoton coupling $C_{\gamma \gamma }^{\mathrm{eff}}/\Lambda \ge 0.35{\mathrm{TeV}}^{-1}(0.1\mathrm{eV}/m_a{)}^2$. A potential test would be the rare decay of the $Z$ boson $Z\to a{H}^{\ast }\to a(b\bar{b})$ 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 $pp\to Z^{\ast }\to aH$ via the same coupling $C_{aZH}^{\mathrm{eff}}/\Lambda$ at the LHC is too small for detection.

Comments: v2: 15 pages, 5 figures, match the journal version