Advances in High Energy Physics

Volume 2018, Article ID 7308935, 10 pages

https://doi.org/10.1155/2018/7308935

## Time Dilation in Relativistic Quantum Decay Laws of Moving Unstable Particles

School of Chemistry and Physics, University of KwaZulu-Natal and National Institute for Theoretical Physics (NITheP), Westville Campus, Durban 4001, South Africa

Correspondence should be addressed to Filippo Giraldi; moc.liamg@oppilif.idlarig

Received 20 March 2018; Accepted 22 April 2018; Published 29 May 2018

Academic Editor: Marek Nowakowski

Copyright © 2018 Filippo Giraldi. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The publication of this article was funded by SCOAP^{3}.

#### Abstract

The relativistic quantum decay laws of moving unstable particles are analyzed for a general class of mass distribution densities which behave as power laws near the (nonvanishing) lower bound of the mass spectrum. The survival probability , the instantaneous mass , and the instantaneous decay rate of the moving unstable particle are evaluated over short and long times for an arbitrary value of the (constant) linear momentum. The ultrarelativistic and nonrelativistic limits are studied. Over long times, the survival probability is approximately related to the survival probability at rest by a scaling law. The scaling law can be interpreted as the effect of the relativistic time dilation if the asymptotic value of the instantaneous mass is considered as the effective mass of the unstable particle over long times. The effective mass has magnitude at rest and moves with linear momentum or, equivalently, with constant velocity . The instantaneous decay rate is approximately independent of the linear momentum , over long times, and, consequently, is approximately invariant by changing reference frame.

#### 1. Introduction

The description of the decay laws of unstable particles via quantum theory has been a central topic of research for decades [1, 2]. Many unstable particles which are generated in astrophysical phenomena or high-energy accelerator experiments are moving in the laboratory frame of the observer at relativistic or ultrarelativistic velocity. For this reason, plenty of studies have been devoted to formulate the decay laws in terms of relativistic quantum theory. See [1, 3–6], to name but a few.

A fundamental subject in the description of the relativistic decays of moving unstable particles is the way the decay laws transform by changing the reference frame. Naturally, it is essential to understand how the decay laws, holding in the rest reference frame of the moving particle, are transformed in the laboratory frame of an observer. The nondecay or survival probability of an unstable particle has been evaluated in [7, 8] for nonvanishing and vanishing values of the linear momentum in terms of the mass distribution density (MDD). The condition of vanishing linear momentum, , provides the survival probability in the reference frame where the particle is at rest, while the condition of nonvanishing linear momentum, , can be referred to the laboratory frame of an observer where the particles move with linear momentum . The effects of the relativistic time dilation in quantum decay laws of moving unstable particles remain a matter of central interest. See [7–17], to name but a few.

As a continuation of the scenario described above, here, we evaluate the survival probability, the instantaneous energy, and the instantaneous decay rate of a moving unstable particle over short and long times for a wide variety of MDDs and for an arbitrary value of the (constant) linear momentum. In light of the short-time transformations of the survival probability according to the relativistic time dilation, we search for further scaling relations in the decay laws which can relate the conditions of nonvanishing and vanishing linear momentum. In this way, we aim to find further descriptions of the ways the decay laws of moving unstable particle transform by changing reference frame.

The paper is organized as follows. Section 2 is devoted to the relativistic quantum decay laws of a general moving unstable particle. In Section 3, the survival probability is evaluated over short and long times for an arbitrary value of the linear momentum and a general class of MDD. In Section 4, the transformation of the long-time survival probability is described via a scaling law. Section 5 is devoted to the evaluation of the instantaneous mass and instantaneous decay rate over short and long times. In Section 6, the scaling law, describing the transformation of the survival probability, is interpreted in terms of the relativistic time dilation and of the instantaneous mass of the moving unstable particle. Summary and conclusions are reported in Section 7.

#### 2. Relativistic Quantum Decay Laws

An extended and detailed description of the relativistic quantum decay laws of moving unstable particles has been recently provided in [14, 17]. Following these references, a brief summary of unstable quantum states, of the survival probability, and of the instantaneous energy and decay rate is reported below for the sake of clarity, by adopting the system of units where . The motion is assumed to be one-dimensional, due to the conservation of the linear momentum [1, 7, 8, 14–17].

