Review Article | Open Access
Application of AdS/CFT in Nuclear Physics
We review some recent progress in studying the nuclear physics especially nucleon-nucleon (NN) force within the gauge-gravity duality, in context of noncritical string theory. Our main focus is on the holographic QCD model based on the AdS6 background. We explain the noncritical holography model and obtain the vector-meson spectrum and pion decay constant. Also, we study the NN interaction in this frame and calculate the nucleonmeson coupling constants. A further topic covered is a toy model for calculating the light nuclei potential. In particular, we calculate the light nuclei binding energies and also excited energies of some available excited states. We compare our results with the results of other nuclear models and also with the experimental data. Moreover, we describe some other issues which are studied using the gauge-gravity duality.
One of the fundamental ingredients of nuclear physics is the nuclear force with which point-like nucleons interact with each other. Since Yukawa, many potential models have been constructed which have been composed to fit the available NN scattering data. The newer potentials have only slightly improved with respect to the previous ones in describing the recent much more accurate data. As it is shown in , all of these potential models do not have good quality with respect to the pp scattering data below 350 MeV and just a few of them are of satisfactory quality. These models are the Reid soft-core potential Reid68 , the Nijmegen soft-core potential Nijm78 , the new Bonn pp potential Bonn89 , and also the parameterized Paris potential Paris80 . These familiar one-boson-exchange potentials (OBEP) contain a relatively small number of free parameters (about 10 to 15 parameters) but do not have a reasonable description of the empirical scattering data. Also, most of these potentials, which have been fitted to the np scattering data, unfortunately do not automatically fit to the pp scattering data even by considering the correction term for the Coulomb interaction . Of course, new versions of these potentials have been constructed such as Nijm I, Nijm II, Reid93 , CD-Bonn , and AV18  which explain the empirical scattering data successfully. But they contain a large number of purely phenomenological parameters. For example, an updated (Nijm92pp ) version of the Nijm78 potential contains 39 free parameters.
On the other hand, there are many attempts to impose the symmetries of QCD using an effective Lagrangian of pions and nucleons [10, 11]. These models only capture the qualitative features of the nuclear interactions and could not compete with the much more successful potential models mentioned above.
Despite many efforts, no potential model has yet been constructed which gives a high-quality description of the empirical data, obeys the symmetries of QCD, and contains only a few number of free phenomenological parameters.
In recent years, holography or gauge-gravity duality gave us a new approach to hadronic physics  and make new progress in understanding the nuclear force.
Nuclear force, the force between nucleons, exhibits a repulsive core of nucleons at short distances. This repulsive core is quite important for large varieties of physics of nuclei and nuclear matter. For example, the well-known presence of nuclear saturation density is essentially due to this repulsive core. However, from the viewpoint of strongly coupled QCD, the physical origin of this repulsive core has not been well understood. Nuclear force especially the repulsive core has been studied using the AdS/CFT correspondence [13–16] and an explicit expression has been obtained for the repulsive core.
Also, there are many attempts to find a geometry dual to nucleus. Since nucleons are described by -branes wrapping a sphere in curved geometry of holographic QCD, on a nucleus with mass number there appears a gauge theory. One can find the dual gravity by taking the large mass number limit and obtained a near horizon geometry corresponding to the heavy nucleus. The corresponding supergravity solution has discrete fluctuation spectra comparable with nuclear experimental data . As we know from the nuclear experiments, the nucleons of a heavy nuclei have coherent excitations which are called Giant resonances. These resonances exhibit harmonic behavior which is explained with phenomenological models such as the liquid drop model. The gauge-gravity duality can reproduce this behavior. Moreover, dependence to the mass number is obtained by using the duality .
Among the holographic QCD models, the Sakai-Sugimoto (SS) [18, 19] and Klebanov-Strassler (KS) models  are the most interesting holographic models to study strong coupling regime of QCD. The SS model is based on ten-dimensional type-IIA string theory, with a background geometry given by 4-branes. They fill four-dimensional Minkowski space-time and extend along a fifth extra dimension compactified on a circle whose circumference is parametrized by the Kaluza-Klein mass. Through this compactified dimension and antisymmetric boundary conditions for fermions supersymmetry is completely broken. Left- and right-handed chiral fermions are introduced by adding 8- and 8-branes which extend in all dimensions except for . In this compact direction, they are separated by a distance . There are two possible background geometries called confined and deconfined phase. For more details about the setup of the model see the original papers by Sakai and Sugimoto [18, 19]. In this model, there is a nice topological interpretation of chiral symmetry breaking.
