Article,
A Theory of Dark Matter
(October 2008)
Abstract
We propose a comprehensive theory of dark matter that explains the recent
proliferation of unexpected observations in high-energy astrophysics. Cosmic
ray spectra from ATIC and PAMELA require a WIMP with mass M\_chi \~ 500 - 800 GeV
that annihilates into leptons at a level well above that expected from a
thermal relic. Signals from WMAP and EGRET reinforce this interpretation. Taken
together, we argue these facts imply the presence of a GeV-scale new force in
the dark sector. The long range allows a Sommerfeld enhancement to boost the
annihilation cross section as required, without altering the weak scale
annihilation cross section during dark matter freezeout in the early universe.
If the dark matter annihilates into the new force carrier, phi, its low mass
can force it to decays dominantly into leptons. If the force carrier is a
non-Abelian gauge boson, the dark matter is part of a multiplet of states, and
splittings between these states are naturally generated with size alpha m\_phi \~
MeV, leading to the eXciting dark matter (XDM) scenario previously proposed to
explain the positron annihilation in the galactic center observed by the
INTEGRAL satellite. Somewhat smaller splittings would also be expected,
providing a natural source for the parameters of the inelastic dark matter
(iDM) explanation for the DAMA annual modulation signal. Since the Sommerfeld
enhancement is most significant at low velocities, early dark matter halos at
redshift \~10 potentially produce observable effects on the ionization history
of the universe, and substructure is more detectable than with a conventional
WIMP. Moreover, the low velocity dispersion of dwarf galaxies and Milky Way
subhalos can greatly increase the substructure annihilation signal.
