Search for nucleon decay into lepton${+K}^0$ final states using Soudan 2

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 ${\mu }^{+}{K}_S^0$ and $e^{+}{K}_S^0,$ zero and one event candidates are observed respectively. The lifetime lower limits $(\tau /B)$ thus implied are $1.5\times {10}^{32}\mathrm{yr}$ and $1.2\times {10}^{32}\mathrm{yr},$ respectively. Lifetime lower limits are also reported for proton decay into $l^{+}{K}_l^0,$ and for neutron decay into $\nu K_S^0.$