SN1987A AND PROPERTIES OF THE NEUTRINO BURST

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SN1987A AND PROPERTIES OF THE NEUTRINO BURST
M.L. Costantinia, A. Iannib, F. Vissanib a
University of LAquila, 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 Cerenkov detector (Irvine- Michigan-Broo
khaven 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 Cerenkov detector)
where 12 events were observed.
Estimation of irradiated energy in anti-ne
ns observations in IMB and Kam-II
ns 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 ex
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.