Soliton models for thick branes

In this work, we present new soliton solutions for thick branes in $4+1$ dimensions. In particular, we consider brane models based on the sine-Gordon ($SG$), $\varphi^{4}$ and $\varphi^{6}$ scalar fields, which have broken $Z_{2}$ symmetry in some cases, and are responsible for supporting and stabilizing the thick branes. The origin of the symmetry breaking in these models resides in the fact that the modified scalar field potential may have non-degenerate vacuua. These vacuua determine the cosmological constant on both sides of the brane. We also study the geodesic equations along the fifth dimension, in order to explore the particle motion in the neighbourhood of the brane. Furthermore, we examine the stability of the thick branes, by determining the sign of the $w^2$ term in the expansion of the potential for the resulting Schrodinger-like equation, where $w$ is the 5-dimensional coordinate. It turns out that the $\varphi^4$ brane is stable, while there are unstable modes for certain ranges of the model parameters in the SG and $\varphi^6$ branes.


I. INTRODUCTION
Since there is no known fundamental principle requiring spacetime to be (3 + 1)−dimensional [1,2], it has been suggested that our observable universe might be a (3 + 1)−dimensional brane in a higher dimensional space [3][4][5]. In most models, there are one or more flat 3-branes embedded discontinuously in the ambient geometry [6]. Moreover, ideas with two 3-branes provide a very elegant description of the large hierarchy between the scales of weak and gravitational forces [6,7] and contain massless modes which reproduce Newtonian gravity at large distances on the brane [6]. In recent years, particle physics extra-dimensional theories beyond the standard model have become a standard part of the array of phenomenological models [8,9]. Although there are still no experimental evidence supporting extra dimensions, due to various theoretical motivations, extra dimensional models continue to be widely considered in the literature [10]. In this context, it would be quite useful to have a set of simple and sufficiently general rules which would allow one to test new models [3]. Most extra-dimensional models require the existence of scalar fields, for instance, to generate a domain-wall which localizes matter fields [2]. The scalar fields also serve to stabilize the size of the compact extra dimensions [3,4], and can also help modify the Randall-Sundrum warped-space [11,12] to a smoothed-out version [5,6], or to cut off the extra dimension at a singularity [7,8]. In order to replace an infinitely thin brane with a thick one, a scalar field with soliton behavior is frequently invoked. The nonlinearity in the scalar field and in particular the existence of discrete vacuua in the self-interaction of the scalar field lead to the appearance of a stable localized solution, which is a good motivation for building thick brane models [10]. A general method for determining the lowest energy configuration has been worked out in [13,14].
Recently, braneworld models have also been considered in higher order curvature gravity and in modified teleparallel gravity. For instance, five-dimensional modified teleparallel gravity was considered in a brane scenario, where analytic domain walls were found to have a double-kink solution in the aftermath of the torsion of spacetime [15]. Furthermore, this model was extended by using a first-order formalism to find analytical solutions for models that include a scalar field with standard and generalized dynamics. In addition to this, it was found that the brane splits, as a result of the deviation from the standard model by controlling specific parameters [16]. Modified gravity in five dimensional spacetime has also been analysed in the Palatini formalism. For instance, a thick Palatini f (R) brane described by an anti-de Sitter warped geometry with a single extra dimension of infinite extent, sourced by a real scalar field was studied in a perturbative scenario [17]. Besides, the model of a domain wall (thick brane) in a non-compact AdS space time with only one extra dimension was further analysed in [18]. The classical tests of General Relativity in thick branes were also studied by studying the motion of test particles in a thick brane scenario and the impact of the brane thickness on the four-dimensional path of massless particles as explored in [19]. More specifically, by applying a confinement mechanism of massive tests particles in the domain wall, for instance, that simulates classically the trapping of the Dirac field in a domain wall, the influence of the brane thickness on the four-dimensional (4D) path of massless particles was analysed. A generalized version of the Randall-Sundrum II model with different cosmological constants on each side of a brane were also discussed, where specific configurations of a scalar field and its stability as a replacing factor of the singular brane were considered [20]. Models of thick branes in noncompact five-dimensional bulk with different anti-de Sitter geometries on each side of the brane were also explored [21]. On the other hand, the existence of brane solutions were also considered as a result of a real scalar field in the presence of five dimensional f (R) gravity [22]. In addition to this, asymmetric thick branworld scenarios were studied, by changing the superpotential of the scalar field [23].
