First calculation of cosmic-ray muon spallation backgrounds for MeV astrophysical neutrino signals in Super-Kamiokande

Abstract

When muons travel through matter, their energy losses lead to nuclear breakup ("spallation") processes. The delayed decays of unstable daughter nuclei produced by cosmic-ray muons are important backgrounds for low-energy astrophysical neutrino experiments, e.g., those seeking to detect solar neutrino or diffuse supernova neutrino background (DSNB) signals. Even though Super-Kamiokande has strong general cuts to reduce these spallation-induced backgrounds, the remaining rate before additional cuts for specific signals is much larger than the signal rates for kinetic energies of about 6 -18 MeV. Surprisingly, there is no published calculation of the production and properties of these backgrounds in water, though there are such studies for scintillator. Using the simulation code FLUKA and theoretical insights, we detail how muons lose energy in water, produce secondary particles, how and where these secondaries produce isotopes, and the properties of the backgrounds from their decays. We reproduce Super-Kamiokande measurements of the total background to within a factor of 2, which is good given that the isotope yields vary by orders of magnitude and that some details of the experiment are unknown to us at this level. Our results break aggregate data into component isotopes, reveal their separate production mechanisms, and preserve correlations between them. We outline how to implement more effective background rejection techniques using this information. Reducing backgrounds in solar and DSNB studies by even a factor of a few could help lead to important new discoveries.

First measurement of radioactive isotope production through cosmic-ray muon spallation in Super-Kamiokande IV

“…11 Be and 15 C are consistent with zero, so we set upper limits at 90% C.L.. The yields from a theoretical calculation [13] are shown in the third column. KamLAND's experimental and simulation results [2] are listed in the fourth and fifth columns, respectively.…”

“…The theoretical calculation is given by Ref. [13], which is based on the simulation code FLUKA [29,30]. 11 Be and 15 C are consistent with zero, so we set upper limits at 90% C.L..…”

“…The time distribution of low-energy events up to 30 s after each muon was used to extract the production rate of radioactive nuclei (in the spallation backgrounds). Table I lists the possible radioactive isotopes induced by cosmic-ray muon spallation at SK [13,22,23].…”

“…However, many longlived isotopes still survived and affected the search for a low-energy upturn of the solar 8 B neutrino spectrum as well as the hunt for evidence of supernova relic neutrinos (SRNs), also known as diffuse supernova neutrino background (DSNB), emitted from past core-collapse supernovae. Recently, theoretical progress has been made in the calculation of the production and properties of these backgrounds in water, shedding light on practical improvements in spallation background rejection [13][14][15]. Providing experimental data should help improve knowledge in this area.…”

Estimate of the theoretical uncertainty of the cross sections for nucleon knockout in neutral-current neutrino-oxygen interactions

“…These studies require that the backgrounds [5] related to atmospheric-neutrino interactions be estimated with a challenging level of accuracy, because the signals are expected to be elusive-a few DSN events per year in the fiducial volume of the Super-Kamiokande (SK) detector.…”