Physica A 268 (1999) 207–213 www.elsevier.com/locate/physa The mechanism of the Zeno eect Knud Saermark a ; ∗ , Jacob Levitan a;b;c , Meir Lewkowicz b a Department of Physics, The Technical University of Denmark, Lyngby, Denmark b Department of Physics, College of Judea and Samaria, Ariel, Israel c Department of Physics, Bar-Ilan University, Ramat-Gan, Israel Received 5 January 1999 Abstract It is shown that the Zeno eect is due to the removal of the decay products. If repetitive screenings take place within the interval of the quadratically decreasing survival possibility the regeneration term does not approach zero and the decay is maintained at a lower pace than the exponential. c  1999 Elsevier Science B.V. All rights reserved. The decay of a quasi-stationary quantum system is governed by an initial non- exponential period succeeded by a large intermediate part with an almost exponential decay and nally followed by a transition to a power law. The extension of the initial short time below which the survival probability decreases at most quadratically has been estimated by Chiu et al. [1] to be of the order t z ≈ 25 E i − E th ; ˜ ≡ 1 ; (1) where E i is the energy of the unstable system and E th is the threshold of the continuous spectrum of the decay products. According to Wigner and Weisskopf [2] the survival probability for a decaying system represented by an ensemble of systems in the initial state  0 after a short time t¡t z is P(t )= |〈 0 |exp(−i Ht )| 0 〉| 2 ≈ 1 − H 2 t 2 (2) * Corresponding author. 0378-4371/99/$ - see front matter c  1999 Elsevier Science B.V. All rights reserved. PII: S0378-4371(99)00016-3