Search for proton decay via $p\to e^+\pi^0$ and $p\to \mu^+\pi^0$ with an enlarged fiducial volume in Super-Kamiokande I-IV

TL;DR

This study searched for proton decay in specific modes using an enlarged fiducial volume in Super-Kamiokande, setting new lower lifetime limits with no significant decay candidates observed.

Contribution

It reports the first search for proton decay modes with an enlarged fiducial volume, improving lifetime limits and utilizing 450 kton·years of data from Super-Kamiokande I-IV.

Findings

No candidates for $p\to e^+\pi^0$ decay.

One candidate for $p\to \mu^+\pi^0$ consistent with background.

Set lower lifetime limits: $2.4 \times 10^{34}$ and $1.6 \times 10^{34}$ years.

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.