Stars as Laboratories for Fundamental Physics - MPP Theory Group

422 Chapter 11
thus be simultaneously attributed to SN 1987A. The events are shown
in the t-E-plane in Fig. 11.13 and for the Cherenkov detectors in the
cos Θ-E-plane in Fig. 11.14 where Θ is the angle relative to the opposite
direction of the SN, i.e. relative to the direction of the neutrino flux.
In principle, the bursts of events observed in the detectors could be
due to rare background fluctuations rather than due to SN 1987A. The
Kamiokande group has performed a detailed analysis of this possibility
by analyzing the multiplicity of events in 10 s time intervals, i.e. the
number of chance events in an arbitrarily chosen 10 s time interval.
They found a probability of about 0.6×10 −7 that the observed burst is
a random fluctuation of a constant background.
However, there are time-correlated backgrounds, notably the spallation
of oxygen induced by primary muons which can cause clusters of
events with large multiplicities; the Kamiokande group found one clus-
Fig. 11.14. SN 1987A neutrinos at Kamiokande and IMB, excluding the ones
which likely are due to background.