High Mass Particles Near Threshold
  A consequence of the Higgs mechanism is that high mass particles, such as the Z-boson, the W-boson and the t-quark, are predicted to have masses that depend on the process by which they are produced. Thus, for example, particles produced singly are predicted to have higher masses than those produced in a pair near threshold. Quantitative details of this prediction are presented and discussed within the context of the current experimental situation. 
S. Reucroft, Y.N. Srivastava, J. Swain and A. Widom