Advances in High Energy Physics

Volume 2018, Article ID 2959354, 8 pages

https://doi.org/10.1155/2018/2959354

## The DKP Oscillator in Spinning Cosmic String Background

Faculty of Physics, Shahrood University of Technology, Shahrood, P.O. Box 3619995161-316, Iran

Correspondence should be addressed to Mansoureh Hosseinpour; moc.liamg@heruosnam.ruopniessoh

Received 11 June 2018; Revised 3 September 2018; Accepted 7 November 2018; Published 30 December 2018

Academic Editor: Diego Saez-Chillon Gomez

Copyright © 2018 Mansoureh Hosseinpour and Hassan Hassanabadi. 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

In this article, we investigate the behaviour of relativistic spin-zero bosons in the space-time generated by a spinning cosmic string. We obtain the generalized beta-matrices in terms of the flat space-time ones and rewrite the covariant form of Duffin-Kemmer-Petiau (DKP) equation in spinning cosmic string space-time. We find the solution of DKP oscillator and determine the energy levels. We also discuss the influence of the topology of the cosmic string on the energy levels and the DKP spinors.

#### 1. Introduction

The Duffin-Kemmer-Petiau (DKP) equation has been used to describe relativistic spin-0 and spin-1 bosons [1–4]. The DKP equation has five- and ten-dimensional representation, respectively, for spin-0 and spin-1 bosons [5]. This equation is compared to the Dirac equation for fermions [6]. The DKP equation has been widely investigated in many areas of physics. The DKP equation has been investigated in the momentum space with the presence of minimal length [7, 8] and for spins 0 and 1 in a noncommutative space [9–12]. Also, the DKP oscillator has been studied in the presence of topological defects [13]. Recently, there has been growing interest in the so-called DKP oscillator [14–23] in particular in the background of a magnetic cosmic string [13]. The cosmic strings and other topological defects can form at a cosmological phase transition [24, 25]. The conical nature of the space-time around the string causes a number of interesting physical effects. Until now, some problems have been investigated in the gravitational fields of topological defects including the one-electron atom problem [26–28]. Spinning cosmic strings are similar usual cosmic string, characterized by an angular parameter that depends on their linear mass density . The DKP oscillator is described by performing the nonminimal coupling with a linear potential. The name distinguishes it from the system called a DKP oscillator with Lorentz tensor couplings of [7–12, 14–16]. The DKP oscillator for spin-0 bosons has been investigated by Guo et al. in [10] in noncommutative phase space. The DKP oscillator with spin-0 has been studied by Yang et al. [11]. Exact solution of DKP oscillator in the momentum space with the presence of minimal length has been analysed in [8]. De Melo et al. construct the Galilean DKP equation for the harmonic oscillator in a noncommutative phase space [29]. Falek and Merad investigated the DKP oscillator of spins 0 and 1 bosons in noncommutative space [9]. Recently, there has been an increasing interest on the DKP oscillator [13–16, 22, 29–31]. The nonrelativistic limit of particle dynamics in curved space-time is considered in [32–36]. Also, the dynamics of relativistic bosons and fermions in curved space-time is considered in [17, 20, 31].

The influence of topological defect in the dynamics of bosons via DKP formalism has not been established for spinning cosmic strings. In this way, we consider the quantum dynamics of scalar bosons via DKP formalism embedded in the background of a spinning cosmic string. We solve DKP equation in presence of the spinning cosmic string space-time whose metric has off diagonal terms which involves time and space. The influence of this topological defect in the energy spectrum and DKP spinor presented graphically.

The structure of this paper is as follows: Section 2 describes the covariant form of DKP equation in a spinning cosmic string background. In Section 3, we introduce the DKP oscillator by performing the nonminimal coupling in this space-time, and we obtain the radial equations that are solved. We plotted the DKP spinor, density of probability, and the energy spectrum for different conditions involving the deficit angle and the oscillator frequency. In the Section 4 we present our conclusions.

