Stars as Laboratories for Fundamental Physics - MPP Theory Group

Supernova Neutrinos 429
The forward events in both detectors have relatively large energies
compared with the isotropic ones at Kamiokande, 66 contrary to what
would be expected from νe → eν where some of the energy is carried
away by the secondary neutrino. Assuming a larger-than-standard flux
of ν e ’s with larger-than-standard energies (LoSecco 1989) does not solve
the problem because above 30−35 MeV the process ν 16 e O → 16 F e −
takes over (Fig. 11.8) which has a backward bias.
Anomalously large fluxes of ν µ,τ or ν µ,τ are difficult to arrange on
energetic grounds—the binding energy of the neutron star is limited.
Even allowing for extreme values of E b and extreme temperature differences
between (anti)electron neutrinos and the other flavors improves
the agreement only marginally; one can achieve an agreement at the
5% CL with the observed angular distribution (Kie̷lczewska 1990). The
simple problem with elastic νe scattering to explain the data is that
this process is too strongly forward peaked, especially for high-energy
neutrinos, hence it does not fit the data very well either. This is especially
true for the IMB events which are selected for high energies by
the detector threshold, and yet are very broadly distributed around the
forward direction.
A very speculative idea was put forth by van der Velde (1989) who
proposed the existence of a new neutral boson X ◦ which could produce
photons when interacting with nucleons. These MeV photons
would look very similar to charged particles in the detectors. In the
forward direction, the X ◦ cross section on 16 O would be coherently enhanced,
causing the observed forward bias for high-energy events while
low-energy ones would naturally follow a more isotropic distribution.
However, the opposite process γ + 4 He → 4 He + X ◦ would then contribute
to the energy-loss of horizontal-branch stars (Raffelt 1988b).
The resulting bound on the interaction cross section (Tab. 2.5), valid
at an energy of about 10 keV, excludes van der Velde’s scenario unless
σ increases with energy at least as E 2 , a scaling which could bring it
up to the requisite level for E in the 10 MeV range, relevant for the SN
detection. The Primakoff conversion of axions or similar particles on
oxygen is much too inefficient in view of the restrictive limits on the
axion-photon interaction strength.
In summary, the angular and energy distributions of the IMB and
Kamiokande events appear to indicate a low-energy isotropic and a
66 The Kamiokande events have an obvious correlation between energy and direction.
The application of Spearman’s rank-ordering test (e.g. Press et al. 1986)
gives a confidence level of 0.06% where event No. 6 was excluded as background.
Therefore, the Kamiokande data alone show a fairly significant “angular anomaly.”