Stars as Laboratories for Fundamental Physics - MPP Theory Group

564 Chapter 15
the present author has not been able to develop a clear view of the
precise range of parameters that can be ruled out or ruled in by the
SN 1987A neutrino signal. 97
15.8 Millicharged Particles
It is commonly assumed that all particles have charges in multiples of
1
e (electron charge); notably neutrinos are thought to be electrically
3
neutral. While gauge invariance and anomaly cancellation constraints
pose limits on the possible charge assignments in the standard model,
electric charge quantization is not entirely assured. Two of the three
neutrino species may carry small charges if one gives up the assumption
that the three fermion families differ only in the mass of their members
(Takasugi and Tanaka 1992; Babu and Volkas 1992; Foot, Lew,
and Volkas 1993). Moreover, if one allows for charge nonconservation,
all neutrinos could have small charges (Babu and Mohapatra 1990;
Maruno, Takasugi, and Tanaka 1991). Finally, the existence of novel
particles with small electric charges is possible and actually motivated
by certain models involving a “mirror sector” where the mirror symmetry
is slightly broken (Holdom 1986; see also Davidson, Campbell, and
Bailey 1991). Therefore, it is interesting to study the experimental, astrophysical,
and cosmological bounds on the existence of particles with
small electric charge. 98
The most severe constraints obtain for nonstandard charge assignments
in the first family of quarks and leptons. The most modelindependent
charge limit on ν e was derived from the absence of an
anomalous dispersion of the SN 1987A neutrino signal (Sect. 13.3.3)
97 An incomplete list of issues that ought to be considered in a study of majorons
in SNe are the following. If the symmetry is broken within the SN core, a derivative
majoron coupling to neutrinos should be used instead of a pseudoscalar one (Choi
and Santamaria 1990). For light neutrinos, the medium-induced dispersion relation
may dominate the cross section result. Besides the medium-induced processes
ν → νχ, the process χ → νν should be included (Sect. 6.8). The decay of heavy
neutrinos outside of the SN could contribute to the measurable signal. The trapping
of neutrinos and majorons due to reactions with each other should be properly understood
along the lines discussed by Dicus et al. (1989); it is dubious, for example,
that a process like χχ → χχ really contributes to the majoron opacity. The effect
of majorons during the infall phase must be understood, especially the possibility
of early deleptonization.
98 Such studies were performed by Dobroliubov and Ignatiev (1990), Davidson,
Campbell, and Bailey (1991), Babu and Volkas (1992), Mohapatra and Nussinov
(1992), and Davidson and Peskin (1994).