Interpretation of excess in H→Zγ using a light axionlike particle

Abstract

We interpret the recent excess in a rare decay of the Higgs boson, H→Zγ, using a light axionlike particle (ALP) in the mass range 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 CaZHeff/Λ∼4×10−5 GeV−1, while the decay of the ALP before reaching the ECAL requires the diphoton coupling Cγγeff/Λ≥0.35 TeV−1(0.1 GeV/ma)2. A potential test would be the rare decay of the Z boson Z→aH*→a(bb¯) 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→Z*→aH via the same coupling CaZHeff/Λ at the LHC is too small for detection. Published by the American Physical Society 2024