Development of Neutron Tagging Algorithm and Search for Supernova Relic Neutrino in SK-Gd Experiment
Sep 25, 2023228 pages
Supervisor:
Thesis: PhD - Okayama U.,
- Okayama University,
- Okayama U.
- Published: 2023and
- Published: Sep 25, 2023
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Abstract:
Observing neutrinos originating from supernovae is vital for deciphering the explosion
mechanisms of core-collapse supernovae. Specifically, detecting supernova relic neutrinos
(SRNs) can elucidate these mechanisms as well as the cosmic star formation
history. The predominant signal within the SRN energy region corresponds to the
inverse-beta decay (¯.e + p . e+ + n).
The Super-Kamiokande experiment, currently the most sensitive neutrino detector
for the SRN energy region, has introduced the delayed coincidence method to the event
selection using a neutron capture signal. This improvement has resulted in a more
stringent upper limit. For the first discovery of SRN, the SK-Gd experiment was
proposed in 2004. In the SK-Gd experiment, the SRN signal efficiency is significantly
improved by loading gadolinium (Gd) in the detector tank. Gd has the largest crosssection
for the thermal neutron capture among natural elements and emits a total of
about 8 MeV gamma rays during the thermal neutron capture. This key advantage
strengthens the correlation between the prompt positron and delayed neutron signal
with enhanced neutron detection efficiency.
The SK-Gd experiment with about 0.011% mass concentration of Gd started in
July 2020 and operated until May 2022. At first, an algorithm for detecting the
neutron signal in SK-Gd is newly introduced. The algorithm is evaluated by the newly
constructed Monte-Carlo simulation and neutron source measurement. Consequently,
the neutron tagging efficiency is estimated to be 40.2%, with an uncertainty of 8.7%.
After that, the initial search was conducted using data from 552.2 days SK-Gd
operation. This analysis employed the newly constructed neutron detection algorithm
and Monte Carlo simulation. After eliminating the major background events, the
remaining were consistent with the expected background within 1.1s according to the
p-value test across all investigated energy bins. In the absence of an SRN signal in
this search, the observed and expected upper limits of SRN fluxes were respectively
extracted to be 0.3–32.3 and 0.3–49.2 cm-2 sec-1 MeV-1, depending on the energy
bins. These limits are commensurate with the most stringent limit determined by the
search result in a pure-water phase of Super-Kamiokande.
For prospects, pragmatic improvements to the detector and analysis are contemplated
for the SK-Gd experiment. The future sensitivity of the SK-Gd experiment is
examined by assuming these enhancements. To demonstrate a potential approach for
extracting physics parameters from the SRN flux, the formation rate of black holes
and heavy neutron stars are highlighted.- supernova
- direct detection
- KAMIOKANDE
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