Large-N thermal QCD laboratories like strongly coupled QGP (sQGP) require not only a large t'Hooft coupling but also a finite gauge coupling [1]. Unlike almost all top-down holographic models in the literature, holographic large-N thermal QCD models based on this assumption, therefore necessarily require addressing this limit from M theory. Using the UV-complete top-down type IIB holographic dual of large-N thermal QCD as constructed in [2] involving a fluxed resolved warped deformed conifold, its delocalized type IIA S(trominger)-Y(au)-Z(aslow) mirror as well as its M-theory uplift constructed in [3], in [4], the type IIB background of [2] was shown to be thermodynamically stable. We also showed that the temperature dependence of DC electrical conductivity mimics a one-dimensional Luttinger liquid, and the requirement of the Einstein relation (ratio of electrical conductivity and charge susceptibility equal to the diffusion constant) to be satisfied requires a specific dependence of the Ouyang embedding parameter on the horizon radius. Any strongly coupled medium behaves like a fluid with interesting transport properties. In [5], we addressed these properties by looking at the scalar, vector and tensor modes of metric perturbations and solve Einstein's equation involving appropriate gauge-invariant combination of perturbations as constructed in [6]. Due to finite string coupling, we obtained the speed of sound, the shear mode diffusion constant and the shear viscosity η (and η s ) upto (N)ext to (L)eading (O)rder in N. The NLO terms in each of the coefficients serve as a the non-conformal corrections to the conformal results.Another interesting result for the temperature dependence of the thermal (and elec-1

It is believed that large-N thermal QCD laboratories like strongly coupled QGP (sQGP) require not only a large 't Hooft coupling but also a finite gauge coupling (Natsuume, String theory and quark-gluon plasma. arXiv: hep-ph/0701201, 2007). Unlike almost all top-down holographic models in the literature, holographic large-N thermal QCD models, based on this assumption, therefore necessarily require addressing this limit from M-theory. This was initiated in Dhuria and Misra (JHEP 1311(JHEP :001, 2013 which presented a local M-theory uplift of the string theoretic dual of large-N thermal QCD-like theories at finite gauge/string coupling of Mia et al. (Nucl. Phys. B 839:187, arXiv:0902.1540 [hep-th] The new insight gained is that it turns out that these corrections show a partial universality in the sense that at NLO in N the same are given by the product of

In this paper we study bulk viscosity in a thermal QCD model with large number of colors at two extreme limits: the very weak and the very strong 't Hooft couplings. The weak coupling scenario is based on kinetic theory, and one may go to the very strong coupling dynamics via an intermediate coupling regime. Although the former has a clear description in terms of kinetic theory, the intermediate coupling regime, which uses lattice results, suffers from usual technical challenges that render an explicit determination of bulk viscosity somewhat difficult. On the other hand, the very strong 't Hooft coupling dynamics may be studied using string theories at both weak and strong string couplings using gravity duals in type IIB as well as M-theory respectively. In type IIB we provide the precise fluctuation modes of the metric in the gravity dual responsible for bulk viscosity, compute the speed of sound in the medium and analyze the ratio of the bulk to shear viscosities. In M-theory, where we uplift the type IIA mirror dual of the UV complete type IIB model, we study and compare both the bulk viscosity and the sound speed by analyzing the quasi-normal modes in the system at strong IIA string coupling. By deriving the spectral function, we show the consistency of our results both for the actual values of the parameters involved as well for the bound on the ratio of bulk to shear viscosities. 65 5.1 The mirror type IIA model and its M-theory uplift 66 5.2 Quasi-normal modes, attenuation constant and the sound speed 70 5.3 The case with a vanishing bare resolution parameter 76 5.4 Shear viscosity, entropy and the bulk viscosity bound 80 6. Type IIA spectral function and the viscosity bound at strong coupling with non-zero flavors 85 6.1 Background gauge fluxes and perturbations on the flavor branes 86 6.2 Equation of motion for gauge field fluctuations 88 6.3 On-shell action and the strong coupling spectral function 95 6.4 The strong string coupling limit and pure classical supergravity 103 -1 -7. Conclusions and discussions 107 A. A Gauge-Invariant Combination of Scalar Modes of Metric Perturbations 112 A.1 The equation of motion for the fluctuation mode H tt 113 A.2 The equation of motion for the combined mode H s 115 A.3 The equations of motion for the remaining fluctuation modes 116 B. A derivation of the on-shell action and the Green's function 118C. Effective number of three-brane charges with background threeforms and the horizon radius 1201 Violation of this bound is seen in the presence of higher derivative terms, discussed first in [23]. In the absence of these terms, the KSS [22] bound continues to hold at strong 't Hooft coupling.2 The non-conformal string theory studied in [30] is different from what we consider here. In [30] it's the N = 2 * supersymmetric gauge theory obtained by a mass deformation of N = 4 Super Yang Mills theory. See also [31] and [32] for an even earlier study on bulk viscosity from first principles.

Meson spectroscopy at finite gauge couplingwhereat any perturbative QCD computation would break down -and finite number of colors, from a top-down holographic string model, has thus far been entirely missing in the literature. This paper fills this gap. Using the delocalized type IIA SYZ mirror (with SU ( (Chin Phys C 38:090001, 2014). Through explicit computations, we verify that the vector and scalar meson spectra obtained by the gravity dual with a black hole for all temperatures (small and large) are nearly isospectral with the spectra obtained by a thermal gravity dual valid for only low temperatures; the isospectrality is much closer for vector mesons than scalar mesons. The black-hole gravity dual (with a horizon a e-mail: viitr.dph2015@iitr.ac.in b e-mail: aalokfph@iitr.ac.in c e-mail: krusldph@iitr.ac.in radius smaller than the deconfinement scale) also provides the expected large-N suppressed decrease in vector meson mass with increase of temperature.

The back reaction imparted by a uniform distribution of heavy static fundamental quarks on large N c strongly coupled gauge theory can be holographically realized as a deformation in AdS blackhole background. The presence of back reaction brings significant changes in to the entanglement structure of the strongly coupled boundary theory at finite temperature. Since the deformed blackhole geometry still remains asymptotically AdS, the gauge/gravity duality allows us to explore the entanglement structure of back reacted plasma in a quantitative way by computing various measures, e.g holographic entanglement entropy (HEE) and entanglement wedge cross section (EWCS). We explicitly study the variation of those entanglement measures with respect to the uniform density of heavy static fundamental quarks present in the boundary theory. In particular, we notice enhancement of both HEE and EWCS with respect to quark density. We also study the effect of back reaction on the holographic subregion volume complexity. In this analysis we observe an occurrence of logarithmic divergence proportional to the quark density parameter.