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Cosmic-ray muons and especially their secondaries break apart nuclei("spallation") and produce fast neutrons and beta-decay isotopes, which arebackgrounds for low-energy experiments. In Super-Kamiokande, these beta decaysare the dominant background in 6--18 MeV, relevant for solar neutrinos and thediffuse supernova neutrino background. In a previous paper, we showed thatthese spallation isotopes are produced primarily in showers, instead of inisolation. This explains an empirical spatial correlation between a peak in themuon Cherenkov light profile and the spallation decay, which Super-Kamiokandeused to develop a new spallation cut. However, the muon light profiles thatSuper-Kamiokande measured are grossly inconsistent with shower physics. We showhow to resolve this discrepancy and how to reconstruct accurate profiles ofmuons and their showers from their Cherenkov light. We propose a new spallationcut based on these improved profiles and quantify its effects. Our results cansignificantly benefit low-energy studies in Super-Kamiokande, and will beespecially important for detectors at shallower depths, like the proposedHyper-Kamiokande.

Tagging Spallation Backgrounds with Showers in Water-Cherenkov Detectors