A measurement of electron antineutrino oscillation by the Daya Bay Reactor Neutrino Experiment is described in detail. Six 2.9-GWth nuclear power reactors of the Daya Bay and Ling Ao nuclear power ...facilities served as intense sources of ν¯e’s. Comparison of the ν¯e rate and energy spectrum measured by antineutrino detectors far from the nuclear reactors (∼1500–1950 m) relative to detectors near the reactors (∼350–600 m) allowed a precise measurement of ν¯e disappearance. More than 2.5 million ν¯e inverse beta-decay interactions were observed, based on the combination of 217 days of operation of six antineutrino detectors (December, 2011–July, 2012) with a subsequent 1013 days using the complete configuration of eight detectors (October, 2012–July, 2015). The ν¯e rate observed at the far detectors relative to the near detectors showed a significant deficit, R=0.949±0.002(stat)±0.002(syst). The energy dependence of ν¯e disappearance showed the distinct variation predicted by neutrino oscillation. Analysis using an approximation for the three-flavor oscillation probability yielded the flavor-mixing angle sin22θ13=0.0841±0.0027(stat)±0.0019(syst) and the effective neutrino mass-squared difference of |Δmee2|=(2.50±0.06(stat)±0.06(syst))×10−3 eV2. Analysis using the exact three-flavor probability found Δm322=(2.45±0.06(stat)±0.06(syst))×10−3 eV2 assuming the normal neutrino mass hierarchy and Δm322=(−2.56±0.06(stat)±0.06(syst))×10−3 eV2 for the inverted hierarchy.
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We have measured the muon flux and production rate of muon-induced neutrons at a depth of 611 m water equivalent. Our apparatus comprises three layers of crossed plastic scintillator hodoscopes for ...tracking the incident cosmic-ray muons and 760 L of a gadolinium-doped liquid scintillator for producing and detecting neutrons. The vertical muon intensity was measured to be I sub(mu)=(5.7+ or -0.6)x10 super(-6)cm super(- 2)s super(-1)sr super(-1). The yield of muon-induced neutrons in the liquid scintillator was determined to be Y sub(n)=(1.19+ or -0.08(stat)+ or -0.21(syst))x10 super(-4)neutrons/(mumiddotgmiddotcm super(-2)). A fit to the recently measured neutron yields at different depths gave a mean muon energy dependence of <E sub(mu)> super(0.76+ or -0.03) for liquid-scintillator targets.
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4.
First search for KL → π0γ Shimizu, N
Physical review. D,
09/2020, Volume:
102, Issue:
5
Journal Article
Peer reviewed
We report the first search for the KL → π0γ decay, which is forbidden by Lorentz invariance, using the data from 2016 to 2018 at the J-PARC KOTO experiment. With a single event sensitivity of (7.1 ± ...0. 3stat ± 1. 6syst) × 10−8, no candidate event was observed in the signal region. The upper limit on the branching fraction was set to be 1.7 × 10−7 at the 90% confidence level.
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In order to collect the events efficiently with a high intensity beam, we built a new trigger electronics system based on the calibrated energy information and the number of electromagnetic shower ...clusters in the calorimeter for the J-PARC KOTO experiment. The dead time of this system is 0.16 µs and the loss was measured to be less than 1%. By performing the online clustering algorithm on the simulated data, the trigger efficiency for the primary mode KL→π0νν¯ is estimated to be 99.6%.
A search for the rare decay KL → π0νν¯ was performed. With the data collected in 2015, corresponding to 2.2 × 1019 protons on target, a single event sensitivity of (1.30 ± 0.01stat ± 0.14syst) × 10–9 ...was achieved and no candidate events were observed. We set an upper limit of 3.0 × 10–9 for the branching fraction of KL → π0νν¯ at the 90% confidence level (C.L.), which improved the previous limit by almost an order of magnitude. An upper limit for KL → π0X0 was also set as 2.4 × 10–9 at the 90% C.L., where X0 is an invisible boson with a mass of 135 MeV /c2.
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