SN1987A AND PROPERTIES OF THE NEUTRINO BURST

1. M.L. Costantinia, A. Iannib, F. Vissanib a University of L’Aquila, Italy b INFN, Gran Sasso Laboratory, Assergi (AQ), Italy A Supernova explosion in the Large Magellanic Cloud at about 50kpc was observed in 1987 (SN1987A). SN1987A was the closest visual SN since 1604 (Kepler). SN1987A marked the beginning of (extra)galactic neutrino astronomy. A neutrino burst was observed by 3 experiments, two water Čerenkov detector (Irvine- Michigan-Brookhaven and Kamiokande-II) and a liquid scintillator detector (Baksan Scintillator Telescope). The burst was detected about 3 hours before the first evidence for optical brigthening. A fourth neutrino observation was claimed by the Mont Blanc Liquid Scintillator Detector (LSD) in correlation with Geograv detector, but about 5 hours earlier than the one reported by IMB, Kam-II and Baksan. Neutrino observations from SN1987A give important information on the Physics of the explosion. As an example we consider the Kam-II experiment (2.14kton fiducial mass water Čerenkov detector) where ~12 events were observed. Estimation of irradiated energy in anti-ne SN1987A AND PROPERTIES OF THE NEUTRINO BURST n’s observations in IMB and Kam-II n’s spectra and cross-sections Inverse Beta Decay Hypothesis Neutrino Oscillations Neutrino Fluxes Experimental observations from solar and atmospheric neutrinos show evidence for neutrino oscillations. Following the (simple minded) theoretical expectations, the emitted fluxes from SN1987A should be modified as follows: Due to the fact that the IBD cross section is the largest, it is reasonable to assume as a first approximation that the most (or all) the observed events are from IBD. We have use this argument to tune the parameters which enter in the SN explosion Mechanism, namely the irradiated energy and the mean energy. A simple model of the Supernova neutrinos fluxes attributes the following spectra (with three different average energies E0) to any species , x being any among muon and tau (anti) neutrinos: A good compromise between observations and predictions can help setting the SN parameters for anti-ne, as shown in the Figure where the two probabilities of survival are: Angular distribution and ES Smearing effect We have used the Smirnov-Cramer-Von Mises statistic to check the observed angular distributions under different Hypotheses (IBD and ES). We have compared the outcome of this test with that of a Maximum Likelihood analysis. Due to multiple scattering and angular dispersion effects the emitted angular distribution for ES, rES, gets smeared. we can take into account these effects considering the following “smearing” distribution: Relative percentage probabilities to have a given number of ES events in Kam-II dataset, estimated from observed angular distribution and theoretical expectations on the fluxes, for 3 hypotheses on εx A significant anomaly of the observed data from the SN1987A comes from the angular distribution which deviates from the approximate isotropy one expects on the basis of the dominant IBD contribution. Moreover, the most forward-peaked events are also the most energetic ones. We have shown that neutrino oscillations could account for this ( ). We have shown that the probability to have one ES event in Kam-II is comparable to the one of having zero events. The probability that one event has an energy larger than 20 MeV is about 16%, as shown in the Figure. At this point, it is interesting to investigate the question under which conditions we can increase the expected number of ES events. We have analyzed certain possibilities. 1) ne have large energy (up to 40 MeV). In this case, though, oscillations transform the emitted ne into nx which, due to the smaller cross section, are not increasing significantly the number of forward events. 2) we can assume that nx are more energetic than expected. 3) a large amount of neutrino radiation comes from and nm (with energy ~30 MeV) are detected as ne due to oscillations.