Stars as Laboratories for Fundamental Physics - MPP Theory Group

574 Chapter 16
Either way, possible charges of all neutrinos must be so small that
the quantization of charge is very accurately realized among the fermions
of the Standard Model. Therefore, charge quantization is probably
exact so that the neutrino electric charges vanish exactly, those of the
quarks are exactly 1e and 2 e, respectively.
3 3
16.3 New Interactions
16.3.1 Majorana Masses
Because of the cosmological limit all neutrino masses are found to be so
small relative to those of the corresponding charged fermions (Fig. 7.1)
that it is hard to maintain the pretense that neutrinos are essentially
like the other fermions. In this sense the assumption of a Minimally
Extended Standard Model is self-defeating. One reaction may be to
return to the assumption of massless two-component neutrinos. It remains
to be seen for how much longer this option remains viable in view
of the expected progress in solar neutrino astronomy and, perhaps, in
laboratory experiments (including atmospheric neutrino observations).
They may soon yield unrefutable evidence for neutrino oscillations.
Another reaction is to embrace the notion of neutrinos being very
different from the charged leptons with a possible wealth of novel and
unexpected properties. The most benign assumption is to maintain the
notion of only two neutrino components per family which are characterized
by a Majorana mass term which may arise by new physics at
some large energy scale. In this case all that was said about neutrino
masses and oscillations in the previous section remains applicable.
16.3.2 “Heavy” Neutrinos and Fast Decays
If one postulates novel neutrino interactions one may speculate about
the possibility of neutrino decays which proceed faster than in the Minimally
Extended Standard Model. Besides accelerated radiative decays
one may speculate about “fast invisible decays” of the form ν → ν ′ ν ′′ ν ′′′
or ν → ν ′ χ where χ is some new boson such as the majoron (Sect. 15.7).
In this case one can escape the cosmological mass limit. In fact, such
fast-decaying neutrinos can be a welcome feature of theories for the
formation of structure in the universe (Sect. 7.1.5).
“Heavy” ν µ ’s or ν τ ’s are not in obvious conflict with normal stellar
evolution even though their emission could now be suppressed by the
mass threshold. Thermal production of neutrinos from a stellar plasma