Phenomenology of Dirac Neutralino Dark Matter
  In supersymmetric models with an unbroken R-symmetry (rather than only
R-parity), the neutralinos are Dirac fermions rather than Majorana. In this
article, we discuss the phenomenology of neutralino dark matter in such models,
including the calculation of the thermal relic abundance, and constraints and
prospects for direct and indirect searches. Due to the large elastic scattering
cross sections with nuclei predicted in R-symmetric models, we are forced to
consider a neutralino that is predominantly bino, with very little higgsino
mixing. We find a large region of parameter space in which bino-like Dirac
neutralinos with masses between 10 and 380 GeV can annihilate through slepton
exchange to provide a thermal relic abundance in agreement with the observed
cosmological density, without relying on coannihilations or resonant
annihilations. The signatures for the indirect detection of Dirac neutralinos
are very different than predicted in the Majorana case, with annihilations
proceeding dominately to $\tau^+ \tau^-$, $\mu^+ \mu^-$ and $e^+ e^-$ final
states, without the standard chirality suppression. And unlike Majorana dark
matter candidates, Dirac neutralinos experience spin-independent scattering
with nuclei through vector couplings (via $Z$ and squark exchange), leading to
potentially large rates at direct detection experiments. These and other
characteristics make Dirac neutralinos potentially interesting within the
context of recent direct and indirect detection anomalies. We also discuss the
case in which the introduction of a small Majorana mass term breaks the
$R$-symmetry, splitting the Dirac neutralino into a pair of nearly degenerate
Majorana states.