Chiral symmetry breaking is realized in the model as follows. A gauge symmetry on the flavor branes corresponds to a global at the boundary. Therefore, the bulk gauge symmetries on the 8- and 8-branes can be interpreted as left- and right-handed flavor symmetry groups in the dual field theory. The Chern-Simons term accounts for the axial anomaly of QCD, such that one is left with the chiral group and the vector part . There is no explicit breaking of this group since the model only contains massless quarks. Spontaneous chiral symmetry breaking is realized when the 8- and 8-branes connect in the bulk. They always connect in the confined phase and whether they connect in the deconfined phase depends on the separation of the 8- and 8-branes in the extra dimension .
The Sakai-Sugimoto model is particularly suited for phenomenon related to chirality as chiral magnetic effect (CME) [21–25] since it has a well-defined concept for chirality and the chiral phase transition. It is straightforward to introduce right- and left-handed chemical potentials independently.
The chiral magnetic effect is a hypothetical phenomenon which states that, in the presence of a magnetic field B, a nonzero axial charge density will lead to an electric current along the direction of the field [26–28]. Analysis of RHIC data appears to favor the presence of a CME in the quark-gluon plasma, although a better understanding of systematic errors and backgrounds is still needed. CME is studied in many holographic systems, following [29–34], including systems without confinement or chiral symmetry breaking in vacuum.
Also, predictions of the SS model are in good agreement with the lattice simulations such as the glueball spectrum of pure QCD [35, 36]. This model describes baryons and their interactions with mesons well [18, 19, 37–39]. It is shown that the baryons can be taken as point-like objects at distances larger than their sizes, so their interactions can be described by the exchange of light particles such as mesons. Therefore, one can find the baryon-baryon potential from the Feynman diagrams using the interaction vertices including baryon currents and light mesons . But there are some inconsistencies. For example, the size of the baryon is proportional to . Consequently in the large ’t Hooft coupling (large ), the size of the baryon becomes zero and the stringy corrections have to be taken into account. Another problem is that the scale of the system associated with the baryonic structure is roughly half the one needed to fit to the masonic data . Also, the holographic models arising from the critical string theory encounter with the some Kaluza-Klein (KK) modes, with the mass scale of the same order as the masses of the hadronic modes. These unwanted modes are coupled with the hadronic modes and there is no mechanism to disentangle them from the hadronic modes yet. In order to overcome this problem, it is possible to consider the color brane configuration in noncritical string theory [41–44].
The noncritical string is not formulated with the critical dimension, but nonetheless has vanishing Weyl anomaly. A worldsheet theory with the correct central charge can be constructed by introducing a nontrivial target space, commonly by giving an expectation value to the dilaton which varies linearly along some spacetime direction. For this reason noncritical string theory is sometimes called the linear dilaton theory. Since the dilaton is related to the string coupling constant, this theory contains a region where the coupling is weak (and so perturbation theory is valid) and another region where the theory is strongly coupled [45, 46].
In such backgrounds the string coupling constant is proportional to , so the large limit corresponds to the small string coupling constant. However, contrary to the critical holographic models, in the large limit, the ’t Hooft coupling is of order one instead of infinity and the scalar curvature of the gravitational background is also of order one. So, it seems that the noncritical gauge-gravity correspondence is not very reliable. But studies show that the results of these models for some low energy QCD properties such as the meson mass spectrum, Wilson loop, and the mass spectrum of glueballs [45–47] are comparable with lattice computations. Therefore, noncritical holographic models still seem useful to study QCD.
One of the noncritical holographic models is composed of a and anti brane in six-dimensional non-critical string theory [43, 47]. The low energy effective theory on the intersecting brane configuration is a four-dimensional QCD-like effective theory with the global chiral symmetry . In this brane configuration, the six-dimensional gravity background is the near horizon geometry of the color -branes. This model is based on the compactified spacetime with constant dilaton. So the model does not suffer from large string coupling as the SS model. The meson spectrum  and the structure of thermal phase  are studied in this model. Some properties, like the dependence of the meson masses on the stringy mass of the quarks and the excitation number, are different from the critical holographic models such as the SS model.
