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Holographic Schwinger effect in a soft wall AdS/QCD model

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Yue Ding and Zi-qiang Zhang. Holographic Schwinger effect in a soft wall AdS/QCD model[J]. Chinese Physics C. doi: 10.1088/1674-1137/abc240
Yue Ding and Zi-qiang Zhang. Holographic Schwinger effect in a soft wall AdS/QCD model[J]. Chinese Physics C.  doi: 10.1088/1674-1137/abc240 shu
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Holographic Schwinger effect in a soft wall AdS/QCD model

    Corresponding author: Zi-qiang Zhang, zhangzq@cug.edu.cn
  • School of Mathematics and Physics, China University of Geosciences, Wuhan 430074, China

Abstract: We perform a potential analysis for the holographic Schwinger effect in a deformed AdS5 model with conformal invariance broken by a background dilaton. We evaluated the static potential by analyzing the classical action of a string attached to a rectangular Wilson loop on a probe D3 brane located at an intermediate position in the bulk AdS space. We observed that the inclusion of the chemical potential tends to enhance the production rate, which is opposite to the effect of the confining scale. In addition, we calculated the critical electric field based on the Dirac-Born-Infeld (DBI) action.

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    I.   INTRODUCTION
    • The Schwinger effect is an interesting phenomenon in quantum electrodynamics (QED), by which virtual electron-position pairs can be materialized and become real particles owing to the presence of a strong electric field. The production rate Γ (per unit time and unit volume) was first calculated by Schwinger for weak-coupling and a weak-field in 1951 [1] as

      Γexp(πm2eE),

      (1)

      where E, m, and e are the external electric field, electron mass, and elementary electric charge, respectively. In this case, there is no critical field trivially. Thirty-one years later, Affleck et al. generalized it to the case of arbitrary coupling and a weak-field [2] as

      Γexp(πm2eE+e24),

      (2)

      where the exponential suppression vanishes when E reaches Ec=(4π/e3)m2137m2/e. Clearly, the critical field Ec does not satisfy the weak-field condition, i.e., eEm2. Thus, it seems that one cannot determine Ec under the weak-field condition. In addition, one cannot determine whether a catastrophic decay actually occurs.

      In fact, the Schwinger effect is not unique to QED but is a universal aspect of quantum field theories (QFTs) coupled to a U(1) gauge field. However, it remains difficult to study this effect in a QCD-like manner or through a confining theory using QFTs because the (original) Schwinger effect must be non-perturbative. Fortunately, the AdS/CFT correspondence [3-5] may provide an alternative approach. In 2011, Semenoff and Zarembo proposed [6] a holographic setup to study the Schwinger effect based on the Higgsed N=4 supersymmetric Yang-Mills theory (SYM). They found that at a large N and a large 't Hooft coupling λ

      Γexp[λ2(EcEEEc)2],Ec=2πm2λ,

      (3)

      Interestingly, the value of Ec coincides with that obtained from the DBI action [7]. Subsequently, Sato and Yoshida argued that [8] the Schwinger effect can be studied using a potential analysis. Specifically, the pair production can be estimated using a static potential, consisting of static mass energies, an electric potential from an external electric-field, and the Coulomb potential between a particle-antiparticle pair. The shapes of the potential depend on the external field E (see Fig. 1). When E<Ec, the potential barrier is present, and the Schwinger effect can occur as a tunneling process. As E increases, the barrier decreases and gradually disappears at E=Ec. When E>Ec, the vacuum becomes catastrophically unstable. Further studies on the Schwinger effect in this direction can be found in [9-19]. The holographic Schwinger effect has also been investigated from the imaginary part of a probe brane action [20-23]. For a recent review on this topic, see [24].

      Figure 1.  V(x) versus x with V(x)=2meExαsx, where αs denotes the fine-structure constant.

      Herein, we present an alternative holographic approach to study the Schiwinger effect using potential analysis. As the motivation of this study, holographic QCD models, such as hard walls [25,26], soft walls [27], and some improved AdS/QCD models [28-34] have achieved considerable success in describing various aspects of hadron physics. In particular, we will adopt the SWT,μ model [35], which is defined by the AdS with a charged black hole to describe the finite temperature and density multiplied by a warp factor to generate confinement. It turns out that such a model can provide a good phenomenological description of quark-antiquark interaction. In addition, the resulting deconfinement line in the μT plane is similar to that obtained by lattice and effective models of QCD (for further studies on models of this type, see [36-41]). Motivated by this, in this study, we considered the Schwinger effect in the SWT,μ model. Specifically, we want to understand how the Schwinger effect is affected by the chemical potential and confining scale. In addition, this study can be considered as an extension of [8] to a case using the chemical potential and confining scale.

      The outline of this paper is as follows. In the next section, we briefly review the SWT,μ model given in [35]. In Section III, we describe the potential analysis for the Schwinger effect in the SWT,μ model and investigate how the chemical potential and confining scale affect the production rate. In addition, we calculate the critical field from the DBI action. Finally, we provide some concluding remarks regarding our results in Section IV.

