Stars as Laboratories for Fundamental Physics - MPP Theory Group

Miscellaneous Exotica 565
which led to (Barbiellini and Cocconi 1987; Bahcall 1989)
e νe ∼ < 3×10 −17 e. (15.7)
If electric charge conservation is assumed to hold in β processes such
as neutron decay, one finds a more restrictive limit of
e νe ∼ < 3×10 −21 e. (15.8)
It is based on a limit for the neutron charge of e n = (−0.4±1.1)×10 −21 e
(Baumann et al. 1988) and on the neutrality of matter which was found
to be e p +e e = (0.8±0.8)×10 −21 e assuming a vanishing neutron charge
(Marinelli and Morpurgo 1984).
The deflection of charged neutrinos in the toroidal magnetic field
in the solar convection zone would modify the observable flux at Earth
(Ignatiev and Joshi 1994, 1995). However, in view of the above limits
an unrealistically large field gradient is required to obtain significant
flux modifications.
Babu and Volkas (1992) derived a limit on the ν µ electric charge
from the measured ν µ e cross section which would receive a contribution
from photon exchange,
e νµ ∼ < 10 −9 e. (15.9)
Similar limits could be derived for ν e .
The following arguments apply to millicharged neutrinos or novel
particles alike. They would appear as virtual states in higher-order amplitudes.
For example, they would contribute to the anomalous magnetic
moment of electrons and muons, and to the Lamb shift between
the 2P 1/2 and 2S 1/2 states of the hydrogen atom. Of these quantitities,
the Lamb shift gives the most restrictive limit (Davidson, Campbell,
and Bailey 1991),
e x < 0.11e m x /MeV, (15.10)
where e x and m x are the charge and mass of the millicharged particle,
respectively. This result applies to m x ∼ > 1 keV.
Davidson, Campbell, and Bailey (1991) have reviewed more restrictive
bounds from a host of accelerator experiments (Fig. 15.3).
A simple astrophysical constraint is based on avoiding excessive energy
losses of stars which can produce millicharged particles by various
reactions, most notably the plasma decay process. To avoid an unacceptable
delay of helium ignition in low-mass red giants, and to avoid an