SNO+

Abstract:

Accurate measurement of the cosmogenic muon-induced neutron yield is crucial for constraining a significant background in a wide range of low-energy physics searches. Although previous underground experiments have measured this yield across various cosmogenic muon energies, $\mathrm{SNO}+$ is uniquely positioned due to its exposure to one of the highest average cosmogenic muon energies at 364 GeV. Using ultrapure water, we have determined a neutron yield of $Y_n=(3.38_{-0.30}^{+0.23})\times {10}^{-4}{\mathrm{cm}}^2{\mathrm{g}}^{-1}{\mu }^{-1}$ at $\mathrm{SNO}+$ . Comparison with simulations demonstrates clear agreement with the neutron production model, highlighting discrepancies with the widely used 4 model. Furthermore, this measurement reveals a lower cosmogenic neutron yield than that observed by the SNO experiment, which used heavy water under identical muon flux conditions. This result provides new evidence that nuclear structure and target material composition significantly influence neutron production by cosmogenic muons, offering fresh insight with important implications for the design and background modeling of future underground experiments.

Abstract:

This paper presents the expected sensitivity to the neutrino oscillation parameters of the Hyper-Kamiokande long-baseline program. The Hyper-Kamiokande experiment, currently under construction in Japan, will measure the oscillations of accelerator-produced neutrinos with thousands of selected events per sample: this corresponds to an increase of statistics of a factor 25–100 with respect to recent results from the currently-running long-baseline neutrino oscillation experiment in Japan, T2K. In the most favorable scenario we will achieve the discovery of Charge-Parity (CP) violation in neutrino oscillation at 5σ$$5\sigma $$ C.L. in less than 3 years. With 10 years of data-taking, and assuming a neutrino : antineutrino beam running ratio of 1:3, a CP violation discovery at 5σ$$5\sigma $$ C.L. is possible for more than 60% of the actual values of the CP-violating phase, δCP.$$\delta _{CP}.$$ Moreover, we will measure δCP$$\delta _{CP}$$ with a precision ranging from 20∘,$$^{\circ },$$ in the case of maximal CP violation, to 6∘,$$^{\circ },$$ in the case of CP conservation. We aim to achieve a 0.5% resolution on the Δm322$$\Delta m^2_{32}$$ parameter, and a resolution between 3% and 0.5% on the sin2θ23$$\sin ^2\theta _{23}$$ parameter, depending on its true value. These results are obtained by extending the analysis methods of T2K with dedicated tuning to take into account the Hyper-Kamiokande design: the larger far detector, the more powerful beam, the upgraded near detector ND280, and the planned additional Intermediate Water Cherenkov Detector.

Abstract:

The $\mathrm{SNO}+$ Collaboration reports the first evidence of ${}^8\mathrm{B}$ solar neutrinos interacting on ${}^{13}\mathrm{C}$ nuclei. The charged current interaction proceeds through ${}^{13}\mathrm{C}+{\nu }_e{\to }^{13}\mathrm{N}+e^{-}$ which is followed, with a 10 minute half life, by ${}^{13}\mathrm{N}{\to }^{13}\mathrm{C}+e^{+}+{\nu }_e$ . The detection strategy is based on the delayed coincidence between the electron and the positron. Evidence for the charged current signal is presented with a significance of $4.2\sigma$ . Using the natural abundance of ${}^{13}\mathrm{C}$ present in the scintillator, 5.7 metric tons of ${}^{13}\mathrm{C}$ over 231 days of data were used in this analysis. The ${5.6}_{-2.3}^{+3.0}$ observed events in the data set are consistent with the expectation of ${4.7}_{-1.3}^{+0.6}$ events. This result is the second real-time measurement of CC interactions of ${}^8\mathrm{B}$ neutrinos with nuclei and constitutes the lowest energy observation of neutrino interactions on ${}^{13}\mathrm{C}$ generally. This enables the first direct measurement of the CC ${\nu }_e$ reaction to the ground state of ${}^{13}\mathrm{N}$ , yielding an average cross section of $({16.1}_{-6.7}^{+8.5}(\mathrm{stat}\mathrm{.}{)}_{-2.7}^{+1.6}(\mathrm{syst}\mathrm{.}))\times {10}^{-43}{\mathrm{cm}}^2$ over the relevant ${}^8\mathrm{B}$ solar neutrino energies.

Abstract:

$\mathrm{SNO}+$ Collaboration reports its second spectral analysis of reactor antineutrino oscillation using 286 ton-yr of new data. The measured energies of reactor antineutrino candidates were fitted to obtain the second-most precise determination of the neutrino mass-squared difference $\mathrm{\Delta }{m}_{21}^2=({7.96}_{-0.42}^{+0.48})\times {10}^{-5}{\mathrm{eV}}^2$ . Constraining $\mathrm{\Delta }{m}_{21}^2$ and ${\sin }^2{\theta }_{12}$ with measurements from long-baseline reactor antineutrino and solar neutrino experiments yields $\mathrm{\Delta }{m}_{21}^2=({7.58}_{-0.17}^{+0.18})\times {10}^{-5}{\mathrm{eV}}^2$ and ${\sin }^2{\theta }_{12}=0.308\pm 0.013$ . This fit also yields a first measurement of the flux of geoneutrinos in the Western Hemisphere, with ${73}_{-43}^{+47}$ TNU at $\mathrm{SNO}+$ .