Stars as Laboratories for Fundamental Physics - MPP Theory Group

428 Chapter 11
Including the pinching effect discussed in Sect. 11.2.2 would enhance
the discrepancy between the signals in the two detectors. Either way,
the IMB detector with its high energy threshold is mostly sensitive to
the high-energy tail of the neutrino spectrum. Therefore, the IMBinferred
⟨E νe ⟩ depends sensitively on the assumed spectral shape and
is thus a poor indicator of the true average energies.
A more conspicuous anomaly is the 7.3 s gap between the first 9
and last 3 events at Kamiokande. Ideas proposed to explain the alleged
pulsed structure of the signal range from the occurrence of a phase
transition in the nuclear medium (pions, quarks) to a secondary collapse
to a black hole. It should be noted, however, that the gap is partially
filled in by the IMB and Baksan data, thus arguing against a physical
cause at the source. The random occurrence of a gap exceeding 7 s with
three or more subsequent events can be as high as several percent, but
naturally it is sensitive to the expected late-time signal (Lattimer and
Yahil 1989).
The most significant and thus the most troubling anomaly is the
remarkable deviation from isotropy of the events in both detectors, in
conflict with the expected signature from ν e p → ne + which actually
predicts a slight (about 10%) backward bias. LoSecco (1989) found
a probability of about 1.5% that the combined Kamiokande and IMB
data set was drawn from an isotropic distribution. Kie̷lczewska (1990)
analyzed the expected signal from standard SN cooling calculations
and found agreement only at the 0.8% CL with the measured angular
distribution. The combined set of IMB plus those Kamiokande
data which are above the IMB threshold, i.e. the combined set of
“high-energy” events is consistent with isotropy only at the 0.07% level
(van der Velde 1989); the four relevant events at Kamiokande are all
very forward.
The IMB collaboration claims that their reconstruction of the event
direction was not seriously impeded by the outage of about a quarter of
their phototubes due to the failure of a high-voltage supply. They conducted
a detailed calibration of their detector to investigate this point
(Bratton et al. 1988). Therefore, one must accept that the forwardpeaked
angular distribution shown in Fig. 11.14 is not a problem of the
detectors or the event reconstruction.
A forward-peaked distribution is expected from νe elastic scattering
which has a much lower cross section than the ν e p process (Fig. 11.8).
One expects far less than one event due to νe scattering from the cooling
phase, although the first Kamiokande event has sometimes been
interpreted as being a scattering event due to the prompt ν e burst.