Requiem for an FCHAMP?
  Fractionally charged massive particles (FCHAMPs) appear in extensions of the
standard model, especially those with superstring constructions. The lightest
FCHAMP would be absolutely stable and any produced during the early evolution
of the Universe would be present today. The production, annihilation, and
survival of L, a lepton with electroweak but no strong interactions, of mass
m_L and charge Q_L (in units of the positron charge) are explored. Since
massive charged particles behave like baryons, primordial nucleosynthesis and
the cosmic background radiation temperature anisotropies limit the FCHAMP relic
density leading to constraints on the Q_L - m_L relation. Further constraints
are provided by the invisible width of the Z and by accelerator searches for
massive, charged particles. We exploit the fact that in the early Universe the
negatively charged L will combine with alpha particles and protons forming
tightly bound, positively charged states. The Coulomb barriers between these
positively charged bound states and the free L+ suppress late time annihilation
in the Galaxy and on Earth, limiting any late-time reduction of relic FCHAMP
pairs. The surviving FCHAMP abundance on Earth is orders of magnitude higher
than the limits from terrestrial searches for fractionally charged particles,
appearing to close the window on FCHAMPs. However, as Q_L approaches an integer
these searches become increasingly insensitive, leaving some "islands" in the
Q_L - m_L plane which may be explored by searching for FCHAMPs in the cosmic
rays.
