Stars as Laboratories for Fundamental Physics - MPP Theory Group

Chapter 8
Neutrino Oscillations
The phenomenon of neutrino oscillations in vacuum and in media as
well as in magnetic fields is studied. Experimental constraints on neutrino
mixing parameters are reviewed.
8.1 Introduction
If neutrinos do not have novel interactions that allow them to decay
fast then they must obey the cosmological mass limit of m < ν ∼ 30 eV.
This is even true for ν τ although it could decay sufficiently fast into
the e + e − ν e channel if it had a mass in the 10 MeV range. However, the
absence of γ rays from SN 1987A in conjunction with the neutrino signal
(Sect. 12.5.2) and independently arguments of big-bang nucleosynthesis
(Fig. 7.2) exclude this option. If neutrino masses are indeed so small
then there is no hope for a direct experimental measurement at the
present time, with the possible exception of m νe which could still show
up in tritium β decay or neutrinoless ββ decay experiments as discussed
in Chapter 7.
Pontecorvo (1967) was the first to realize that the existence of several
neutrino flavors (two were known at the time) allows even very
small masses to become visible. 46 A “weak-interaction eigenstate”
which is produced, say, in the neutron decay n → pe − ν e is in general
expected to be a mixture of neutrino mass eigenstates. The phenomenon
of particle mixing (Sect. 7.2.1) is familiar from the quarks
46 Pontecorvo’s (1957, 1958) original discussion referred to ν ↔ ν oscillations in
analogy to the experimentally observed case of K ◦ ↔ K ◦ . For a historical overview
see Pontecorvo (1983). In this book I will not discuss ν ↔ ν oscillations any
further—see Akhmedov, Petcov, and Smirnov (1993) for a recent reexamination of
“Pontecorvo’s original oscillations.”
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