  We identify a benchmark point in the CMSSM's heavy stau-coannihilation
region, which is favored by experiments, and demonstrate that it could be
accessible to the LHC at $\sqrt{s}=14$ TeV with 300/fb of integrated luminosity
via a golden decay measurement. With Monte-Carlo, we simulate sparticle
production and subsequent golden decay at the event level and perform
pseudo-measurements of sparticle masses from kinematic endpoints in invariant
mass distributions. We find that two lightest neutralino masses and the first
and second generation left-handed slepton and squark masses could be rather
precisely measured with correlated uncertainties. We investigate whether from
such measurements one could determine the CMSSM's Lagrangian parameters by
including a likelihood from our pseudo-measurements of sparticle masses in a
Bayesian analysis of the CMSSM's parameter space. We find that the CMSSM's
parameters can be accurately determined, with the exception of the common
trilinear parameter. Experimental measurements of the relic density by Planck
and the Higgs boson's mass slightly improve this determination, especially for
the common trilinear parameter. Finally, within our benchmark scenario, we show
that the neutralino dark matter will be accessible to direct searches in future
one tonne detectors.