We study the gauge field and its mode expansion in this noncritical holography model and obtain the effective pion action . The model has a mass scale like the SS model in which we set its value by computing the pion decay constant. Then, we study the baryon  and obtain its size. We show that the size of the baryon is of order one with respect to the ’t Hooft coupling, so the problem of the zero size of the baryon in the critical holography model is solved. But the size of the baryon is still smaller than the mass scale of holographic QCD, so we treat it as a point-like object and introduce an isospin Dirac field for the baryon . We write a 5 effective action for the baryon field and reduce it to 4 using the mode expansion of gauge field and baryon field and obtain the NN potential in terms of the meson exchange interactions. We calculate the meson-nucleon couplings using the suitable overlapping wave function integrals and compare them with the results of SS model. Also, we compare the nucleon-meson couplings obtained from noncritical holographic model with the results of SS model and predictions of some phenomenological models. Our study shows that the noncritical results are in good agreement with the other available models.
On the other hand, one of the oldest problems of nuclear physics is the nuclear binding energies: the interactions between nucleons are very strong, while the nuclear matter is not relativistic. Nuclear binding energies are experimentally known with high accuracy while they are not predicted with sufficient accuracy using different theoretical models. Since, prediction of nuclear binding energy is a useful tool to test the goodness of a theoretical nucleon-nucleon (NN) interaction model, we use our NN holography potential to obtain the light nuclei binding energies. We construct a nuclear holographic model [50–53] in the noncritical base and calculate the nuclei potentials as the sum of their NN interactions. The minimum of the ground state potential is considered as the binding energy. Also, difference between this energy and the minimum of the excited state potential presents the excited energy for each state. In order to compute the potentials, we use the values of nucleon-meson coupling constants obtained from both the critical and noncritical holography models.
This paper is organized as follows. In Section 2, we briefly review the AdS/CFT correspondence. The noncritical holographic model is introduced in Section 3 and NN potential is constructed in this section. In Section 4, we construct a simple model to study light nuclei such as , , , and and obtain their potential of ground and excited states and respective binding energies. Section 5 is devoted to a brief summary and conclusions. Also, some other topics, which are studied using the duality, are introduced in this section.
2. Review of AdS/CFT Correspondence
2.1. Historical Notes
Quantum chromodynamics (QCD) is the quantum field theory of the strong interactions which has two important properties, asymptotic freedom and confinement. Various analytical and numerical methods have been developed to study QCD. One example is perturbative QCD which works at small distances where the coupling is weak but fails to work at larger distances where the coupling becomes relatively strong in which case the problem is said to become nonperturbative. Examples of methods that study nonperturbative problems are effective field theories such as chiral perturbation theory, lattice QCD , Dyson-Schwinger equations (DSE) formalism , and gauge-gravity duality [12, 66, 67].
Before QCD, in the 1960’s string theory was introduced as a model to describe the strong interactions . It was able to explain the organization of hadrons in Regge trajectories, describing them as rotating strings. After the formulation of QCD, string theory took a different direction, becoming a possible candidate for a unified theory of all the forces. Nevertheless, some string interpretation of hadron spectra was not abandoned; for example, a meson is sometimes described as a quark and an antiquark connected by a tube of strong interaction flux [69, 70]. This picture establishes a link between QCD and string theory, which becomes even more evident in the limit of large number of colors . ’t Hooft proposed that in this limit the gauge theory may have a description in terms of a tree level string theory; in particular, the leading Feynman diagrams in the 1/ expansion are planar and look like the worldsheet of a string theory. For example, a meson can be represented by two quark lines propagating in time connected by a dense sheet of gluons, reminding the worldsheet swept out by a string through time. In 1997, these studies found a possible new framework in the so-called AdS/CFT correspondence , a conjecture introduced by Maldacena relating a supergravity theory in ten dimensions to a supersymmetric gauge theory in four dimensions. This correspondence has been extended to a gauge theory as , thus proving some link between QCD and a higher dimensional theory in a curved space-time.
2.2. D-Branes and AdS Space
The most important property of -branes is that they contain gauge theories on their world volume. In particular, the massless spectrum of open strings living on a -brane contains a (maximally supersymmetric) gauge theory in dimensions. Moreover, it appears that if we consider the stack of coincident -branes, then there are different species of open strings which can begin and end on any of the -branes, allowing us to have (maximally supersymmetric) gauge theory on the world-volume of these -branes. Now, if is sufficiently large, then this stack of -branes is a heavy object embedded into a theory of closed strings that contains gravity. This heavy object curves the space which can then be described by some classical metric and other background fields.
Thus, we have two absolutely different descriptions of the stack of coincident -branes. One description is in terms of the supersymmetric gauge theory on the world volume of the -branes, and the other is in terms of the classical theory in some gravitational background. It is this idea that lies at the basis of gauge-gravity duality.
