Definitions and upper bounds for unstable boson masses

We consider the problem of a proper definition of the mass of a heavy unstable boson. It is shown how various definitions are related by the renormalization-scheme independence of the full resummed amplitude. This is made explicit for the W boson in the large Nf limit, where we find an upper bound on the peak position of the lineshape and on the mass, provided a sensible definition of the mass is used. In more realistic models, we conjecture that under certain conditions the mass is similarly bounded. Using this approach an upper limit of about 1 TeV on the mass of the Minimal-Standard-Model Higgs boson is found, independent of any breakdown of perturbation theory.