Hi there, my question is along the lines of the following. I understand that in an experiment we obtain a distribution of masses for a given particle, due to the finite resolution of the detectors. In terms of a fundamental (if that even makes sense) decay width, what is the decay width? Is it simply the following: Peak value is the particle rest mass; distribution at higher energy is particles with larger velocity; distribution at lower energy are 'off mass shell' particles? I'm having quite a bit of trouble really getting around what a decay width really is and how to think about it. I understand (to some extent) the point about resolution in experiments, and the relation of width to lifetime, but I don't truly understand. Thanks for any replies.