#### 2. Covariant Form of the DKP Equation in the Spinning Cosmic String Background

We choose the cosmic string space-time background, where the line element is given byThe space-time generated by a spinning cosmic string without internal structure, which is termed ideal spinning cosmic string, can be obtain by coordinate transformation as

With this transformation, the line element ((1)) becomes [37–43]

with , , and . From this point on, we will take . The angular parameter which runs in the interval is related to the linear mass density of the string as and corresponds to a deficit angle . We take where is the universal gravitation constant and is the angular momentum of the spinning string; thus is a length that represents the rotation of the cosmic string. Note that, in this case, the source of the gravitational field relative to a spinning cosmic string possesses angular momentum and the metric (1) has an off diagonal term involving time and space.

The DKP equation in the cosmic string space-time (1) reads [13, 17, 31]The covariant derivative in (4) iswhere are the spinorial affine connections given byThe matrices are the standard Kemmer matrices in Minkowski space-time.The Kemmer matrices are an analogous to Dirac matrices in Dirac equation. There has been an increasing interest Dirac equation for spin half particles [44–47]. The matrices satisfies the DKP algebra,The conserved four-current is given byand the conservation law for takes the formThe adjoint spinor is defined as with , in such a way that . The factor which multiplies is of no importance for the conservation law and ensures the charge density is compatible with the one used in the Klein-Gordon theory and its nonrelativistic limit. Thus, if is Hermitian with respect to and the curved-space beta-matrices are covariantly constant, then the four-current will be conserved if [30]The algebra expressed by these matrices generates a set of 126 independent matrices whose irreducible representations comprise a trivial representation, a five-dimensional representation describing the spin-zero particles, and a ten-dimensional representation associated with spin-one particles. We choose the beta-matrices as follows [31]:In (7), denote the tetrad basis that we can choose asFor the specific tetrad basis given by (13), we find from (7) that the curved-space beta-matrices readand the spin connections are given byWe consider only the radial component in the nonminimal substitution. Since the interaction is time-independent, one can write , where is the energy of the scalar boson, is the magnetic quantum number, and is the wave number. The five-component DKP spinor can be written as , and the DKP equation (4) leads to the fiveThen we obtain the following equation of motion for the first component of the DKP spinor:Then (17) changes to

By the change of variable , we can write (19) in the formwhere and . The physical solution of (20) is function. Therefore the general solution to (20) is given bywhere is the Bessel function of the second kind. Sometimes this family of functions is also called Neumann functions or Weber functions. is the Bessel function of the first kind, given byand is the Bessel function of the second kind, given byBy considering the boundary condition for (21) such that , we find

#### 3. The DKP Oscillator in Spinning Cosmic String Background

The DKP oscillator is introduced via the nonminimal substitution [17, 30, 31]where is the oscillator frequency, is the mass of the boson already found in (4), and is defined in (5). We consider only the radial component in the nonminimal substitution. The DKP equation (4) leads to the five equations:By solving the above system of (26) in favour of we get

Combining these results we obtain (23) of motion for the first component of the DKP spinor:Let us take asThen (28) changes toIn order to solve the above equation, we employ the change of variable: ; thus we rewrite the radial equation (34) in the formIf we compare with this second-order differential equation with the Nikiforov-Uvarov (NU) form, given in (A.1) of Appendix, we see thatwhich gives the energy levels of the relativistic DKP equation fromwhereAs the final step, it should be mentioned that the corresponding wave function iswhere is the normalization constant. In limit a 0 we have the usual metric in cylindrical coordinates where described by the line elementas pointed out by authors in [17] dynamic of DKP oscillator in the presence of this metric describe byand the corresponding wave function iswhere A,B,C, and D are constant and denotes the generalized Laguerre polynomial. In Figure 1, is plotted versus for different quantum number with the parameters listed under it. The density of probability is shown in Figure 2. The negative and positive solution of energy versus is shown in Figures 3 and 4 for and 10. As in Figures 3 and 4, we observe that the absolute value of energy decreases with . Also in Figure 5 energy is plotted versus for quantum numbers. We see that absolute value of energy increases with . The negative and positive solution of energy versus is shown in Figure 6 for different parameter . We obtained the energy levels of the DKP oscillator in that background and observed that the energy increases with the level number. In Figure 7, energy is plotted versus for different quantum numbers. We see energy increases with parameter . Also we observed that the energy levels of the DKP oscillator in that background increases with the level number.