Search for neutral Q-balls in Super-Kamiokande II

A search for Q-balls has been carried out in Super-Kamiokande II with 541.7 days of live time. A neutral Q-ball passing through the detector can interact with nuclei to produce pions, generating a signal of successive contained pion events along a track. No candidate for successive contained event groups has been found in Super-Kamiokande II, so we obtain upper limits on the flux of such Q-balls.