Stars as Laboratories for Fundamental Physics - MPP Theory Group

Chapter 9
Oscillations
of Trapped Neutrinos
In a supernova (SN) core or in the early universe neutrinos scatter on
the background medium and on each other. Therefore, oscillations of
mixed neutrinos are frequently interrupted, leading to flavor equilibrium
if there is enough time. A Boltzmann-type kinetic equation is
derived that accounts simultaneously for neutrino oscillations and collisions
for arbitrary neutrino degeneracy. It is applied to a SN core,
yielding an estimate of the time scale to reach flavor equilibrium. On
the basis of the SN 1987A neutrino observations a limit on the mixing
of sequential neutrinos with a hypothetical sterile flavor is derived.
9.1 Introduction
So far oscillations were discussed in the context of “beam experiments”
where neutrinos of a known flavor are produced at a certain location,
for example in a power reactor or in the Sun, then propagate over a
distance, and are then detected with a device that allows one to distinguish
between different flavors. It was assumed that there were no
interactions between the production and detection point, with the possible
exception of decays, allowing one to treat neutrino oscillations as
a simple propagation phenomenon fully analogous to light propagation
in an optically active medium.
This approach is not adequate for the early universe and a young SN
core. In both cases there are frequent neutrino collisions which affect
the free evolution of the phases. The impact of these collisions can
be understood if one assumes for the purpose of illustration that one
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