Naturalness of Light Neutralino Dark Matter in pMSSM after LHC, XENON100
  and Planck Data
  We examine the possibility of a light (below 46 GeV) neutralino dark matter
(DM) candidate within the 19-parameter phenomenological Minimal Supersymmetric
Standard Model (pMSSM) in the light of various recent experimental results,
especially from the LHC, XENON100, and Planck. We also study the extent of
electroweak fine-tuning for such a light neutralino scenario in view of the
null results from the searches for supersymmetry so far. Using a Markov Chain
Monte Carlo likelihood analysis of the full pMSSM parameter space, we find that
a neutralino DM with mass > 10 GeV can in principle still satisfy all the
existing constraints. Our light neutralino solutions can be broadly divided
into two regions: (i) The solutions in the 10 - 30 GeV neutralino mass range
are highly fine-tuned and require the existence of light selectrons (below 100
GeV) in order to satisfy the observed DM relic density. We note that these are
not yet conclusively ruled out by the existing LEP/LHC results, and a dedicated
analysis valid for a non-unified gaugino mass spectrum is required to exclude
this possibility. (ii) The solutions with low fine-tuning are mainly in the 30
- 46 GeV neutralino mass range. However, a major portion of it is already ruled
out by the latest XENON100 upper limits on its spin-independent direct
detection cross section, and the rest of the allowed points are within the
XENON1T projected limit. Thus, we show that the allowed MSSM parameter space
for a light neutralino DM below the LEP limit of 46 GeV, possible in
supersymmetric models without gaugino mass unification, could be completely
accessible in near future. This might be useful in view of the recent claims
for positive hints of a DM signal in some direct detection experiments.