One important example is 3-branes which can also be seen as solutions of ten-dimensional type IIB supergravity at low energies, with metric of the form : where Here, is the string coupling constant which is related to the constant dilaton as (). Also, there is units of flux. is the only length scale in the solution. This metric interpolates between a throat geometry and a ten-dimensional Minkowski region.
If we take the near horizon limit of the solution given in (1), , and redefine , we can completely decouple the Minkowski region and are left with a throat geometry which is given by which is the Poincaré wedge of the direct product of five-dimensional anti-de-Sitter space and a five-sphere (). The isometry group of this space is given by , though if we include fermions, the full supersymmetric isometry group is . Note that this is exactly the same as the full global symmetry group of the low energy limit of the open string sector (i.e., SYM theory).
We see that the radius , of both the AdS throat and the , in string units is given in terms of the gauge theory parameters as
Therefore, in order that the stringy modes be unimportant, , which translates into gauge theory language as .
2.3. = 4 Super Yang-Mills Theory
4 supersymmetric Yang-Mills theory (SYM) in four dimensions (the dimensionality of the world volume of the 3-branes) has one vector field, , six scalars fields (), and four fermions , () which are in the and representations of the -symmetry group.
This theory naturally arises on the surface of a 3-brane in type IIB superstring theory. Open strings generate a massless gauge field in ten dimensions. When the open string ends are restricted to a dimensional subspace the ten components of the gauge field naturally break into a dimensional gauge field and 6 scalar fields. The fermionic superpartners naturally separate to complete the dimensional supermultiplets.
The beta function of 4 SYM theory vanishes to all orders in perturbation theory, . This implies that the theory is conformal with conformal symmetry group also at the quantum level. Moreover, this theory has a global symmetry group. The complete superconformal group is , of which both and are bosonic subgroups.
2.4. The AdS/CFT Correspondence
The AdS/CFT correspondence, which was first suggested by Maldacena  in 1997, states that type IIB string theory on plus some appropriate boundary conditions (and possibly also some boundary degrees of freedom) is dual to 4, = 3 + 1 super Yang-Mills. There are three different versions of this conjecture , depending on the precise form of the limits taken. In the strong version, type IIB string theory on is dual to SYM theory. The mild version relates classical type IIB strings on to planar SYM theory. But the mostly adopted form of the conjecture is the weak regime (in the SUGRA limit) which specializes further to the case in which is large. In this limit, strongly coupled 4 Yang-Mills theory is mapped to supergravity on ; the inverse string tension goes to zero.
A precise way in which the two theories can be mapped into each other was proposed independently by Gubser et al.  and by Witten . Since the boundary of the space, namely, , is equivalent to , which is a copy of the Minkowski space, plus a point at infinity, the authors suggested a recipe to link the gravity theory in the bulk (AdS space) to the field theory on the boundary (Minkowski space). In this sense, the AdS/CFT correspondence can be considered as a holographic projection of the supergravity theory in the bulk to the field theory on the boundary.
Despite the fact that there is no proof of the AdS/CFT correspondence taking account of its string-theoretical origin yet, the huge amount of symmetry present almost guarantees that the AdS/CFT correspondence should hold. When proceeding to less symmetrical situations below, generalized gauge-gravity dualities remain a conjecture though.
2.5. QCD versus SYM
It would be useful if the four-dimensional theory on the boundary was QCD, since this would allow us to explore its nonperturbative regime by studying a perturbative dual theory. However, the field theory described by the correspondence is a supersymmetric theory with conformal invariance, while QCD has none of these features. The most important differences between the two theories are as follows .(i)QCD confines while SYM is not confining.(ii)QCD has a chiral condensate while SYM has no chiral condensate.(iii)QCD has a discrete spectrum while that of SYM is continuous.(iv)QCD has a running coupling while SYM has a tunable coupling and is conformal.(v)QCD has quarks while SYM has adjoint matter.(vi)QCD is not supersymmetric while SYM is maximally supersymmetric.(vii)QCD has in real life, while the AdS/CFT correspondence holds for large .
