Publications

PhD Thesis: New Methods to Reduce Cosmogenic Backgrounds of Super-Kamiokande in the Solar Neutrino Energy Regime

Abstract:
Solar neutrinos pass through the Earth’s surface with a flux on the order of 10¹¹ cm²/s. Super-Kamiokande (SK) searches for neutrinos from the sun with an energy above 4.0 MeV. Neutrinos rarely interact with matter, and removing backgrounds that would otherwise overwhelm the signal is imperative. SK observes about 2 muons a second at its depth of 2700m water equivalent, and a fraction of these muons shower within the detector with a possibility to create radioactive isotopes (spallation) which live from milliseconds to tens of seconds. Radioactive isotopes from any source are by far the largest source of background in the solar neutrino energy region observed in SK, and spallation is the dominant background to neutrino interactions between 6 and 20 MeV. Spallation in SK is mostly caused by neutrons and pions interacting with the Oxygen nucleus. Recently, new techniques involving identifying neutron in the showers after muons and effective tagging of multiple spallation, along with improvements and additions to former spallation tagging methods have reduced deadtime by ~55% (~45%) where neutron data is available (unavailable). This increases measured solar neutrino signal by 12% and reduces relative statistical error by 6.6% for the entire SK-IV solar neutrino sample.

Submitted Paper (First Author): New Methods and Simulations for Cosmogenic Induced Spallation Removal in Super-Kamiokande-IV

Abstract:
Radioactivity induced by cosmic muon spallation is a dominant source of backgrounds for O(10) MeV neutrino interactions in water Cherenkov detectors. In particular, it is crucial to reduce backgrounds to measure the solar neutrino spectrum and find neutrino interactions from distant supernovae. In this paper we introduce new techniques to locate muon-induced hadronic showers and efficiently reject spallation backgrounds. Applying these techniques to the solar neutrino analysis with an exposure of 2790×22.5~kton.day increases the signal efficiency by 12.6%, approximately corresponding to an additional year of detector running. Furthermore, we present the first spallation simulation at SK, where we model hadronic interactions using FLUKA. The agreement between the isotope yields and shower pattern in this simulation and in the data gives confidence in the accuracy of this simulation, and thus opens the door to use it to optimize muon spallation removal in new data with gadolinium-enhanced neutron capture detection.