Stars as Laboratories for Fundamental Physics - MPP Theory Group

What Have We Learned from SN 1987A 517
could produce these “wrong-helicity” states. 86 The main possibilities
are the existence of novel r.h. interactions which couple directly to r.h.
neutrinos, the existence of neutrino magnetic or electric dipole moments
which allow for left-right scatterings or magnetic oscillations, and the
existence of neutrino Dirac masses. Of course, these possibilities are
not necessarily distinct as the existence of r.h. currents or a Dirac mass
would usually also induce magnetic dipole moments.
Beginning with the assumption that neutrinos have a Dirac mass,
the mismatch between chirality and helicity for massive fermions implies
that in purely l.h. interactions a final-state neutrino or antineutrino
sometimes has the “wrong” helicity and thus nearly r.h. chirality.
Then it is essentially noninteracting and thus may escape almost freely.
In principle, there are two production channels, the spin-flip scattering
of trapped l.h. states, and the production of pairs ν L ν R or ν R ν L by
the medium. In Sect. 4.10 it was shown that the neutrino phase space
favors the spin-flip process by a large margin. Moreover, in a nonrelativistic
medium the spin-flip scattering rate was found to be simply
the nonflip scattering rate times the factor (m ν /2E ν ) 2 . The energyloss
rate was then given by Q scat in Eq. (4.94) in terms of a dynamic
structure function of the medium. 87
In a dilute medium consisting of only one species of nucleons one
has S(ω) = (CV 2 + 3CA) 2 2πδ(ω), leading to an energy-loss rate of
ϵ R = 3(C2 V + 3CA) 2 G 2 Fm 2 νT 4
2π 3 m N
( )
≈ 0.7×10 19 erg g −1 s −1 mν 2 ( )
T 4
, (13.14)
30 keV 30 MeV
where C 2 V +3C 2 A ≈ 1 was used (see Appendix B). Even though the axialvector
structure function is not a δ function, Eq. (13.14) is probably a
reasonable estimate because the results of the previous section indicate
that the neutrino scattering rate in a dense medium is probably not
86 Such bounds naturally can be avoided if one assumes that the r.h. neutrinos
have other novel interactions which are strong enough to trap them efficiently in a
SN core. Explicit models were constructed, for example, by Babu, Mohapatra, and
Rothstein (1992) or Rajpoot (1993). These authors aimed at avoiding the SN 1987A
bound on Dirac neutrino masses.
87 Besides the neutrino spin-flip rate due to the neutrino weak interactions with
nucleons or other particles, there is also a spin-flip scattering term in the gravitational
field of the entire neutron star (Choudhury, Hari Dass, and Murthy 1989).
However, the resulting energy loss was found to be small except for low-energy
neutrinos.