It has been shown by Polchinski and Strassler that the scaling of high energy QCD scattering amplitudes can be obtained from string theory. They considered an AdS slice as an approximation for the dual space of a confining gauge theory. Here we use this approximation to estimate in a very simple way the ratios of scalar glueball masses imposing Dirichlet boundary conditions on the string dilaton field. These ratios are in good agreement with the results in the literature. We also find that they do not depend on the size of the slice.

Recently Polchinski and Strassler reproduced the high energy QCD scaling at fixed angles from a gauge string duality inspired in AdS/CFT correspondence. In their approach a confining gauge theory is taken as approximately dual to an AdS space with an IR cut off. Considering such an approximation (AdS slice) we found a one to one holographic mapping between bulk and boundary scalar fields. Associating the bulk fields with dilatons and the boundary fields with glueballs of the confining gauge theory we also found the same high energy QCD scaling. Here, using this holographic mapping we give a simple estimate for the mass ratios of the glueballs assuming the AdS slice approximation to be valid at low energies. We also compare these results to those coming from supergravity and lattice QCD. Recently Polchinski and Strassler reproduced important observed properties of QCD from string theory in AdS space [1,2]. In these articles they used a model for the dual of a confining gauge theory which is approximately an AdS slice with an infrared cut off. Using this model they were able[1] to obtain the high energy scaling of QCD scattering amplitudes for fixed angles [3,4] as well as the Regge regime. Further they proposed[2] a way of analyzing the deep inelastic scattering and Bjorken scaling in terms of string theory. Using the same kind of AdS slice we proposed[5] a one to one holographic mapping between low energy string dilaton states in AdS bulk and massive composite operators on its boundary. From this mapping we also obtained a scaling for high energy amplitudes at fixed angles similar to that of QCD and of Polchinski and Strassler (see also [6,7,8]).The gauge/string duality considered in refs. [1, 2] was inspired in the AdS/CFT correspondence proposed recently by Maldacena[9] where SU(N) conformal gauge theory with N = 4 supersymmetry is dual to string theory in AdS space (times a compact manifold). The prescriptions for realizing of this correspondence obtaining boundary correlation functions in terms of bulk fields were proposed in [10,11] (see also [12] for a review). The AdS/CFT correspondence can be understood as a realization of the holographic principle [13,14,15,16]. This principle asserts that the degrees of freedom of a theory with gravity defined in a given space can be mapped on the corresponding boundary.In the AdS/CFT correspondence the higher the energy of a given boundary process, the closer to the horizon is the bulk dual. Restricting boundary process to energies higher than some IR cut off would then correspond to restricting the bulk to some region in the neighborhood of the horizon. That is a motivation for taking an AdS slice as an approximation for the space dual to a boundary confining gauge theory. Such a gauge theory with an * Electronic address: boschi@if.ufrj.br † Electronic address: braga@if.ufrj.br infrared cut off can be related to N = 1 * supersymmetric Yang Mills theory[1, 2] (see also [17]). This model leads to QCD like behavior at high energies. An AdS slice was used before in [18,...

The spectrum of light baryons and mesons has been reproduced recently by Brodsky and Teramond from a holographic dual to QCD inspired in the AdS/CFT correspondence. They associate fluctuations about the AdS geometry with four dimensional angular momenta of the dual QCD states. We use a similar approach to estimate masses of glueball states with different spins and their excitations. We consider Dirichlet and Neumann boundary conditions and find approximate linear Regge trajectories for these glueballs. In particular the Neumann case is consistent with the Pomeron trajectory.

We use the holographic AdS/QCD soft-wall model to investigate the spectrum of scalar glueballs in a finite temperature plasma. In this model, glueballs are described by a massless scalar field in an AdS 5 black hole with a dilaton soft-wall background. Using AdS/CFT prescriptions, we compute the boundary retarded Green's function. The corresponding thermal spectral function shows quasiparticle peaks at low temperatures. We also compute the quasinormal modes of the scalar field in the soft-wall black hole geometry. The temperature and momentum dependences of these modes are analyzed. The positions and widths of the peaks of the spectral function are related to the frequencies of the quasinormal modes. Our numerical results are found employing the power series method and the computation of Breit-Wigner resonances.

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