    II.   SETUP
    • This section is devoted to a short introduction of the SWT,μ model proposed in [35]. The metric of the model in the string frame takes the following form:

      ds2=R2z2h(z)(f(z)dt2+dx2+dz2f(z)),

      (4)

      with

      f(z)=1(1+Q2)(zzh)4+Q2(zzh)6,h(z)=ec2z2,

      (5)

      where R is the AdS radius. Here Q represents the charge of a black hole; z denotes the fifth coordinate with z=zh as the horizon, which is defined by f(zh)=0. The warp factor h(z), characterizing the soft wall model, distorts the metric and brings about a confining scale c (see [28] for an analytical way to introduce the warp factor through a potential reconstruction approach).

      The temperature of the black hole is

      T=1πzh(1Q22),0Q2.

      (6)

      In addition, the chemical potential is

      μ=3Q/zh.

      (7)

      Note that, for Q=0, the SWT,μ model reduces to the Andreev model [42]. For c=0, it becomes an AdS-Reissner Nordstrom black hole [43,44]. For Q=c=0, it returns to an AdS black hole.

    III.   POTENTIAL ANALYSIS IN (HOLOGRAPHIC) SCHWINGER EFFECT
    • In this section, we follow the argument in [8] to study the behavior of the Schwinger effect in the SWT,μ model. Because the calculations of [8] were performed using the radial coordinate r=R2/z, for contrast, we also use coordinate r.

      The Nambu-Goto action is

      S=TFdτdσL=TFdτdσg,TF=12πα,

      (8)

      where α is related to λ by R2α=λ, and g represents the determinant of the induced metric

      gαβ=gμνXμσαXνσβ,

      (9)

      where gμν is the metric, and Xμ is the target space coordinate.

      Supposing the pair axis is aligned in one direction, e.g., the x1 direction,

      t=τ,x1=σ,x2=0,x3=0,r=r(σ).

      (10)

      Under this ansatz, the induced metric reads as follows:

      g00=r2h(r)f(r)R2,g01=g10=0,g11=r2h(r)R2+R2h(r)r2f(r)(drdσ)2,

      (11)

      and the Lagrangian density then becomes

      L=M(r)+N(r)(drdσ)2,

      (12)

      with

      M(r)=r4h2(r)f(r)R4,N(r)=h2(r).

      (13)

      Because L does not depend on σ explicitly, the Hamiltonian is conserved,

      LL(drdσ)(drdσ)=Constant.

      (14)

      Imposing the boundary condition at the tip of the minimal surface,

      drdσ=0,r=rc(rt<rc<r0),

      (15)

      one gets

      drdσ=M2(r)M(r)M(rc)M(rc)N(r),

      (16)

      with M(rc)=M(r)|r=rc. Here, r=rt is the horizon, and r=r0 is an intermediate position, which can yield a finite mass [6]. The configuration of the string world-sheet is depicted in Fig. 2.

      Figure 2.  Configuration of string world-sheet.

      Integrating (16) with the boundary condition (15), the following inter-distance of the particle pair is obtained:

      x=2r0rcdσdrdr=2r0rcdrM(rc)N(r)M2(r)M(r)M(rc).

      (17)

      In contrast, plugging (12) into (8), the sum of the Coulomb potential and static energy is given by the following:

      VCP+E=2TFr0rcdrM(r)N(r)M(r)M(rc).

      (18)

      The next task is to calculate the critical field. The DBI action is

      SDBI=TD3d4xdet(Gμν+Fμν),

      (19)

      with

      TD3=1gs(2π)3α2,Fμν=2παFμν,

      (20)

      where TD3 is the D3-brane tension.

      The induced metric is

      G00=r2h(r)f(r)R2,G11=G22=G33=r2h(r)R2.

      (21)

      Supposing the electric field is turned on along the x1 direction [8],

      Gμν+Fμν=(r2h(r)f(r)R22παE002παEr2h(r)R20000r2h(r)R20000r2h(r)R2),

      (22)

      which results in

      det(Gμν+Fμν)=r4h2(r)R4[(2πα)2E2r4h2(r)f(r)R4].

      (23)

      Substituting (23) into (19) and locating the probe D3-brane at r=r0, one finds

      SDBI=TD3r20h(r0)R2d4xr40h2(r0)f(r0)R4(2πα)2E2,

      (24)

      with f(r0)=f(r)|r=r0, h(r0)=h(r)|r=r0.

      To avoid (24) being ill-defined, one should get

      r40h2(r0)f(r0)R4(2πα)2E20,

      (25)

      yielding

      ETFr20h(r0)R2f(r0).

      (26)

      As a result, the critical field is as follows:

      Ec=TFr20h(r0)R2f(r0),

      (27)

      and it can be seen that Ec depends on T, μ, and c.

      Next, we calculate the total potential. For simplicity, we introduce the following dimensionless parameters

      αEEc,yrrc,arcr0,brtr0.