It is widely practiced that in brane world scenarios Z 2 symmetry is assumed [10,[24][25][26], which is originally motivated from the Z 2 symmetry considered in M-theory [24]. Under this symmetry the bulk metric on the two sides of the brane should be the same [26]. Moreover, under such a symmetry the empty bulk on either sides of the brane have the same negative cosmological constant and as a result they are AdS [25]. Note that these conditions are satisfied in the Randall-Sandrum model. There are, however, brane models in which there is no Z 2 symmetry and the bulk is different on both sides of the brane [24]. In the latter, the Friedmann equation for a positive brane tension situated between two bulk spacetimes that posses the same 5D cosmological constant, but which does not possess a Z 2 symmetry of the metric itself was derived, and the possible effects of dropping the Z 2 symmetry on the expansion of our Universe were examined. In some of these models, the cosmological constant differ on both sides of the brane [27], where the effects of including a Gauss-Bonnet combination of higher-order curvature invariants in the bulk action are taken into account. In fact, by considering braneworld scenarios including the Gauss-Bonnet term, it was found that the cosmological dynamics have the same form as those in Randall-Sundrum scenarios but with time-varying four-dimensional gravitational and cosmological constants [26]. Motivated by such a possibility, we consider in this paper several models, namely, the sine-Gordon (SG), ϕ 4 and ϕ 6 brane models which have broken Z 2 symmetry in some cases. In several cases, the Z 2 symmetry can be restored by a proper choice of model parameters. The origin of symmetry breaking in our models reside in the fact that the modified scalar field potential may have non-degenerate vacuua. These vacuua determine the cosmological constant on both sides of the brane.
Relative to the stability issue, topological solitons are known for their non-singular structure and a natural localization mechanism which are highly stable. Zeldovich et al. [14,28] suggested that the soliton of the φ 4 model is a reasonable source for the formation of domain walls. Vilenkin extended this idea to incorporate the general theory of relativity [14,[29][30][31]. Since there is a close similarity between domain walls in 3+1 dimensions and branes in 4+1 dimensions, it is natural to think that the soliton idea might have something to do with the existence and stability of branes. Motivated by this idea, we consider soliton models for thick branes and extend some of the existing works.
This paper is outlined in the following manner: In Section II, we briefly review the thick brane formalism, by presenting the action and the field equations. Furthermore, we also study the geodesic equations along the fifth dimension, in order to explore the particle motion in the neighbourhood of the brane. In Section III, we present new soliton models and discuss their fundamental properties, for instance, by exploring the broken Z 2 -symmetry character of the solutions and the confining effects of the scalar field on the brane. In Section IV, we analyse the stability of these brane models, where the metric and the scalar field are perturbed about the static brane and the resulting equations of motion are linearized in the proper gauge. Finally, in Section V, we present our concluding remarks.

II. THICK BRANE FORMALISM
We consider a thick brane, embedded in a five-dimensional (5D) bulk spacetime, modelled by the following action where g (5) is the metric and R[g (5) ] the scalar curvature in the bulk; ϕ is a dilaton field living on the bulk and V (ϕ) is a general potential energy. The simplest line element of the brane, embedded in the 5D bulk spacetime can be written as [14]: where A is the warp factor which depends only on the 5D coordinate w. For the scalar field ϕ with the potential V (ϕ), the 5D energy-momentum tensor is given by where g AB and ϕ depend only on w.
The 5D gravitational and scalar field equations take the following forms respectively, where the prime denotes a derivatives with respect to w.
