  We measured the neutron time-of-flight spectrum in the ^4He(stopped K^-, n)
reaction by stopping negative kaons in a superfluid helium target.
  A clear enhancement was observed in the neutron spectrum, which indicates the
formation of a strange tribaryon of charge +1 with a mass and width of M = 3141
+/- 3 (stat.) +4/-1 (sys.) MeV/c^2 and G < 23 MeV/c^2. This state, denoted as
S^+(3140), is about 25 MeV/c^2 higher than the previously observed S^0(3115)
T=1.

Nuclear Experiment