Stars as Laboratories for Fundamental Physics - MPP Theory Group

516 Chapter 13
By the same token, N IMB is quite sensitive to spectral pinching, an effect
not included in these calculations where an equilibrium neutrino
transport scheme was used. Therefore, the total number of events,
notably at IMB, is a poor measure to characterize the neutrino signal.
In the calculations of Keil, Janka, and Raffelt (1995) the mass of
the initial neutron-star model as well as its temperature profile and the
equation of state were varied. While such modifications cause changes
in the predicted signal durations and event counts, none of these parameters
appears likely to be able to compensate for an extreme suppression
of the neutrino opacities. The SN 1987A neutrino signal excludes
a suppression effect stronger than, say, a > ∼ 0.5.
These findings are in agreement with those of the previous section
where the number of neutrino flavors had been increased. Essentially
that procedure amounted to increasing the efficiency of neutral-current
energy transfer and so it is not very different from the decreased opacities
used here.
Therefore, it appears that the “standard” opacities which ignore
fast spin fluctuations provide a reasonable representation of what is
observed. This result appears to imply that the spin-fluctuation rate
Γ σ does not exceed O(T ) in a SN core—see Sect. 4.6.7 for a discussion
of these matters. However, it is surprising that the best fit is achieved
by the naive opacities because the spin-fluctuation effect is only one
reason to expect reduced axial-vector opacities. Other reasons include
reduced effective values for C A in a nuclear medium, and spin-spin
correlations which tend to “pair” the spins and thus tend to reduce the
opacities. The question of the appropriate neutrino opacities in a SN
medium remains worrisome.
However, with regard to particle bounds the effect of reduced opacities
goes in the direction of making those constraints more conservative
as a reduction of the opacities, like an anomalous energy loss, shortens
the neutrino signal.
13.8 Right-Handed Neutrinos
13.8.1 Dirac Mass
Right-handed neutrinos (helicity-minus neutrinos, helicity-plus antineutrinos)
do not interact by the standard weak interactions and so
they would not be trapped in the interior of a SN core. Therefore, the
SN 1987A neutrino signal allows one to constrain any mechanism that