We explore the photoemission rate and conductivity in dense QCD by taking the AdS/CFT methods. Specifically, we take three QCD gravity dual models to do comparative studies: D4/D6 model, noncritical Sakai-Sugimoto model and soft-wall AdS/QCD model. We turn on the time component of flavor U(1) gauge field in the bulk side to model the baryon density in QCD. For all three models, we numerically solve the U(1) fluctuation equation to plot the spectral function and photoemission rate for lightlike momenta as well as the AC conductivity. We find that the results for the former two models are very similar to previous studies while the last one show some differences. The conductivity in the soft-wall model seems to be similar to that of holographic superconductor constructed from five-dimensional bulk gravity, i.e. the pseudogap structure. This should be owing to that the infrared (IR) soft-wall (an effective cutoff near the IR of the geometry) produces a mass gap in the dual field theory, which has imprinting in the conductivity as a pseudogap formation. However, the DC conductivity in our computations is finite while it should be a delta peak in holographic superconductor models. Our results show the importance of dilaton running in affecting physical quantities.
Motivated by recent studies of deep inelastic scattering (DIS) off the N = 4 super-Yang-Mills (SYM) plasma, holographically dual to AdS 5 × S 5 black hole, we in this note use the spacelike flavor current to probe the internal structure of one holographic quark-gluon plasma, which is described by the Sakai-Sugimoto model at high temperature phase (i.e., the chiral symmetric phase). The plasma structure function is extracted from the retarded flavor current-current correlator. Our main aim here is to explore the effect of non-conformality on these physical quantities. As usual, our study is under the supergravity approximation and the limit of large color number. Although the Sakai-Sugimoto model is non-conformal, which makes the calculations more involved than the well-studied N =4 SYM case, the result seems to indicate that the non-conformality has little essential effect on the physical picture of the internal structure of holographic plasma, which is consistent with the intuition from the asymptotic freedom of QCD at high energy. While the physical picture underlying our investigation is same as the DIS off the N =4 SYM plasma with(out) flavor, the plasma structure functions are quantitatively different, especially their scaling dependence on the temperature, which can be recognized as model-dependent. As a comparison, we also do the same analysis for the non-critical version of the Sakai-Sugimoto model which is conformal in the sense that it has a constant dilaton vacuum. The result for this non-critical model is much similar to the conformal N = 4 SYM plasma.We therefore attribute the above difference to the effect of non-conformality of the Sakai-Sugimoto model.
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