Spallation Backgrounds in Super-Kamiokande Are Made in Muon-Induced Showers

Understand

Crucial questions about solar and supernova neutrinos remain unanswered.

• Super-Kamiokande has the exposure needed for progress, but detector backgrounds are a limiting factor.
• A leading component is the beta decays of isotopes produced by cosmic-ray muons and their secondaries, which initiate nuclear spallation reactions.
• Cuts of events after and surrounding muon tracks reduce this spallation decay background by $\simeq 90\%$ (at a cost of $\simeq 20\%$ deadtime), but its rate at 6--18 MeV is still dominant.