The Diffuse Supernova Neutrino Background

The diffuse supernova neutrino background (DSNB) is the weak glow of megaelectronvolt neutrinos and antineutrinos from distant core-collapse supernovae. The DSNB has not been detected yet, but the Super-Kamiokande (SK) 2003 upper limit on the flux is close to predictions, now quite precise, that are based on astrophysical data. If SK is modified with dissolved gadolinium to reduce detector backgrounds and increase the energy range for analysis, then it should detect the DSNB at a rate of a few events per year, providing a new probe of supernova neutrino emission and the cosmic core-collapse rate. If the DSNB is not detected, then new physics will be required. Neutrino astronomy, although uniquely powerful, has proven extremely difficult—only the Sun and the nearby Supernova 1987A have been detected to date—so the promise of detecting new sources soon is exciting indeed.