      (28)

      Given the above, the total potential reads as follows:

      Vtot(x)=VCP+EEx=2ar0TF1/a1dyA(y)B(y)A(y)A(yc)2ar0TFαr20h(y0)R2f(y0)×1/a1dyA(yc)B(y)A2(y)A(y)A(yc),

      (29)

      with

      A(y)=(ar0y)4h2(y)f(y)R4,A(yc)=(ar0)4h2(yc)f(yc)R4,B(y)=h2(y),h(y)=ec2R4(ar0y)2,f(y)=1(1+μ2R43r2t)(bay)4+μ2R43r2t(bay)6,h(yc)=ec2R4(ar0)2,f(yc)=1(1+μ2R43r2t)(ba)4+μ2R43r2t(ba)6,h(y0)=ec2R4r20,f(y0)=1(1+μ2R43r2t)b4+μ2R43r2tb6,

      (30)

      we have checked that, by taking c=μ=0 in (29), the result of N=4 SYM [8] is regained.

      Before going further, we should discuss the value of c. In this study, we considered the behavior of the holographic Schwinger effect in a class of models parametrized by c. To this end, we make c dimensionless by normalizing it at fixed temperatures and express other quantities in units of μ. In [45], the authors found that the range of 0c/T2.5 is the most relevant for a comparison with QCD. We therefore use this range.

      In Fig. 3, we plot Vtot(x) as a function of x for μ/T=1 and c/T=0.1 (other cases with different values of μ/T and c/T have a similarity), where we set b=0.5 and TFr0=R2/r0=1 as in [8]. From these figures, it can be seen that a critical electric field exists at α=1 (E=Ec), and for α<1 (E<Ec), the potential barrier is present, which is in agreement with [8].

      Figure 3.  (color online) Vtot(x) versus x with μ/T=1 and c/T=0.1. In the plots, from top to bottom, α=0.8,0.9,1.0,1.1, respectively.

      To see how the chemical potential modifies the Schwinger effect, we plot Vtot(x) versus x with a fixed c/T for different values of μ/T in Fig. 4. The left panel is for c/T=0.1, and the right one is for c/T=2.5. In both panels, from top to bottom, μ/T=0,1,5, respectively. One can see that, at a fixed c/T, as μ/T increases, both the height and width of the potential barrier decrease. As we know, the higher or wider the potential barrier is, the more difficult it is for the produced pairs to escape to infinity. Thus, it can be concluded that the inclusion of the chemical potential decreases the potential barrier, thus enhancing the Schwinger effect, in accordance with the findings of [14].

      Figure 4.  (color online) Vtot(x) versus x with α=0.8 and fixed c/T for different values of μ/T. In both plots, from top to bottom, μ/T=0,1,5, respectively.

      In addition, we plot Vtot(x) against x with a fixed μ/T for different values of c/T in Fig. 5. It can be seen that, at a fixed μ/T, both the height and width of the potential barrier increase as c/T increases, implying that the presence of a confining scale reduces the Schwinger effect, inverse to the effect of the chemical potential.

      Figure 5.  (color online) Vtot(x) versus x with α=0.8 and fixed μ/T for different values of c/T. In both plots, from top to bottom, c/T=2.5,1,0, respectively.

      Finally, to understand how the chemical potential and confining scale affect the critical electric field, we plot Ec/Ec0 versus μ/T (c/T) in the left (right) panel of Fig. 6, where Ec0 denotes the critical electric field of the SYM. It can be seen that Ec/Ec0 decreases as μ/T increases, indicating that the chemical potential decreases Ec, thus enhancing the Schwinger effect. Meanwhile, the confining scale has an opposite effect, consistent with the potential analysis. Furthermore, it can be seen that Ec/Ec0 can be larger or smaller than 1, which means that the SWT,μ model may provide a wider range of the Schwinger effect in comparison to SYM.

      Figure 6.  (color online) Left: Ec/Ec0 versus μ/T; from top to bottom, c/T=2.5,1,0, respectively. Right: Ec/Ec0 versus c/T; from top to bottom, μ/T=0,1,5, respectively.

    IV.   CONCLUSION
    • The study of the Schwinger effect in non-conformal plasma under the influence of the chemical potential may shed some light on heavy ion collisions. In this paper, we investigated the effect of the chemical potential and confining scale on the holographic Schwinger effect in a soft wall AdS/QCD model. We analyzed the electrostatic potentials by evaluating the classical action of a string attached to the rectangular Wilson loop on a probe D3 brane located at an intermediate position in the bulk AdS and calculated the critical electric field from the DBI action. We found that the inclusion of the chemical potential tends to decrease the potential barrier, thus enhancing the production rate, opposite to the effect of the confining scale. Moreover, we observed that, with some chosen values of μ/T and c/T,Ec can be larger or smaller than the counterpart in SYM, implying that the SWT,μ model may provide a theoretically wider range of the Schwinger effect in comparison to SYM.

      However, there are some questions that need to be addressed further. First, the potential analysis is basically within the Coulomb branch, related to the leading exponent corresponding to the on-shell action of the instanton and not the full decay rate. In addition, the SWT,μ model is not a consistent model because it does not solve the full set of equations of motion. Performing such an analysis in some consistent models, e.g., those in [28-34], would be instructive (usually the metrics of such models are only known numerically, and thus, the calculations are more challenging).

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