Search for proton decay via $p\to e^{+}\ddot{I}{€}^0$ and $p\to \hat{I}{¼}^{+}\ddot{I}{€}^0$ with an enlarged fiducial volume in Super-Kamiokande I-IV

Abstract

We have searched for proton decay via $p\to e^{+}\ddot{I}{€}^0$ and $p\to \hat{I}{¼}^{+}\ddot{I}{€}^0$ 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 $p\to e^{+}\ddot{I}{€}^0$ and one candidate remains for $p\to \hat{I}{¼}^{+}\ddot{I}{€}^0$ 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: $\ddot{I}„/B(p\to e^{+}\ddot{I}{€}^0)>2.4\times 10^{34}$ years and $\ddot{I}„/B(p\to \hat{I}{¼}^{+}\ddot{I}{€}^0)>1.6\times 10^{34}$ years at 90% confidence level.

16 pages, 10 figures