ON THE DECAY WIDTH OF MULTIPLY MEASURED UNSTABLE STATE

In the case of multiple measurements we have found that there is nonze ro decay width GAMMA(DELTA) (even for bound states) and that, for rela tively long time intervals DELTA separating successive measurements, G AMMA(DELTA) tends like DELTA-k (k greater-than-or-equal-to 1 according to the investigated model) to the asymptotic value GAMMA0 for unstabl e states and to zero for bound states. We have shown that the detector efficiency Z not-equal 1 is responsible for this effect and that the evolution operator acting in the considered subspace of states should be renormalized to avoid this effect. As an illustration we have consi dered electromagnetic levels in the Coulomb field as an example of uns table states.