In order to obtain a first-order equation, we introduce an auxiliary function W according to [6,[32][33][34][35], which demands: while V (ϕ) takes the following form [6,[32][33][34][35]: The energy density distribution on the bulk, T 00 , which will be analysed in detail below, is given by [10] T 00 = e 2A 1 2 It can also be shown that, for models with an infinitely thin brane and Dirac delta distributions, the energy density is equal to the cosmological constant of the bulk plus the energy density on the brane, i.e., ε = Λ ± 5 + kδ(w). Moreover, it is also interesting to calculate the geodesic equation along the fifth dimension in a thick brane, in order to investigate the particle motion near the brane [36]. To this end, we start with the geodesic equation: which leads toẅ where c 1 is a constant of integration and the factor f (w) is defined as Equation (12) is a second order differential equation for w and its solution depends critically on whether f (w) is positive or negative. For positive (negative) values of f (w) one obtains exponential (periodic) solutions, respectively. Note that the periodic motion indicates particle confinement near the brane, while the exponential solutions implies that the reference point is unstable. However, this may point to the possibility that the reference point is different from the localization of the brane. In a periodic situation, by introducing a new quantity F (w) = −c 2 1 A (w)e −2A(w) , one can write the geodesic equation in the following form One can show that in the exact stable point, i.e., w 0 , then F (w 0 )=0. On the other hand, by expanding F (w) around w 0 , we have and the geodesic equation leads toẅ Taking into account a change of variablew = w − w 0 , the geodesic equation reduces tö where Ω = F (w 0 ). In the following section, we explore several models for thick branes and their characteristics.

III. SOLITON MODELS FOR THE BRANE
A. Sine-Gordon-based models The sine-Gordon (SG) model is a well-known integrable model which has found interesting applications in various disciplines [37,38]. In fact, single and multiple (topological) soliton solutions of this system are found analytically through different mathematical methods [37,38]. The self-interaction potential for this model reads where a and b are free parameters of the model. When considered as the brane potential, however, this potential should be modified to become consistent with the Einstein equations.
The SG system has the following exact static kink solution [37]: which is plotted in Fig. 1(a), for various values of parameters a and b, that shows the brane with different thickness. Taking into account the scalar field given by Eq. (19), and plugging it into the field equations, we obtain the following quantities where the warp factor is plotted in Fig. 2(a). The corresponding potential for this model to support the brane is given by which is depicted in Fig. 3(a). Notice that this potential has two series of non-degenerate vacuua, as in the DSG (double sine-Gordon) system potential [38]. However, in the limit of b a these vacuua tend to the same value (become degenerate), such as the potentials used in [10,34].
This system leads to the following energy density, T 00 , which is plotted in Fig. 4(a). Note that the energy density is localized at the brane and the thickness of the latter is given by In this model, the Ricci and Kretschmann scalars are given by and K = 16384 81 respectively. It can be seen that there is no singularity in the Ricci scalar and/or Kretschmann scalar. In the limits of w → ±∞, the Ricci scalar becomes and the limit of w → 0 yields Moreover, the mixed Einstein tensor components are given by: respectively. Note that all the components of the Einstein tensor in the limits w → ±∞ become However, in the limit of w → 0 (for µ = ν = 0, 1, 2, 3) the Einstein tensor is given by and one can interpret it as the cosmological constant on the brane, i.e., G i j ∝ Λδ i j . These results can be interpreted in that we have a broken Z 2 -symmetry in the bulk, as the two sides of the brane differ completely. On the right and in the limit of w → +∞, the Einstein tensor and consequently the cosmological constant of the bulk vanish, so the bulk is asymptotically Minkowski. However, on the other side of the brane, these quantities are nonzero and equal to the constant value 512a/(3b 3 ), and as a result the bulk would be de Sitter. The Ricci scalar and the Einstein tensor component G 00 are plotted Fig. 5(a) and Fig. 6(a), respectively. Furthermore, by calculating the field equations, one verifies that κ 2 5 = 2, which is consistent with the usual normalization notation [32,34,39]. As pointed out in DeWolfe et al. [6], the limit of the domain wall in the stiff limit where ab → ∞, the wall reduces to the step function and approaches a δ-function (see Figs. 1(a) and 4(a)).