However, the gauge-gravity duality can be expanded to more field theories by changing the supergravity theory. This gives a possibility to search for a field theory that is closer to QCD and has a gravity dual.(i)For example, considering multiple 3-branes on curved backgrounds leads to an interesting family of superconformal field theories [74, 75] which contain adjoint matter fields. Also, one can introduce the confinement and break the conformal symmetry by deforming the background further. This leads to chiral symmetry breaking and a running coupling constant .(ii)Also, theories looking like supersymmetric Yang-Mills theory in the IR can be obtained by considering higher dimensional -branes wrapped on certain submanifolds of the ten-dimensional geometry [76, 77].(iii)Deformations of the geometry lead to nonsupersymmetric, nonconformal gauge theories which display confinement and chiral symmetry breaking [18, 19, 78–81].(iv)Fundamental matter can be added to the gauge theory by introducing 7-branes . In the quenched approximation, , their effect on the background geometry is ignored. Also, dynamical quarks can be added to this geometry .(v)Recently, some phenomenological models have been suggested which are motivated by the AdS/CFT but not within the full string theory framework. These models are known as AdS/QCD [83–86].(vi)Also, an approach similar to AdS/QCD is introduced based on the noncritical string theory in dimensions [42, 86, 87].
3. Holographic QCD from the Noncritical String Theory
The key idea of construction of holographic models with flavors was given by Karch and Katz . In these models, two stacks of flavor branes, branes, and antibranes are added to the geometry as a probe, so that the back reaction of the flavor branes is negligible (probe approximation). This approximation is reliable when , where and refer to the number of colors and flavors, respectively.
Of course, the brane/antibrane system is unstable, since the branes and antibranes will tend to annihilate. This is reflected in the presence of tachyons in the spectrum. But, it should make sense within the context of perturbation theory. The point where the tachyon field vanishes corresponds to a local maximum of the tachyon potential, and thus it is part of a classical solution. The one-loop effective action in an expansion around this solution should be well defined, even though the solution is unstable, and, in particular, it should have a well-defined phase. It was conjectured that at the minimum of the tachyon potential, the negative contribution to the energy density from the potential exactly cancels the sum of the tensions of the brane and the antibrane, thereby giving a configuration of zero energy density (and hence restoring spacetime supersymmetry). Therefore, the various gauge and gravitational anomalies, which arise as one-loop effects, cancel and as we expected theory is perturbatively well-behaved [72–92].
In this section, we study a model which is similar in many aspects to the SS model , a holographic model based on the critical string theory. But, we try to solve some inconsistencies of the SS model in describing the baryons via the noncritical model.
3.1. AdS6 Model
In the presented noncritical model, the gravity background is generated by near-extremal -branes wrapped over a circle with the antiperiodic boundary conditions. Two stacks of flavor branes, namely, -branes and anti--branes, are added to this geometry and are called flavor probe branes. The color branes extend along the directions , , , , and while the probe flavor branes fill the whole Minkowski space and stretch along the radius which is extended to infinity. The strings attaching a color 4-brane to a flavor brane transform as quarks, while strings hanging between a color and a flavor transform as antiquarks. The chiral symmetry breaking is achieved by a reconnection of the brane and antibrane pairs. Under the quenched approximation , the reactions of flavor branes and the color branes can be neglected. Just like the SS model, the coordinate is wrapped on a circle and the antiperiodic condition is considered for the fermions on the thermal circle. The final low energy effective theory on the background is a four-dimensional QCD-like effective theory with the global chiral symmetry .
In this model, the near horizon gravity background at low energy is  where is the radius of the AdS space. Also and RR six-form field strength, , are defined by the following relations:
In order to obtain solutions of near extremal flavored , the values of dilaton and are considered as
This relation indicates that the and dilaton depend on the ratio of the number of colors and flavors . Under the quenched approximation, the values of the dilaton and AdS radius can be rewritten as where is proportional to the number of color branes, .
To avoid singularity, the coordinate satisfies the following periodic condition:
Also, the Kaluza-Klein mass scale of this compact dimension is and dual gauge field theory for this background is nonsupersymmetric. Also, the Yang-Mills coupling constants can be defined as a function of string theory parameters using the DBI action as follows: where is the Regge slope parameter and is the string length. Also, the ’t Hooft coupling is .
3.2. Meson Sector
In AdS/QCD, there is a gauge field living in the bulk AdS whose dynamics is dual to the meson sector of QCD such as pions and higher resonances. The gauge field on the -brane includes five components, () and . The -brane action is given by Pahlavani et al. : where , is the field strength tensor, and is the gauge field on the -brane. The second term in the above action is the Chern-Simons action and . It is useful to define the new variable as
Then by neglecting the higher order of in the expansion, the -brane action can be written as  where is
The gauge fields () and have a mode expansion in terms of complete sets and as
After calculating the field strengths, the action (14) is rewritten as where the over dot denotes the derivative respect to the coordinate.
