Search for supernova relic neutrino at Super-Kamiokande

Supernova relic neutrinos (SRN) is diffuse supernova neutrino background from all past supernova. No experiments has succeeded in detecting SRN yet. In this paper a search for SRN was conducted using Super-Kamiokande (SK) data. SK is a large water Cherenkov detector in Kamioka, Japan. In this analysis, 791 days' data taken in the second phase of SK (SK-II) was used and antineutrino signals in the neutrino energy range above 19.3MeV were searched. The observed spectrum is consistent with the expected background spectrum and a flux upper limit of 3.68 /cm2 /sec was obtained with 90% confidence level. A similar search using SK-I data gave a flux upper limit of >1.25 /cm2 /sec. By the combined analysis of SK-I and SK-II, a flux upper limit of > 1.08 /cm2/sec was obtained. For future we are planning to improve this analysis. As a first try of the improvement, we started study of fiducial volume enlargement. Now the volume within 2m from the inner detector wall is not used for this analysis. If we can expand 1m from current boarder line, 20% larger volume can be obtained. We show the status of this improvement too.