Search for nucleon decay via $p\rightarrowνπ^{+}$ and $n\rightarrowνπ^{0}$ in 0.484 Mton-year of Super-Kamiokande data

TL;DR

This work reports a search for nucleon decays p→νπ⁺ and n→νπ⁰ using 0.484 Mt·yr of Super-Kamiokande I–V data. It employs a spectrum-based fit to the reconstructed total momentum with a comprehensive treatment of systematic uncertainties, including atmospheric neutrino flux and pion production, and benefits from an expanded fiducial volume and updated pion-nucleus interaction models. No evidence for nucleon decay is found, and 90% CL lower limits on partial lifetimes are established, tightening constraints on grand unified theories. The results provide robust constraints on baryon-number-violating processes and inform the design of future large-volume detectors and theoretical models of nucleon decay.

Abstract

We present the results of searches for nucleon decays via $p\rightarrowνπ^{+}$ and $n\rightarrowνπ^{0}$ using a 0.484 Mt$\cdot$yr exposure of Super-Kamiokande I-V data covering the entire pure water phase of the experiment. Various improvements on the previous 2014 nucleon decay search, which used an exposure of 0.173 Mt$\cdot$yr, are incorporated. The physics models related to pion production and nuclear interaction are refined with external data, and a more comprehensive set of systematic uncertainties, now including those associated with the atmospheric neutrino flux and pion production channels is considered. Also, the fiducial volume has been expanded by 21\%. No significant indication of a nucleon decay signal is found beyond the expected background. Lower bounds on the nucleon partial lifetimes are determined to be $3.5\times10^{32}$~yr for $p\rightarrowνπ^{+}$ and $1.4\times10^{33}$~yr for $n\rightarrowνπ^{0}$ at 90\% confidence level.

Figures (3)

• Figure 1: True neutral pion initial momentum by $n\rightarrow\nu\pi^0$ MC (left) and cumulative fractions of nuclear interactions for neutral pion as a function of its true momentum predicted by previous model (center) PhysRevLett.113.121802 and updated model (right) PhysRevD.95.012004. The fraction for each nuclear interaction is labeled at the corresponding color-filled region. In the figures, the "No interaction" means the neutral pion escapes the nucleus without nuclear interaction. For the nuclear interactions, the "Absorption" is denoted for pion absorption, "Scattering" indicates pion scattering events, the "Charge exchange" is the case where the pion charge is exchanged, and the "Particle Production" is for the production of multiple hadrons.
• Figure 2: Expected breakdown of background spectra for $p\rightarrow\nu\pi^{+}$ search with zero-decay-electron sample (left) and one-decay-electron sample (right). The breakdown by neutrino interactions includes charged-current quasi-elastic scattering with one-nucleon knockout (green) and two-nucleon knockout (red) by meson exchange current, single-$\pi$ production (blue), deep-inelastic scattering (magenta), and neutral-current interactions (orange). The MC event rate for each SK phase is scaled based on the SK I-V livetime, respectively.
• Figure 3: Reconstructed momentum distributions for SK data (black dots), the best fit for atmospheric neutrino background and nucleon decay Monte Carlo simulation (red line), 90% confidence level allowed amount of nucleon decay (blue hatched histogram). The dashed blue line shows how a positive signal of nucleon decay would look, corresponding to five times the limit we set on the decay partial lifetimes. The plots correspond to the fitted spectra and data points for the $p\rightarrow\nu\pi^{+}$ search, with 0-decay-e sample (left), 1-decay-e sample (center), and the $n\rightarrow\nu\pi^{0}$ search (right).