In the Hilbert space of the quantum states which describe the unstable particle, let the state kets be the common eigenstates of the linear momentum operator and the Hamiltonian self-adjoint operator, and , for every value of the mass parameter and of the linear momentum . The mass parameter belongs to the spectrum of the Hamiltonian which is supposed to be continuous with lower bound , which means . In the rest reference frame of the moving particle the linear momentum vanishes and the mentioned state kets become , while the eigenstates of the Hamiltonian are . Let be the ket of the Hilbert space which describes the quantum state of the unstable particle. Such state can be expressed in terms of the eigenstates of the Hamiltonian as , via the expansion function . The survival amplitude is defined in the rest reference frame of the moving unstable particle as and is given by the integral expressionwhere is the imaginary unit. The function represents the MDD and reads . The probability that the decaying particle is in the initial state at the time , i.e., the survival probability, is given by the following form, , in the rest reference frame of the moving unstable particle.

Let be the Lorentz transformation which relates the reference frame where the unstable moving particle is at rest, to the one with velocity , where is the corresponding relativistic Lorentz factor. Let be an unitary representation of the transformation acting on the Hilbert space such that for every value of the mass parameter in the Hamiltonian spectrum. The state ket describes the moving unstable particle with nonvanishing linear momentum . Such state is related to the state ket as follows: . The form is obtained by considering the energy-momentum -vector and the Lorentz invariance [6, 18]. The quantity is defined as and represents the survival amplitude in the reference frame where the particle has linear momentum . The approach described above leads to the following integral expression of the survival amplitude:The quantity represents the survival probability that the decaying particle is in the initial state at the time in the reference frame where the unstable particle has linear momentum and is given by the square modulus of the survival amplitude, . See [7, 8, 17] for details.

The decay laws of the unstable particles are obtained from the MDD via (1) and (2). In literature, the MDD is usually represented via the Breit-Wigner function [19]:where is a normalization factor, is the Heaviside unit step function, is the rest mass of the particle, and is the decay rate at rest. A detailed analysis of the survival amplitude of a moving unstable particle has been performed in [8] by considering the Breit-Wigner form of the MDD [7]. The long-time behavior of the survival amplitude results in dominant inverse power laws, besides additional decaying exponential terms. Refer to [8] for details. In [17, 20, 21] general forms of MDD are considered with power-law behaviors near the lower bound of the mass spectrum. The long-time decay of the survival amplitude is described by inverse power laws which are determined by the low-mass profile of the MDD [20, 21]. Additional removable logarithmic singularities in the low-mass form of the MDD lead to logarithmic-like relaxations of the survival amplitude at rest which can be arbitrarily slower or faster than inverse power laws [22, 23].

Since the unstable particle decays, the initial state is not an eigenstate of the Hamiltonian and the instantaneous mass (energy) is not defined during the time evolution. For this reason, instantaneous mass (energy) and decay rate are defined in the rest reference frame of the moving particle in terms of an effective Hamiltonian. Such operator acts on the subspace of the Hilbert space which is spanned by the initial state. Refer to [14, 17, 20, 21, 24] for details. In the same way, the instantaneous mass (energy) and decay rate of the moving unstable particle are defined in the frame system where the particle has linear momentum . For both vanishing and nonvanishing values of the linear momentum , the instantaneous mass and decay rate are obtained from the survival amplitude via the following forms [14, 17, 20–23]:The long-time behavior of the instantaneous mass and decay rate has been evaluated in [14, 15] for the Breit-Wigner form of the MDD.

##### 2.1. Relativistic Time Dilation

The interpretation of decay processes via the theory of special relativity suggests that the lifetime which is detected in the rest reference frame of the unstable particle increases in the reference frame where the particle is moving. The increase is due to the relativistic dilation of times and is determined by the relativistic Lorentz factor [25].

The appearance of the relativistic time dilation in quantum decays is a matter of great interest and fruitful discussions. See [7–17], to name but a few. A broadly shared opinion is that the relativistic time dilation influences the survival probability uniquely in the short-time exponential decay. Briefly, this means that the survival probability and the survival probability at rest are given by the exponential forms and , over short times. Under this assumption, the survival probability obeys the scaling lawover short times. The parameter represents the lifetime of the particle at rest, while is the lifetime which is detected in the reference frame where the particle is moving with linear momentum . According to the relativistic time dilation, the lifetimes are related as follows: , where is the corresponding relativistic Lorentz factor. Refer to [7, 8, 12–17] for an extended explanation. In this context, we intend to study the survival probability and the survival probability at rest over short and long times for a wide variety of MDDs. We search for further ways to describe the transformations of the decay laws which occur by changing reference frame.

