Atmospheric axionlike particles at Super-Kamiokande

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 pi(+/-) -> mu(+/-)va, via the ALP-muon coupling g(a)(mu mu). After propagating tens of kilometers, the ALPs decay with a -> gamma gamma 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),10(2)], assuming the proper decay length c tau(a) in [10(-3) km, 10(6) km] either dependent on or independent of g(a)(mu mu). We conclude that atmospheric searches of such exotic states can be complementary to collider and beam-dump experiments as well as astrophysical probes.