Cosmic star formation history and the future observation of supernova relic neutrinos

We investigate the flux and event rate of supernova relic neutrinos (SRNs) and discuss their implications for the cosmic star formation rate. Since SRNs are diffuse neutrino background emitted from past core-collapse supernova explosions, they contain fruitful information on the supernova rate in the past and present universe, as well as on the supernova neutrino spectrum itself. As reference models, we adopt the supernova rate model based on recent observations and the supernova neutrino spectrum numerically calculated by several groups. In the detection energy range E-e > 10 MeV, which will possibly be a background-free region in the near future, the SRN event rate is found to be 1 - 2 yr(-1) at a water Cerenkov detector with a fiducial volume of 22.5 kton, depending on the adopted neutrino spectrum. We also simulate the expected signal with one set of the reference models by using the Monte Carlo method and then analyze these pseudodata with several free parameters, obtaining the distribution of the best-fit values for them. In particular, we use a parameterization such that R-SN(z) R-SN(0)(1+z)(alpha), where R-SN(z) is the comoving supernova rate density at redshift z and R-SN(0) and alpha are free parameters, assuming that the supernova neutrino spectrum and luminosity are well understood by way of a future Galactic supernova neutrino burst or the future development of numerical supernova simulations. The obtained 1 sigma errors for these two parameters are found to be deltaalpha/[alpha] = 30% (7.8%) and deltaR(SN)(0)/[R-SN(0)] = 28% (7.7%) for a detector with an effective volume of 22.5 kton x 5 yr (440 kton x 5 yr), where one of the parameters is fixed. On the other hand, if we fix neither of the values for these two parameters, the expected errors become rather large, deltaalpha/[alpha] = 37% and deltaR(SN)(0)[R-SN(0)] 55%, even with an effective volume of 440 kton x 5 yr.