The confining gravitational field of the brane is best observed by looking at the geodesic equation of a test particle moving only in the direction of the extra dimension, which is given bÿ This proves the confining effect of the scalar field, and c 1 is an integration constant (see Eq. 12). If interpreted as a quantum oscillator, one can assign an energy to each quantum state given by E n = (n + 1/2) ω, where B. ϕ 4 -based model The φ 4 models are well known for having simple soliton-like solutions, although it is not strictly integrable like the SG system. This model is also the central ingredient in the Goldtone and Higgs mechanisms. Spontaneous breakdown of the Z 2 symmetry in the complex version of the φ 4 model leads to the appearance of the Goldstone mode and once coupled with a Gauge field, it causes the Gauge boson to acquire mass [40]. This model is therefore frequently used for building thick branes.
Moreover, one can show that the Ricci and Kretschmann scalars are given by which is depicted in Fig. 5(b), and K = 64 6561 α 4 β 4 cosh 12 (βw) 90α 4 cosh 4 (βw) − 80α 4 cosh 6 (βw) − 240α 4 cosh 8 (βw) + 160α 4 cosh 12 (βw) +60α 4 cosh 2 (βw) + 10α 4 + 324α 2 cosh 4 (βw) + 108α 2 cosh 2 (βw) − 432α 2 cosh 8 (βw) + 729 cosh 4 (βw) , (45) respectively. The mixed Einstein tensor components take the following form where the G 0 0 component is depicted in Fig. 6(b). As for the previous SG model, we determine the limits w → ±∞ for all the components of the Einstein tensor components, which are given by: and in the limit of w → 0 the Einstein tensor components (for µ = ν = 0, 1, 2, 3) takes the form Note that in this model the cosmological constant on the brane Λ would be −2α 2 β 2 . Taking into account all of the above considerations, we verify that the Einstein equations are given consistently by G AB = κ 2 5 T AB where κ 2 5 = 2. The geodesic equation for a test particle moving in the direction of the fifth dimension one obtains which corresponds to a linearized quantum mode of energy ω = Ω = F (w 0 ) = 2 3 c 1 αβ. It is seen that the resulting potential for the ϕ 4 model is an odd function with respect to w, while the A function is even. This property is not verified in the SG and ϕ 6 models, where the latter is discussed below. This results in an asymmetry in the corresponding properties (such as the energy density). However, by an appropriate selection of the model parameters, one can restore the Z 2 symmetry in the φ 4 and φ 6 cases, which is commonly used in brane models.

C. ϕ 6 -based model
For this model, we have the following potential: and as a result, the kink solution is given by [41] φ(w) = α  where α and β are constant (as in the ϕ 4 model). The kink solution is depicted in Fig. 1(c). For this model the potential W , and the warp factor A are given by the following expressions respectively. Thus, for the ϕ 6 system the self-interaction potential takes the form . 3: The plots depict the modified soliton potential as a function of the fifth dimension for (a) SG with a = b = 10, (b) φ 4 with α = β = 1 and (c) φ 6 with α = β = 1 systems. The potential of the SG and φ 6 systems have non-degenerate vacuua. In contrast, the φ 4 potential has degenerate vacuua and this leads to stable, topological solitonic brane. which is depicted in Fig. 3(c). Note that V (ϕ) has raised to O(ϕ 8 ).
The energy density, T 00 , is given by: which is plotted in Fig. 4(c). The thickness of this brane is given by = ( √ 2αβ) −1 .  with α = β = 1 and (c) φ 6 with α = β = 1 systems. Note that this quantity approaches different constant values for the SG and ϕ 6 systems, while the ϕ 4 system is Z2-symmetric.