We introduce the following dimensionless parameters: and find that the functions () satisfy the normalization condition as
Also, we suppose that the functions () satisfy the following condition:
The orthonormal condition for are as follows:
We find that the functions and are related together. In fact, we can consider (). Also, there exists a function which is orthogonal to for all :
We use the normalization condition to obtain the normalization constant . Finally by using an appropriate gauge transformation, the action (14) becomes where is a massive vector meson of mass for all and is the pion field, which is the Nambu-Goldstone boson associated with the chiral symmetry breaking .
It is useful to make another gauge choice, namely, the gauge. Actually, we can transform to the new gauge through a suitable gauge transformation and obtain the following new gauge fields:
Function is calculated through where is well-known hypergeometric function. It should be noted that the massless pseudoscalar meson appears in the asymptotic behavior of , since we have
Since (21) is invariant under , we can assume to be an even or odd function. In fact, the is a four-dimensional vector and axial vector if is an even or odd function, respectively. Equation (21) is solved numerically using the shooting method to obtain the mass of lightest mesons. Our results are compared with the results of the SS, KS, and DKS models and experimental data in Table 1. As is clear, our results are in good agreement with the experimental data .
3.3. Pion Effective Action
Now, we just consider the pion field in the gauge field expansion and use the non-Abelian generalization of the DBI action to find the effective pion action : where the coefficients and are defined by the following relations :
It is clear from the above equations that the parameters and depend on as and , respectively. It is coincident with the result obtained from the SS model and also QCD in large . We fix the such that the MeV for . So, we obtain MeV for our holographic model . It should be noted that is the only mass scale of the noncritical model below which the theory is effectively pure Yang-Mills in four dimensions.
3.4. Baryon in AdS6
In this section we aim to introduce baryon configuration in the noncritical holographic model. As is known, in the SS model the baryon vertex is a -brane wrapped on a cycle. Here in six-dimensional configuration, there is no compact sphere. So, we introduce an unwrapped -brane as a baryon vertex instead . In analogy with the SS model, there is a Chern-Simons term on the vertex world volume as which induces units of electric charge on the unwrapped -brane. In accordance with the Gauss constraint, the net charge should be zero. So, one needs to attach fundamental strings to the -brane. In turn, the other side of the strings should end up on the probe -branes. The baryon vertex looks like an object with electric charge with respect to the gauge field on the -brane whose charge is the baryon number. This -brane dissolves into the -brane and becomes an instanton solution . It is important to know the size of the instanton in our model. In the SS model, it is shown that the size of an instantonic baryon goes to zero at large ’t Hooft coupling limit which is one of the problems of the SS model in describing the baryons .
Let us consider the DBI action in the Yang-Mills approximation for the -brane:
The induced metric on the -brane is
It is useful to define the new coordinate
Using this coordinate, the metric (34) transforms to a conformally flat metric:
Also, the coordinate can be rewritten in terms of the coordinate introduced in (13) as
Note that in the new conformally flat metric, the fifth direction is a finite interval because
We can approximate near the origin as and using relation (10), we obtain or, equivalently,
In analogy with the SS model, this relation implies that is the only mass scale that dictated the deviation of the metric from the flat configuration and it is the only mass scale of the theory in the low energy limit. (It should be noted that the -branes come with two asymptotic regions at corresponding to the ultraviolet and infrared region near the ).
Thus, the position dependent electric coupling of this five-dimensional Yang-Mills is equal to 
Also, for a unit instanton we have
Inserting the above relations in (42), we obtain the energy of a point-like instanton localized at as
By increasing the size of the instanton, more energy is needed because is an increasing function of . So the instanton tends to collapse to a point-like object. On the other hand, fundamental strings attached to the -branes behave as units of electric charge on the brane. The Coulomb repulsions among them prefer a finite size for the instanton. Therefore, there is a competition between the mass of the instanton and Coulomb energy of fundamental strings. For a small instanton of size with the density , the Yang-Mills energy is approximated as and the five-dimensional Coulomb energy is
The size of a stable instanton is obtained by minimizing the total energy :
As it is stated in the previous section, in the SS model (the critical version of dual QCD) the size of the instanton goes to zero because of the large ’t Hooft coupling limit. However in the noncritical string theory, the ’t Hooft coupling is of order one. So, the size of the instanton is also of order 1 but it is still smaller than the effective length of the fifth direction of the dual QCD.
3.5. Nucleon-Nucleon Potential
In the previous section, we demonstrated that the size of the baryon in the noncritical holographic model is smaller than the scale of the dual QCD and we can assume that the baryon is a point-like object in five dimensions. Thus as a leading approximation, we can treat it as a point-like quantum field in five dimensions. In the rest of this paper, we will restrict ourselves to fermionic baryons because we intend to study the nucleons. So, we consider odd to study a fermionic spin baryon. We choose in our numerical calculations for realistic QCD. Also, we will assume and consider the lowest baryons which form the proton-neutron doublet under . All of these assumptions lead us to introduce an isospin Dirac field, for the five-dimensional baryon.
