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Stars as Laboratories for Fundamental Physics - MPP Theory Group

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Nonstandard Neutrinos 277<br />

7.5.2 Spin-Flip Scattering in Supernovae<br />

Neutrino scattering by photon exchange can also be important in <strong>as</strong>trophysical<br />

settings, notably if neutrinos are Dirac particles. The magnetic<br />

or electric dipole coupling is such that it flips the helicity of relativistic<br />

neutrinos, i.e. the final state is r.h. <strong>for</strong> an initial l.h. neutrino. This<br />

spin flip is of no importance in experiments where the electron recoil is<br />

me<strong>as</strong>ured, but it can have dramatic consequences in supernovae where<br />

l.h. neutrinos are trapped by the standard weak interactions. The spinflip<br />

scattering by a electromagnetic dipole interaction would produce<br />

“wrong-helicity” states that could freely escape unless they scattered<br />

again electromagnetically. The SN 1987A neutrino signal indicates that<br />

this anomalous cooling channel cannot have been overly effective, yielding<br />

a constraint of around µ < ν ∼ 3×10 −12 µ B on all Dirac diagonal or<br />

transition moments in the sense of Eq. (7.25); see Sect. 13.8.3 <strong>for</strong> a<br />

more detailed discussion.<br />

One should keep in mind, however, that <strong>for</strong> dipole moments in this<br />

range the spin precession in the strong macroscopic magnetic fields that<br />

are believed to exist in and near SN cores could also cause significant<br />

left-right transitions. Notably, the back conversion of r.h. neutrinos<br />

could cause a transfer of energy between widely separated regions of the<br />

SN core, and might even help at the explosion (Sect. 13.8.3). The role<br />

of relatively large neutrino dipole moments in SN physics h<strong>as</strong> not been<br />

elaborated in enough depth to arrive at reliable regions of parameters<br />

that are ruled out or ruled in by SN physics and the SN 1987A neutrino<br />

signal.<br />

7.5.3 Spin-Flip Scattering in the Early Universe<br />

Neutrino spin-flip scattering h<strong>as</strong> important consequences in the early<br />

universe <strong>as</strong> it can bring some or all of the “wrong-helicity” Dirac neutrino<br />

degrees of freedom into thermal equilibrium. The usual big bang<br />

nucleosynthesis (BBN) argument previously mentioned in Sect. 7.1.5<br />

allows one to exclude this possibility because even one additional thermally<br />

excited neutrino degree of freedom appears to be <strong>for</strong>bidden by the<br />

spectacular agreement between the predicted and observed primordial<br />

light-element abundances.<br />

This argument w<strong>as</strong> first advanced by Morgan (1981a,b). Un<strong>for</strong>tunately,<br />

he used an unrealistically small cutoff <strong>for</strong> the Coulomb divergence<br />

of the spin-flip scattering cross section, leading to an overestimate<br />

of the efficiency by which r.h. Dirac neutrinos can be brought into ther-

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