Search for nucleon decay into lepton + K0 final states using Soudan 2

TL;DR

This study searches for nucleon decay into lepton and K0 meson final states using the Soudan 2 detector, setting new lower limits on proton and neutron decay lifetimes based on observed event candidates.

Contribution

It provides the first experimental search for nucleon decay into lepton plus K0 states with the Soudan 2 detector, establishing new lifetime lower bounds.

Findings

No candidate events for proton decay into ^+K^0_s.

One candidate event for proton decay into e^+K^0_s.

Lower limits on nucleon decay lifetimes are set at 1.5 ^{32} and 1.2 ^{32} years.

Abstract

A search for nucleon decay into two-body final states containing K^0 mesons has been conducted using the 963 metric ton Soudan 2 iron tracking calorimeter. The topologies, ionizations, and kinematics of contained events recorded in a 5.52 kiloton-year total exposure (4.41 kton-year fiducial volume exposure) are examined for compatibility with nucleon decays in an iron medium. For proton decay into the fully visible final states \mu^+K^0_s and e^+K^0_s, zero and one event candidates are observed respectively. The lifetime lower limits (\tau /B) thus implied are 1.5 \times 10^{32} years and 1.2 \times 10^{32} years, respectively. Lifetime lower limits are also reported for proton decay into l^+K^0_l, and for neutron decay into \nu K^0_s.