Search for neutron decay into an antineutrino and a neutral kaon in 0.401 megaton-years exposure of Super-Kamiokande

We searched for bound neutron decay via <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mi>n</a:mi> <a:mo stretchy="false">→</a:mo> <a:mover accent="true"> <a:mi>ν</a:mi> <a:mo stretchy="false">¯</a:mo> </a:mover> <a:mo>+</a:mo> <a:msup> <a:mi>K</a:mi> <a:mn>0</a:mn> </a:msup> </a:math> predicted by the grand unified theories in <f:math xmlns:f="http://www.w3.org/1998/Math/MathML" display="inline"> <f:mrow> <f:mn>0.401</f:mn> <f:mtext> </f:mtext> <f:mtext> </f:mtext> <f:mi>Mton</f:mi> <f:mo>·</f:mo> <f:mi>years</f:mi> </f:mrow> </f:math> exposure of all pure water phases in the Super-Kamiokande detector. About 4.4 times more data than in the previous search have been analyzed by a new method including a spectrum fit to kaon invariant mass distributions. No significant data excess has been observed in the signal regions. As a result of this analysis, we set a lower limit of <h:math xmlns:h="http://www.w3.org/1998/Math/MathML" display="inline"> <h:mn>7.8</h:mn> <h:mo>×</h:mo> <h:msup> <h:mn>10</h:mn> <h:mn>32</h:mn> </h:msup> <h:mtext> </h:mtext> <h:mtext> </h:mtext> <h:mi>years</h:mi> </h:math> on the neutron lifetime at a 90% confidence level.