SHORT-TIME BEHAVIOR OF UNSTABLE SYSTEMS IN-FIELD THEORY AND PROTON DECAY

The short-time behavior of an unstable particle is examined in a reali stic field theoretical model. The onset of linear decreasing of the no ndecay probability is shown to be extremely rapid, so rapid as to excl ude any relevance of regimes quadratic in time (''Zeno'' quantum parad ox). The result applies to super-renormalizable, renormalizable, and n onrenormalizable cases, particularly to the proton decay problem. We d iscuss also the deviations from the exponential decay law in small-Q-v alue decays.