Diffuse supernova neutrino background search at Super-Kamiokande

Abstract

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.

Neutrino flavor evolution in dense environments and the r-process

“…The sensitivity of the combined analysis (90 % C.L.) of SK-IV and previous phases, is on par with DSNB rates from four models, whereas more conservative predictions are lower about a factor of 4 [32]. The DSNB measurement is expected with the running Super-Kamiokande (SK)+Gd and the upcoming JUNO and Hyper-Kamiokande experiments.…”

Prospects for the Detection of the Diffuse Supernova Neutrino Background with the Experiments SK-Gd and JUNO

“…Since the start of data taking on 1st April 1996, the Super-Kamiokande experiment has spent the last quarter century conducting ground-breaking studies of neutrinos from the Earth's atmosphere [26], the Sun [27,28], and long-baseline accelerator-generated beams from KEK [29] and J-PARC [30], while also searching for nucleon decay [31][32][33][34], dark matter [35,36], and both galactic [37,38] and diffuse supernova neutrinos [12,[39][40][41]. The discovery of neutrino oscillations in SK's atmospheric neutrino data resulted in a share of the 2015 Nobel Prize in physics, while those results plus SK's solar and long-baseline neutrino measurements led to a share of two 2016 Breakthrough Prizes.…”

“…Based on these numbers, it becomes possible to estimate the experimental sensitivities of the individual and combined measurements. While the eventual DSNB analyses will apply more sophisticated techniques, here we restrict ourselves to a simple count rate analysis for signal and background in the energy window of interest (12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30). As a measure of sensitivity, we adopt the ratio S/ √ S + B, i.e., the significance of the signal strength over the expected statistical variation of the count rate.…”

“…Detector target masses on the order of ∼10 kilotons are required to obtain one signal event per year. The current best upper limit on the DSNB's νe flux component is held by the Super-Kamiokande (Super-K, SK) water Cherenkov experiment at 2.7 cm −2 s −1 above 17.3 MeV [12]. This result is already cutting into the parameter range predicted by current DSNB models (e.g., [11]).…”

“…Therefore, the 9 Li/ 8 He background can be effectively reduced by muon veto strategies. Moreover, excellent energy resolution at JUNO will ensure most of the 9 Li/ 8 He background below 12 MeV of the visible energy and can be safely neglected if 12 MeV is chosen as the lower boundary of the search window.…”