For this system the Ricci scalar is given by
The mixed Einstein tensor components are given by where the G 0 0 component is depicted in Fig. 6(c). These tensor components reduce to the following in the limit of w −→ ±∞: and in the limit of w → 0, the Einstein tensor is (for µ = ν = 0, 1, 2, 3): As in the previous cases, the Einstein equation in the bulk G AB = κ 2 5 T AB is found self-consistently with κ 2 5 = 2, and which leads to the following geodesic equation The quantum mode energy is thus given by

IV. BRANE STABILITY
In this section, we examine small perturbations about the soliton branes obtained in the previous sections. To this end, the metric and the scalar field are perturbed about the static brane and the resulting equations of motion are linearized in the proper gauge [6,[32][33][34][35]. The linearized equation turns out to be a Schrodinger-like equation with a potential U (z) which determines the linear modes. Unfortunately, for the models considered, this potential is too complicated to be reproduced analytically, or to be used for finding the corresponding modes. Therefore, we will only examine the potential near its minimum up to second order in z.
In order to study the stability of the branes, we choose an "axial gauge" where the metric is perturbed as [6,[32][33][34][35]: where g µν represents the four-dimensional AdS or dS metric, h µν represents the metric perturbations, and ε is a small parameter [33]. Moreover, in order to render the metric conformally flat, one can choose dz = e −A(w) dw. In this case, the corresponding Schrödinger equation takes the form [6,[32][33][34][35]: where the potential is given by: Note that Λ is a cosmological constant on the brane, which could be positive, negative or zero corresponding to the 4D spacetime being de Sitter (dS 4 ), anti-de Sitter (AdS 4 ) or Minkowski (M 4 ), respectively [32,33]. The corresponding potentials for the SG, ϕ 4 and ϕ 6 systems are calculated to be as follows: (i) SG model: (70) (ii) ϕ 4 model: (iii) ϕ 6 model: As in quantum mechanical systems, we check for the stability of the system via the existence of a real frequency, bound ground state. Since the potentials (70)-(72) are too complicated to be solved analytically, we found the corresponding ground state eigenvalues via expansions in terms of the fifth coordinate w. One can deduce the stability up to O(w 2 ) by looking at the sign of the w 2 term. It is seen in Figures 7-9 that this coefficient is everywhere positive (stability) for the φ 4 system, while there are regions of the parameter space where the coefficient is negative (instability) for the SG and φ 6 systems.

V. CONCLUSION
In this work, we obtained exact thick brane models inspired by well-known nonlinear systems, namely, the sine-Gordon (SG), ϕ 4 and ϕ 6 models. The confining effect of the scalar field in all these three models were confirmed by examining the geodesic equation for a test particle moving normal to the brane. In particular, it turns out that the modified potential for the SG system resembles that of the double sine-Gordon (DSG) system, while those of ϕ 4 and ϕ 6 became ϕ 6 and ϕ 8 , respectively. We have extended previous brane models [10,14,34] based on SG and φ 4 solitons taking into account different parametrizations. The similarity of the φ 4 model with the generic Higgs model makes this choice particularly interesting, especially as the resulting potential is an odd function of the fifth coordinate and the Z 2 symmetry is respected. We have studied the φ 6 model for the first time. This model is interesting by its own right, since unlike the φ 4 model, we have two pairs of solitions and anti-solitons which live in different topological sectors.
In the case of the SG model, we have used a more general form of the potential compared to the one used in [10] and [34]. The resulting brane does not have Z 2 symmetry, in general, where the center of the brane may be displaced from w = 0 and the potential will not be an odd function of w in general. However, by a suitable choice of the model parameters it is possible to make the vacuua of the effective potential degenerate, in which case the Z 2 symmetry is restored. In the case of the φ 6 model, however, we could not restore this symmetry via re-parametrization. Finally, using standard procedures, we examined the stability of the thick branes, by determining the sign of the w 2 term in the expansion of the potential for the resulting Schrodinger-like equation. It turns out that the φ 4 brane is stable, while there are unstable modes for certain ranges of the model parameters in the SG and φ 6 branes.
We considered the limiting case in which the brane tends to zero thickness and approaches a thin brane. It should be noted that the topological stability of the soliton brane remains valid even in this limit (at least at the classical level). An interesting question would be whether the thick brane continuous metric develops a discontinuity and whether the Israel junction conditions will be satisfied in this limit. Although one would expect intuitively that this is the case, we have not worked out the detailed calculations. This issue will be explored in separate paper.