Atmospheric axionlike particles at Super-Kamiokande

Abstract

We consider a muonphilic axionlike particle (ALP), denoted as a, lighter than twice the muon mass. ALPs of this mass range dominantly decay into a pair of photons, induced by a triangular muon loop. Such light muonphilic ALPs are naturally long lived. At the atmosphere, the ALPs are copiously produced from charged-meson decays in air showers, such as π±→μ±νa, via the ALP-muon coupling gaμμ. After propagating tens of kilometers, the ALPs decay with a→γγ inside large-volume Cherenkov detectors near Earth's surface, such as Super-Kamiokande (SK). We find the present SK observation constrains on muonphilic ALPs of mass range [1 MeV, 30 MeV] and ALP-muon coupling [10-3,102], assuming the proper decay length cτa in [10-3 km, 106 km] either dependent on or independent of gaμμ. We conclude that atmospheric searches of such exotic states can be complementary to collider and beam-dump experiments as well as astrophysical probes.