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 μ and A0> 0 , whilst the extent of the stop coannihilation strip is limited for A0< 0 and either sign of μ. 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 ≳ 8 TeV.
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Acknowledgements We would like to thank S. Heinemeyer for his continuing help with our implementation of FeynHiggs. The work of J.E. was supported in part by STFC (UK) via the research Grant ST/L000326/1, and in part by the Estonian Research Council via a Mobilitas Pluss Grant. F.L. was supported by the World Premier International Research Center Initiative (WPI), MEXT, Japan. The work of K.A.O. was supported in part by DOE Grant DE-SC0011842 at the University of Minnesota. The work of J. Z. was supported in part by KAKENHI Grant number JP26104009.