Naturalness Lowers the Upper Bound on the Lightest Higgs Boson Mass in Supersymmetry

We quantify the extent to which naturalness is lost as experimental lower bounds on the Higgs boson mass increase, and we compute the natural upper bound on the lightest supersymmetric Higgs boson mass. We find that it would be unnatural for the mass of the lightest supersymmetric Higgs boson to saturate it's maximal upper bound. In the absence of significant fine-tuning, the lightest Higgs boson mass should lie below $120$ GeV, and in the most natural cases it should be lighter than $108$ GeV. For modest $tan \beta$, these bounds are significantly lower. Our results imply that a failure to observe a light Higgs boson in pre-LHC experiments could provide a serious challenge to the principal motivation for weak-scale supersymmetry.