Stars as Laboratories for Fundamental Physics - MPP Theory Group

Nonstandard Neutrinos 263
Flavor mixing is the only possibility for members of one family (one
row in Fig. 7.1) to transform into those of a different family. This phenomenon
is known as the absence of flavor-changing neutral currents;
it means that the Z ◦ coupling to quarks and leptons, like the photon
coupling, leaves a given superposition of fermions unaltered. For example,
muons decay only by the flavor-conserving mode µ − → ν µ e − ν e
(Fig. 7.4); the experimental upper limits on the branching ratios for
µ − → e − γ and µ − → e − e + e − are 5×10 −11 and 1.0×10 −12 , respectively.
In the absence of neutrino masses and mixing the individual lepton flavor
numbers are conserved: a lepton can be transformed only into its
partner of the same family, or it can be created or annihilated together
with an antilepton of the same family.
Fig. 7.4. Allowed and forbidded µ decays.
When neutrinos have masses and mixings, flavor-violating lepton
decays become possible, but their rate would be so small that their
experimentally observed absence does not yield interesting constraints
on neutrino parameters. Because neutrino masses must be very small if
they exist at all, the most significant observable effect is that of neutrino
oscillations.
7.2.2 Standard-Model Decays of Mixed Neutrinos
For massive neutrinos it is kinematically possible to decay according
to ν → ν ′ γ, ν → ν ′ γγ, or ν → ν ′ ν ′′ ν ′′ . In the absence of mixing, of
course, all of these modes are forbidden. Even in the presence of mixing,
Fig. 7.5. Allowed and forbidden tree-level decays of mixed neutrinos.