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arXiv:1204.1896 · International Winter Meeting on Nuclear Physics (Bormio)

(Oscillating) non-exponential decays of unstable states

Francesco Giacosa

Published Oct 12, 2012• 8 citations
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Abstract

We discuss deviations from the exponential decay law which occur when going beyond the Breit-Wigner distribution for an unstable state.In particular, we concentrate on an oscillating behavior, remisiscent of the Rabi-oscillations, in the short-time region.We propose that these oscillations can explain the socalled GSI anomaly, which measured superimposed oscillations on top of the exponential law for hydrogen-like nuclides decaying via electron-capture.Moreover, we discuss the possibility that the deviations from the Breit-Wigner in the case of the GSI anomaly are (predominantely) caused by the interaction of the unstable state with the measurement apparatus.The consequences of this scenario, such as the non-observation of oscillations in an analogous experiment perfromed at Berkley, are investigated.

DOI
10.22323/1.160.0028

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We discuss deviations from the exponential decay law which occur when going beyond the Breit-Wigner distribution for an unstable state.In particular, we concentrate on an oscillating behavior, remisiscent of the Rabi-oscillations, in the short-time region.We propose that these oscillations can explain the socalled GSI anomaly, which measured superimposed oscillations on top of the exponential law for hydrogen-like nuclides decaying via electron-capture.Moreover, we discuss the possibility that the deviations from the Breit-Wigner in the case of the GSI anomaly are (predominantely) caused by the interaction of the unstable state with the measurement apparatus.The consequences of this scenario, such as the non-observation of oscillations in an analogous experiment perfromed at Berkley, are investigated.
We study the deviations from the usual exponential decay law for quantum mechanical systems. We show that simple and physically motivated deviations from the Breit-Wigner energy distribution of the unstable state are sufficient to generate peculiar deviations from the exponential decay law. Denoting with p(t)p(t)p(t) the survival probability, its derivative h(t)h(t)h(t) shows typically an oscillating behavior on top of the usual exponential function. We argue that this can be a viable explanation of the observed experimental results at GSI Darmstadt, where the function h(t)h(t)h(t) has been experimentally measured for electron capture decays of Hydrogen-like ions. Moreover, if our interpretation is correct, we predict that by measuring h(t)h(t)h(t) at times close to the initial one, the number of decays per second rapidly drops to zero.
An effect generated by the nonexponential behavior of the survival amplitude of an unstable state in the long time region is considered.We find that the instantaneous energy of the unstable state for a large class of models of unstable states tends to the minimal energy of the system E min as t → ∞ which is much smaller than the energy of this state for t of the order of the lifetime of the considered state.Analyzing the transition time region between the exponential and non-exponential form of the survival amplitude, we find that the instantaneous energy of the considered unstable state can take large values, much larger than the energy of this state for t from the exponential time region.Taking into account results obtained for a considered model, it is hypothesized that this purely quantum mechanical effect may be responsible for the properties of broad resonances such as σ meson as well as having astrophysical and cosmological consequences Key words: decay law, non exponential decay
We study the survival probability of moving relativistic unstable particles and we found the integral representation of the survival amplitude for such particles moving with constant momentum ~ 6 0. Using this representation of the amplitude we found decay curves of such particles for the quantum mechanical models considered. These model studies show that late time deviations of the survival probability of these particles from the exponential form of the decay law, that is the transition times region between exponential and non-exponential form of the survival probability, occur much earlier than it follows from the classical standard approach resolving itself into replacing time t by t/γ (where γ is the relativistic Lorentz factor) in the formula for the survival probability and that the survival probabilities tend to zero as t → ∞ much slower than one would expect using classical time dilation relation. Here we show also that for some physically admissible models of unstable states the properly computed decay curves of the moving particles have analogous fluctuating form as the decay curves obtained in the GSI experiment.
The quantum-mechanical theory of the decay of unstable states is revisited. We show that the decay is non-exponential both in the short-time and long-time limits using a more physical definition of the decay rate than the one usually used. We report results of numerical studies based on Winter's model that may elucidate qualitative features of exponential and non-exponential decay more generally. The main exponential stage is related to the formation of a radiating state that maintains the shape of its wave function with exponentially diminishing normalization. We discuss situations where the radioactive decay displays several exponents. The transient stages between different regimes are typically accompanied by interference of various contributions and resulting oscillations in the decay curve. The decay curve can be fully oscillatory in a two-flavor generalization of Winter's model with some values of the parameters. We consider the implications of that result for models of the oscillations reported by GSI.
Late time properties of moving relativistic particles are studied. Within the proper relativistic treatment of the problem we find decay curves of such particles and we show that late time deviations of the survival probability of these particles from the exponential form of the decay law, that is the transition times region between exponential and non-exponential form of the survival amplitude, occur much earlier than it follows from the classical standard approach boiled down to replace time t by t/γL (where γL is the relativistic Lorentz factor) in the formula for the survival probability. The consequence is that fluctuations of the corresponding decay curves can appear much earlier and much more unstable particles have a chance to survive up to these times or later. It is also shown that fluctuations of the instantaneous energy of the moving unstable particles have a similar form as the fluctuations in the particle rest frame but they are seen by the observer in his rest system much earlier than one could expect replacing t by t/γL in the corresponding expressions for this energy and that the amplitude of these fluctuations can be even larger than it follows from the standard approach. All these effects seem to be important when interpreting some accelerator experiments with high energy unstable particles and the like (possible connections of these effects with GSI anomaly are analyzed) and some results of astrophysical observations.
In this article, I provide a pedagogical discussion of the GSI anomaly. Using two different formulations, namely the intuitive quantum-field-theory language of the second quantized picture as well as the language of amplitudes, I clear up the analogies and differences between the GSI anomaly and other processes (the double-slit experiment using photons, ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$ scattering, and charged-pion decay). In both formulations, the conclusion is reached that the decay rate measured at GSI cannot oscillate if only standard model physics is involved and the initial hydrogen-like ion is no coherent superposition of more than one state (in case there is no new, yet unknown, mechanism at work). Furthermore, a discussion of the quantum beat phenomenon will be given, which is often assumed to be able to cause the observed oscillations. This is, however, not possible for a splitting in the final state only.