Abstract
We have searched for proton decay via $p\to e^+\pi^0$ and $p\to \mu^+\pi^0$ modes with the enlarged fiducial volume data of Super-Kamiokande from April 1996 to May 2018, which corresponds to 450 kton$\cdot$years exposure. We have accumulated about 25% more livetime and enlarged the fiducial volume of the Super-Kamiokande detector from 22.5 kton to 27.2 kton for this analysis, so that 144 kton$\cdot$years of data, including 78 kton$\cdot$years of additional fiducial volume data, has been newly analyzed. No candidates have been found for $p\to e^+\pi^0$ and one candidate remains for $p\to \mu^+\pi^0$ in the conventional 22.5 kton fiducial volume and it is consistent with the atmospheric neutrino background prediction. We set lower limits on the partial lifetime for each of these modes: $\tau/B(p\to e^+\pi^0) > 2.4 \times 10^{34}$ years and $\tau/B(p\to \mu^+\pi^0) > 1.6 \times 10^{34}$ years at 90% confidence level.
Highlights
Grand unified theories (GUTs) [1] extend the Standard Model gauge symmetry to larger symmetry groups and provide explanations for the quantization of electric charge and predict the convergence of the electromagnetic, weak, and strong interaction couplings at energies around ∼1016 GeV [2]
Since recent theoretical studies [9,10] show that the preferred models of GUTs may be revealed by the first signs of proton decay, it is important to search for both modes
This paper describes a search for these two proton decay modes using this updated dataset and is organized as follows
Summary
Grand unified theories (GUTs) [1] extend the Standard Model gauge symmetry to larger symmetry groups and provide explanations for the quantization of electric charge and predict the convergence of the electromagnetic, weak, and strong interaction couplings at energies around ∼1016 GeV [2]. Since recent theoretical studies [9,10] show that the preferred models of GUTs may be revealed by the first signs of proton decay, it is important to search for both modes Both modes produce back-to-back event topologies in which all final state particles are visible making it possible to cleanly separate a proton decay signal from atmospheric neutrino backgrounds in water Cherenkov detectors. Free protons (hydrogen nuclei) in their water provide enhanced background rejection capabilities since a decay therein would be free from the effects of the Fermi motion and intranuclear scattering processes that alter the final state particles from decays within 16O These decay modes have been the target of several experimental searches, but there have been no positive observations so far [11].
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