Prof Dr Armin Reichold

Abstract:

This paper demonstrates a novel method to extract photomultiplier tube (PMT) calibration timing constants in large liquid scintillation detectors from physics data using the machinery of unsupervised deep learning. The approach uses a simplified physical model of optical photon transport in the loss function, with PMT calibration constants treated as free parameters, and the simple assumption that individual events represent point-like emission. The problem is, thus, effectively reduced to that of regression on a very large scale, made tractable by deep learning architectures and automatic differentiation frameworks. Using data from the 9300 PMTs in the SNO+ detector, the method has been shown to reliably extract 3 calibration constants for each of the over 7500 online PMTs using radioactive background events. We believe that this basic approach can be straightforwardly generalized for a wide range of applications.

Abstract:

The SNO+ Collaboration reports new results on reactor antineutrino oscillations using data acquired from May 2022 through July 2025. The spectral analysis of a flux dominated by nuclear reactors at 240, 340, and 350 km yields the mass-squared difference $\mathrm{\Delta }{m}_{21}^2=(7.91_{-0.25}^{+0.22})\times {10}^{-5}{\mathrm{eV}}^2$ . This result is compatible with and approaches the precision of the only other long-baseline reactor antineutrino measurement, by KamLAND. Combining these measurements, along with those from solar neutrino experiments, the global values of the neutrino mixing parameters become $\mathrm{\Delta }{m}_{21}^2=(7.59\pm 0.17)\times {10}^{-5}{\mathrm{eV}}^2$ and ${\sin }^2{\theta }_{12}=0.310\pm 0.012$ . The analysis of geoneutrinos at $\mathrm{SNO}+$ is also improved, with a measured signal of ${48}_{-12}^{+14}$ TNU.

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.