Stars as Laboratories for Fundamental Physics - MPP Theory Group

Miscellaneous Exotica 567
For a narrow range of charges, these limits can be extended to larger
masses by the SN 1987A cooling argument (Sect. 13.8.4). Accordingly,
10 −9 e ∼ < e x ∼ < 10 −7 e (15.12)
is excluded for m x up to several MeV, perhaps up to 10 MeV. On
the trapping side of this range (large e x ) these particles surely would
have an important impact on SN physics even though they cannot be
excluded on the basis of the simple cooling argument.
If millicharged particles reach thermal equilibrium in the early universe
before nucleosynthesis they contribute to the energy density and
thus to the expansion rate. If they are one of the sequential neutrinos,
this means that the right-handed degrees of freedom of that species
are excited (they must be Dirac particles!), adding an effective neutrino
degree of freedom. If they are nonneutrinos, even more energy
is contributed, depending on their spin degrees of freedom. Even one
additional effective neutrino degree of freedom is excluded and so for
any millicharged particle Davidson, Campbell, and Bailey (1991) found
e x ∼ < 3×10 −9 e (15.13)
if m < x ∼ 1 MeV. In certain models where the millicharged particles are
associated with a shadow sector, more stringent limits apply (Davidson
and Peskin 1994).
Additional regions in the mass-charge plane can be excluded by
the requirement that the novel objects do not overclose the universe.
However, these arguments depend on the annihilation cross section in
the early universe so that one needs to know all of their interactions
apart from the millicharge. It is hard to imagine novel particles which
interact only by their small electric charge! For certain specific cases
the excluded regime was derived by Davidson, Campbell, and Bailey
(1991) and Davidson and Peskin (1994).
All of these constraints leave the possibility open that ν τ ’s have a
mass in the 1−24 MeV range and a charge in the 10 −5 −10 −3 e range.
Then they would annihilate sufficiently fast before nucleosynthesis to
actually reduce their effective contribution to the expansion rate (Foot
and Lew 1993). In the standard model with small neutrino charges,
however, two sequential neutrino species must carry a millicharge of
equal but opposite magnitude (Babu and Volkas 1992; Takasugi and
Tanaka 1992). Because the large charges required for Foot and Lew’s
scenario are excluded for ν e and ν µ one would need to require that only
ν τ carries a relatively large charge, forcing one to espouse even more
exotic particle-physics models.