Fine tuning in the constrained exceptional supersymmetric standard model

Supersymmetric unified models in which the Z' couples to the Higgs doublets, as in the E-6 class of models, have large fine tuning dominated by the experimental mass limit on the Z'. To illustrate this, we investigate the degree of fine tuning throughout the parameter space of the constrained exceptional supersymmetric standard model (cE(6)SSM) that is consistent with a Higgs mass m(h) similar to 125 GeV. Fixing tan beta = 10, and taking specific values of the mass of the Z' boson, with M-Z' similar to 2-4 TeV, we find that the minimum fine tuning is set predominantly from the mass of Z' and varies from similar to 200 to 400 as we vary M-Z' from similar to 2 to 4 TeV. However, this is significantly lower than the fine tuning in the constrained minimal supersymmetric standard model, of O(1000), arising from the large stop masses required to achieve the Higgs mass.