The leading 5 kinetic term for is the standard Dirac action in the curved background along with a position dependent mass term for the baryon. Moreover, there is a coupling between the baryon field and the gauge filed living on the flavor branes that should be considered. Therefore, a complete action for the baryon reads as  where is a covariant derivative, is the size of the stable instanton, and is an unknown function with a value as of . are the standard matrices in the flat space and .
The factor is used for convenience. Usually, the first two terms in the action are called the minimal coupling and the last term in the first integral refers to the magnetic coupling.
A four-dimensional nucleon is the localized mode at which is the lowest eigenmode of a five dimensional baryon along the direction. So, the action of the five-dimensional baryon must be reduced to four dimensions. In order to do this, one should perform the KK mode expansion for the baryon field and the gauge field . The gauge field has a KK mode expansion presented in (16). The baryon field also can be expanded as where is the chiral component of the four-dimensional nucleon field. Also the profile functions, , satisfy the following conditions: in the range , and the eigenvalue is the mass of the nucleon mode, . Moreover, the eigenfunctions obey the following normalization condition:
It is more useful to consider the following second-order differential equations for
As we approach , diverges as and the above equations have normalizable eigenfunctions with a discrete spectrum of . Note that the term is asymmetric under . It causes that tends to shift to the positive side of and the opposite behavior happens for . It is important in the axial coupling of the nucleon to the pions.
The gauge field can be expanded in gauge as follows : where and are related to the pion field by the following relations:
Here, we use the above expansion along with the properties of and and under the transformation to calculate the four-dimensional action. It is worthwhile to note that again is even, while is odd under , corresponding to vector and axial-vector mesons , respectively. For simplicity, we neglect the Chern-Simons term in the baryon action (49).
By inserting the mode expansion of the nucleon field and gauge field into the baryon action , where
Also, stands for all the couplings. We neglect the derivative couplings in the following calculations as a leading approximation. The various minimal couplings constants and as well as the pion-nucleon axial coupling are calculated by the following suitable overlap integrals of wave functions:
Also, we can compute the magnetic couplings using the following integrals : where we define as
Since the instanton carries only the non-Abelian field strength, the isoscalar mesons couple to the nucleon in a different formalism than the isovector mesons. Therefore, for the isoscalar mesons, such as the meson, only the minimal couplings contribute to
However, the isovector mesons couple to the nucleon from both the minimal and magnetic channels. Thus, isovector meson couplings are 
The is-scalar and isovector mesons have the same origin in the five-dimensional dynamics of the gauge field. In fact, if we write the gauge field in the fundamental representation, we could decompose the massive vector mesons as where and are the isoscalar and the isovector parts of a vector meson, respectively. Since the baryon is made out of product quark doublets, the above composition for nucleon should be written as
Therefore, there is an overall factor between the isoscalar, and isovector, mesons. Indeed, there is a universal relation between the Yukawa couplings involving the isoscalar and isovector mesons:
We solve the eigenvalue (51) numerically using the shooting method to obtain the wave function, , and the mass, , of the nucleon. In order to do the numerical calculation, we assume for realistic QCD. Also as was mentioned in the previous section, we choose the value of GeV to have the pion decay constant GeV. We obtain the various couplings by evaluating integrals (58) and (59) and compare some of our results with the results of the SS model  in Table 2.
|presented model results; SS model results. |
Also, using this noncritical model, the axial couplings are obtained as while in the previous analysis  using the SS model, these couplings are reported as
If we choose , then the SS model predicts and . It should be noted that the higher order of corrections can be used to improve this result but the lattice calculations indicate that higher order of corrections are suppressed. Our results are a good approximation of the experimental data at leading order .
3.5.1. Nucleon-Meson Couplings
Our holographic NN potential contains just the vector, axial-vector, and pseudoscalar meson exchange potentials which have the isospin dependent and isospin independent components. The vector meson (), axial-vector meson (), and pseudoscalar meson () couplings are related to the minimal and magnetic couplings as follows:
All of the leading order meson-nucleon couplings are calculated numerically and compared with the predictions of the four modern phenomenological NN interaction models such as the AV 18 , CD-Bonn , Nijmegen , and Paris  potentials in Table 3. Also, results of the SS model are presented in this table. It is necessary to mention here that the components of the phenomenological models are very different in strength, and if parameterized in terms of single meson exchange give rise to effective meson-nucleon coupling strengths, which also are similar. We explain different components of the NN potential below in detail.