#### 3. Survival Probability versus Linear Momentum

In the present section the short- and long-time behaviors of the survival probability are studied for a general value of the constant linear momentum of the moving unstable particle. The analysis performed in the whole paper is based entirely on form (2) of the survival amplitude [7, 12]. For the sake of convenience, the survival amplitude is expressed via the dimensionless variables , , , and . These variables are defined in terms of a generic scale mass as follows: , , , and . The MDD is expressed in terms of the auxiliary function via the scaling law , for every , where . In this way, the survival amplitude results in the expression below:

The MDDs under study are defined over the infinite support by auxiliary functions of the following form:In order to study the long-time behavior of the decay laws of the moving unstable particle, the MDDs are requested to obey the conditions below. The lower bound of the mass spectrum is chosen to be nonvanishing, . The constraints and are also requested. The function and the derivatives are required to be summable, for every , and continuously differentiable in the whole support , for every . Consequently, the limits must exist as finite and be for every . The functions have to decay sufficiently fast as , so that the auxiliary function and the derivatives vanish as , for every .

As far as the short-time behavior of the survival amplitude is concerned, let the auxiliary function decay as follows: for , with . Under this condition, the survival amplitude evolves algebraically over short times:for . The constants , , and are given by the expressions below:The short-time evolution of the survival probability is derived from (7) and (9) and is algebraic:for , where .

The long-time behavior of the survival amplitude is obtained from (7) and from the following equivalent form:where . Notice that the constraint of nonvanishing lower bound of the mass spectrum, , is fundamental for the equivalence of expressions (7) and (12) of the survival amplitude. The asymptotic analysis [26, 27] of the integral form, appearing in (12) for , provides the expression of the survival amplitude over long times:for , where , andAsymptotic form (13) of the survival amplitude holds for every value of the linear momentum , nonvanishing, arbitrarily large or small, or vanishing; for the variety of MDDs under study; for every ; and for every value of the lower bound of the mass spectrum such that . The last condition is crucial and will be interpreted in the next section in terms of the instantaneous mass at rest of the moving unstable particle. The square modulus of asymptotic expression (13) approximates the survival probability over long times:for . Notice that the time scale and, consequently, the short or long times, or , are independent of the auxiliary function and are determined uniquely by the MDD.

#### 4. Scaling Law for the Survival Probability

Expression (15) of the survival probability holds for every value of the constant linear momentum . Such arbitrariness allows evaluating the survival probability in the nonrelativistic and ultrarelativistic limits. For vanishing value of the linear momentum, , or, equivalently, in the rest reference frame of the unstable particle, the survival probability is approximated over long times as follows:for . Instead, consider large values of the linear momentum, . In such condition the survival probability is approximated over long times as follows:for . By comparing (15) and (16), we observe that, for the MDDs under study, the survival probability obeys, approximately over long times, the following scaling law:for . This is the main result of the paper. In fact, the above scaling law describes how the survival probability at rest transforms, approximately over long times, in the reference frame where the particle moves with constant linear momentum . The transformation can be interpreted, approximately, as the effect of a time dilation which is determined by the scaling factor . Notice that the scaling factor, given by (14), diverges in the limit . In Section 6, the scaling law (18) is interpreted, via the special relativity, as the effect of the relativistic time dilation. This interpretation holds if the asymptotic value of the instantaneous mass is considered as the effective mass of the moving unstable particle over long times.

The correction to the scaling law (18) can be estimated via the expression . For the MDDs under study, such correction vanishes inversely quadratically over long times:for , where

Numerical analysis of the survival probability has been displayed in Figures 1, 2, 3, 4, and 5. The computed MDDs are given by the following toy form of the auxiliary function:where is a normalization factor and reads . Different values of the nonnegative power and linear momentum are considered. The corresponding MDDs belong to the class under study which is defined in Section 3 via (8). The asymptotic lines appearing in Figure 3 agree with the long-time inverse-power-law decays of the survival probability, given by (15). The ordinates of the asymptotic horizontal lines of Figures 4 and 5 are in accordance with form (14) of the scaling factor . The long-time dilation in the survival probability, given by the scaling law (18), is confirmed by the common horizontal asymptotic line, at ordinate , which appears in Figures 6 and 7.