Astroparticle Physics

6.2 Neutrino Astronomy 101In the hot phase of the collapse corresponding to a temperatureof 10 MeV (≈ 10 11 K), the thermal photons producedelectron–positron pairs which, however, were immediatelyabsorbed because of the high density of the surroundingmatter. Only the weak-interaction process with a virtual Z,e + + e − → Z → ν α +¯ν α , (6.39)allowed energy to escape from the hot stellar nucleus in theform of neutrinos. In this reaction all three neutrino flavoursν e ,ν µ ,andν τ were produced ‘democratically’ in equal numbers.The total neutrino burst comprised 10 58 neutrinos andeven at Earth the neutrino flux from the supernova was comparableto that of solar neutrinos for a short period.Actually, the neutrino burst of the supernova was the firstsignal to be registered on Earth. The large water Cherenkovcounters of Kamiokande and IMB (Irvine–Michigan–Brookhaven) recorded a total of 20 out of the emitted 10 58neutrinos. The energy threshold of the Kamiokande experimentwas as low as 5 MeV. In contrast, the IMB collaborationcould only measure neutrinos with energies exceeding19 MeV. The Baksan liquid scintillator was lucky to record– even though their fiducial mass was only 200 t – five coincidentevents with energies between 10 MeV and 25 MeV.Since the neutrino energies in the range of 10 MeV areinsufficient to produce muons or taus, only electron-typeneutrinos were recorded via the reactions¯ν e + p → e + + n,¯ν e + e − →¯ν e + e − , (6.40)ν e + e − → ν e + e − .In spite of the low number of measured neutrinos onEarth some interesting astrophysical conclusions can bedrawn from this supernova explosion. If Eν i is the energy ofindividual neutrinos measured in the detector, ε 1 the probabilityfor the interaction of a neutrino in the detector, and ε 2the probability to also see this reaction, then the total energyemitted in form of neutrinos can be estimated to bemeasurement of neutrinosenergy outputE total =20∑i=1E i νε 1 (E i ν )ε 2(E i ν ) 4πr2 f(ν α , ¯ν α ), (6.41)where the correction factor f takes into account that thewater Cherenkov counters are sensitive not to all neutrino