Disentangling axionlike particle couplings to nucleons via a delayed signal in Super-Kamiokande from a future supernova

In this work, we show that, if axionlike particles (ALPs) from core-collapse supernovae (SNe) couple to protons, they would produce very characteristic signatures in neutrino water Cherenkov detectors through their scattering off free protons via ap→pγ interactions. Specifically, sub-MeV ALPs would generate photons with energies ∼30 MeV, which could be observed by Super-Kamiokande and Hyper-Kamiokande as a delayed signal after a future detection of SN neutrinos. We apply this to a hypothetical neighboring SN (at a maximum distance of 100 kpc) and demonstrate that the region in the parameter space with ALP masses between 10−4 and 1 MeV and ALP-proton couplings in the range 3×10−6–4×10−5 could be probed. We argue that this new signature, combined with the one expected at ∼7 MeV from oxygen deexcitation, would allow us to disentangle ALP-neutron and ALP-proton couplings. Published by the American Physical Society 2025