Search for the Decay K0L → π0νν

This thesis describes the results of a search for the decay KL0 →π0νν performed by the E391a experiment at KEK 12 GeV Proton Synchrotron. We analyzed full data taken in the second run period in 2005. A blind analysis technique was employed, and we established the background level estimation before examining the candidate events in the signal region. No events were observed with the expected background of 0.42 ± 0.14, and we set an upper limit of 6.7 × 10−8 at the 90% confidence level, which represents the improvement by a factor of 3.1 over the past limit.