The isospin dependent component of the vector potential which arises from a meson exchange is roughly three times weaker than the isospin independent component. In a chiral quark model, it is expected to have , but the value of the differs from the one in the above phenomenological interaction models. It is 1.66 for the CD-Bonn, 1.5 for the Nijmegen, and 0.77 in the Paris model. This ratio is about 1.2 in the SS model and equal to in our model. Actually, the NN phase shifts uniformly require a larger than the chiral quark model prediction which is a mystery. However, in the resultant potential of the holographic QCD model, it can be explained by the contribution of the magnetic coupling in the vector channel.
4. Holographic Light Nuclei
In the holographic models, baryon is introduced as a -brane wrapped on a higher dimensional sphere in the curved spacetime . According to the fact that each nucleus is a set of nucleons, so the collection of the baryon -branes can describe a nucleus with the mass number . Then the dual gravity for the nucleus can be obtained by applying the AdS/CFT correspondence. The gauge theory living in the gravity dual of QCD is difficult to treat; hence, the large limit is considered for this dual geometry which corresponds to the heavy nuclei . On the other hand, it is necessary to use the nucleon-nucleon potential to study the properties of light nuclei. In this section, we aim to study the holographic light nuclei such as , , , and . For this purpose, we consider a set of instantonic baryons as a nucleus. It is known that the nucleons are stabilized at a certain distance in nuclei because of a binding force and a strong repulsive force due to the light meson exchanges. We assume that the nucleons have a uniform distribution in nucleus. Therefore, we consider a homogeneous distribution of -branes in the space. In order to study the potential of nucleus, we should regard the interaction between these -branes. It was shown that the size of baryon (instanton) is small and the interaction between two instantons can be explained by the OBEP potential . In this section, we use this nucleon-nucleon potential to obtain the potentials of light nuclei. Also we calculate the binding energy of these nuclei. Then we impose different conditions on nucleon spins in order to obtain some excited states of the nucleus. Finally, we calculate the energy of these excited states and estimate their excited energy.
4.1. Nucleon-Nucleon Holography Potential
Two particle scattering Phase shift in different partial waves as well as the bound state properties of deuteron are experimental data for a two-nucleon system which identify the main properties of nucleon-nucleon interaction. But the potentials attained phenomenologically have many free parameters which are determined by fitting to the experimental data. Various mesons and their resonances play a special role in producing the nucleon-nucleon potential with the following rules.(i)The long range part of the NN potential is mostly due to the one pion exchange mechanism.(ii)Isoscalar mesons are responsible for the attractive interaction in the intermediate range of the potential .(iii)Exchanging the vector meson can explain the small attractive behavior of the odd-triplet state.(iv)Vector mesons produce the strong short range repulsion.
Then by considering these facts the general one boson exchange nucleon-nucleon potential is written as  which contains the pseudoscalar , vector , and axial vector meson exchange potentials, respectively. It should be noted that despite of the phenomenological NN interaction model, here we compute all of the nucleon-meson couplings contributing to the above potential using the noncritical holography model.
In our calculations, the leading parts of the potential come from the pseudoscalar meson , isoscalar vector meson , isovector vector meson , and isovector axial vector meson exchange interactions:
One pion exchange potential (OPEP) has the following form:
Also, the holographic potentials for isospin singlet vector mesons , isospin triplet vector mesons , and the triplet axial-vector mesons are
In the above equations we have
The masses of all mesons are of the order and . Also, the mass of pion in the holographic model is zero and its coupling constant to the nucleon in our approach is 15.7.
It is shown that in the SS model, at the large enough distances, is an acceptable value for these potentials. We consider the ten first terms of the above potentials in our numerical calculations both in SS and models.
In order to calculate the NN potential, the nucleon-meson coupling constants are needed. These couplings are calculated using the SS model at the large limit and presented in Table 4.
Also, we calculate the coupling values in the noncritical background. The obtained results are presented in Tables 5 and 6. In the following, we calculate the light nuclei potentials using the NN holography potentials coming from both SS and models.
4.2. Holographic Deuteron
Deuteron is the only bound state of two-nucleon system with the isospin , total spin , spin parity , and binding energy . In our holographic model, we suppose that deuteron is made of two instantonic baryons with and which are located at relative distance in the space and consider the following potential for the deuteron: where , and are presented in (75), (76), and (77), respectively. The super selection rules propose that