91̽»¨

Abstract:

The charge-conjugation and parity-reversal (CP) symmetry of fundamental particles is a symmetry between matter and antimatter. Violation of this CP symmetry was first observed in 19641, and CP violation in the weak interactions of quarks was soon established2. Sakharov proposed3 that CP violation is necessary to explain the observed imbalance of matter and antimatter abundance in the Universe. However, CP violation in quarks is too small to 91̽»¨ this explanation. So far, CP violation has not been observed in non-quark elementary particle systems. It has been shown that CP violation in leptons could generate the matter–antimatter disparity through a process called leptogenesis4. Leptonic mixing, which appears in the standard model’s charged current interactions5,6, provides a potential source of CP violation through a complex phase δCP, which is required by some theoretical models of leptogenesis7,8,9. This CP violation can be measured in muon neutrino to electron neutrino oscillations and the corresponding antineutrino oscillations, which are experimentally accessible using accelerator-produced beams as established by the Tokai-to-Kamioka (T2K) and NOvA experiments10,11. Until now, the value of δCP has not been substantially constrained by neutrino oscillation experiments. Here we report a measurement using long-baseline neutrino and antineutrino oscillations observed by the T2K experiment that shows a large increase in the neutrino oscillation probability, excluding values of δCP that result in a large increase in the observed antineutrino oscillation probability at three standard deviations (3σ). The 3σ confidence interval for δCP, which is cyclic and repeats every 2Ï€, is [−3.41, −0.03] for the so-called normal mass ordering and [−2.54, −0.32] for the inverted mass ordering. Our results indicate CP violation in leptons and our method enables sensitive searches for matter–antimatter asymmetry in neutrino oscillations using accelerator-produced neutrino beams. Future measurements with larger datasets will test whether leptonic CP violation is larger than the CP violation in quarks.

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 $Y_n=(3.38_{−0.30}^{+0.23})×{10}^{−4}{\mathrm{cm}}^2{\mathrm{g}}^{−1}{\mathrm{μ}}^{−1}$ 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:

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 ${}^{13}\mathrm{C}+{\mathrm{ν}}_e{→}^{13}\mathrm{N}+e^{−}$ which is followed, with a 10 minute half life, by ${}^{13}\mathrm{N}{→}^{13}\mathrm{C}+e^{+}+{\mathrm{ν}}_e$ . 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 $4.2\mathrm{σ}$ . 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 ${\mathrm{ν}}_e$ reaction to the ground state of ${}^{13}\mathrm{N}$ , yielding an average cross section of $({16.1}_{−6.7}^{+8.5}(\mathrm{stat}\mathrm{.}{)}_{−2.7}^{+1.6}(\mathrm{syst}\mathrm{.}))×{10}^{−43}{\mathrm{cm}}^2$ 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 $\mathrm{Δ}{m}_{21}^2=({7.96}_{−0.42}^{+0.48})×{10}^{−5}{\mathrm{eV}}^2$ . Constraining $\mathrm{Δ}{m}_{21}^2$ and ${\sin }^2{\mathrm{θ}}_{12}$ with measurements from long-baseline reactor antineutrino and solar neutrino experiments yields $\mathrm{Δ}{m}_{21}^2=({7.58}_{−0.17}^{+0.18})×{10}^{−5}{\mathrm{eV}}^2$ and ${\sin }^2{\mathrm{θ}}_{12}=0.308Â\pm 0.013$ . This fit also yields a first measurement of the flux of geoneutrinos in the Western Hemisphere, with ${73}_{−43}^{+47}$ TNU at $\mathrm{SNO}+$ .