  Stop coannihilation may bring the relic density of heavy supersymmetric dark
matter particles into the range allowed by cosmology. The efficiency of this
process is enhanced by stop-antistop annihilations into the longitudinal
(Goldstone) modes of the $W$ and $Z$ bosons, as well as by Sommerfeld
enhancement of stop annihilations and the effects of bound states. Since the
couplings of the stops to the Goldstone modes are proportional to the trilinear
soft supersymmetry-breaking $A$-terms, these annihilations are enhanced when
the $A$-terms are large. However, the Higgs mass may be reduced below the
measured value if the $A$-terms are too large. Unfortunately, the
interpretation of this constraint on the stop coannihilation strip is clouded
by differences between the available Higgs mass calculators. For our study, we
use as our default calculator FeynHiggs 2.13.0, the most recent publicly
available version of this code. Exploring the CMSSM parameter space, we find
that along the stop coannihilation strip the masses of the stops are severely
split by the large $A$-terms. This suppresses the Higgs mass drastically for
$\mu$ and $A_0 > 0$, whilst the extent of the stop coannihilation strip is
limited for $A_0 < 0$ and either sign of $\mu$. However, in sub-GUT models,
reduced renormalization-group running mitigates the effect of the large
$A$-terms, allowing larger LSP masses to be consistent with the Higgs mass
calculation. We give examples where the dark matter particle mass may reach
$\gtrsim 8$ TeV.
