Interpretation of excess in $H \to Z γ$ using a light axion-like particle Article Swipe
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· 2024
· Open Access
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· DOI: https://doi.org/10.48550/arxiv.2402.05678
· OA: W4391709895
We interpret the recent excess in a rare decay of the Higgs boson, $H\to Zγ$, 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 $C^{\rm eff}_{aZH} / Λ\sim 4 \times 10^{-5}\;{\rm GeV}^{-1}$, while the decay of the ALP before reaching the ECAL requires the diphoton coupling $C^{\rm eff}_{γγ}/ Λ\ge 0.35 \,{\rm TeV}^{-1} (0.1\,{\rm eV}/m_a)^2$. 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 $C^{\rm eff}_{aZH} / Λ$ at the LHC is too small for detection.
