The RENO experiment reports more precisely measured values of θ13 and |∆m 2 ee | using ∼2 200 live days of data. The amplitude and frequency of reactor electron antineutrino (νe) oscillation are measured by comparing the prompt signal spectra obtained from two identical near and far detectors. In the period between August 2011 and February 2018, the far (near) detector observed 103 212 (850 666) νe candidate events with a background fraction of 4.8% (2.0%). A clear energy and baseline dependent disappearance of reactor νe is observed in the deficit of the measured number of νe. Based on the measured far-to-near ratio of prompt spectra, we obtain sin 2 2θ13 = 0.0896 ± 0.0048(stat) ± 0.0047(syst) and |∆m 2 ee | = [2.68 ± 0.12(stat) ± 0.07(syst)] × 10 −3 eV 2 .
A new search for the diffuse supernova neutrino background (DSNB) flux has been conducted at Super-Kamiokande (SK), with a 22.5 × 2970-kton•day exposure from its fourth operational phase IV. The new analysis improves on the existing background reduction techniques and systematic uncertainties and takes advantage of an improved neutron tagging algorithm to lower the energy threshold compared to the previous phases of SK. This allows for setting the world's most stringent upper limit on the extraterrestrial νe flux, for neutrino energies below 31.3 MeV. The SK-IV results are combined with the ones from the first three phases of SK to perform a joint analysis using 22.5 × 5823 kton•days of data. This analysis has the world's best sensitivity to the DSNB νe flux, comparable to the predictions from various models. For neutrino energies larger than 17.3 MeV, the new combined 90% C.L. upper limits on the DSNB νe flux lie around 2.7 cm −2 •sec −1 , strongly disfavoring the most optimistic predictions. Finally, potentialities of the gadolinium phase of SK and the future Hyper-Kamiokande experiment are discussed.
We report a fuel-dependent reactor electron antineutrino (νe) yield using six 2.8 GW th reactors in the Hanbit nuclear power plant complex, Yonggwang, Korea. The analysis uses 850 666 νe candidate events with a background fraction of 2.0 % acquired through inverse beta decay (IBD) interactions in the near detector for 1807.9 live days from August 2011 to February 2018. Based on multiple fuel cycles, we observe a fuel 235 U dependent variation of measured IBD yields with a slope of (1.51 ± 0.23) × 10 −43 cm 2 /fission and measure a total average IBD yield of (5.84 ± 0.13) × 10 −43 cm 2 /fission. The hypothesis of no fuel-dependent IBD yield is ruled out at 6.6 σ. The observed IBD yield variation over 235 U isotope fraction does not show significant deviation from the Huber-Mueller (HM) prediction at 1.3 σ. The measured fuel-dependent variation determines IBD yields of (6.15 ± 0.19) × 10 −43 cm 2 /fission and (4.18 ± 0.26) × 10 −43 cm 2 /fission for two dominant fuel isotopes 235 U and 239 Pu, respectively. The measured IBD yield per 235 U fission shows the largest deficit relative to the HM prediction. Reevaluation of the 235 U IBD yield per fission may mostly solve the Reactor Antineutrino Anomaly (RAA) while 239 Pu is not completely ruled out as a possible contributor of the anomaly. We also report a 2.9 σ correlation between the fractional change of the 5 MeV excess and the reactor fuel isotope fraction of 235 U.A definitive measurement of the smallest neutrino mixing angle θ 13 is a tremendous success in neutrino physics during the last decade [1,2]. The measurement has been achieved by comparing the observed ν e fluxes with detectors placed at two different distances from the reactors. As reactor ν e experiments suffer from large reactor related uncertainties of the expected ν e flux and energy spectrum [3][4][5][6][7], identical detector configuration is essential to cancel out the systematic uncertainties. The RAA, ∼6 % deficit of measured ν e flux compared to the HM prediction, is an intriguing mystery in current neutrino physics research and needs to be understood [4][5][6][8][9][10][11]. There have been numerous attempts to explain this anomaly by incorrect inputs to the fission β spectrum conversion, deficiencies in nuclear databases, underestimated uncertainties of reactor ν e model, and the existence of sterile neutrinos [3,[12][13][14][15][16][17][18][19]. Moreover, all of ongoing reactor ν e experiments have observed a 5 MeV excess in the IBD prompt spectrum with respect to the expected one [8,9,20,21]. This suggests that reactor ν e model is not complete at all.In commercial nuclear reactor power plants, almost all (> 99 %)ν e 's are produced through thousands of β-decay branches of fission fragments from 235 U, 239 Pu, 238 U, and 241 Pu. The ν e flux calculation is based on the inversion of spectra of the β-decay electrons of the thermal fissions which were measured in 1980s at ILL [10,11]. The reactor ν e models using these measurements as inputs have large uncertainties [5][6][7]. Therefore, ree...
Core-collapse supernovae are among the most magnificent events in the observable universe. They produce many of the chemical elements necessary for life to exist and their remnants-neutron stars and black holes-are interesting astrophysical objects in their own right. However, despite millennia of observations and almost a century of astrophysical study, the explosion mechanism of core-collapse supernovae is not yet well understood.
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