Strongly correlated quantum fluids are phases of matter that are intrinsically quantum mechanical and that do not have a simple description in terms of weakly interacting quasiparticles. Two systems that have recently attracted a great deal of interest are the quark-gluon plasma, a plasma of strongly interacting quarks and gluons produced in relativistic heavy ion collisions, and ultracold atomic Fermi gases, very dilute clouds of atomic gases confined in optical or magnetic traps. These systems differ by 19 orders of magnitude in temperature, but were shown to exhibit very similar hydrodynamic flows. In particular, both fluids exhibit a robustly low shear viscosity to entropy density ratio, which is characteristic of quantum fluids described by holographic duality, a mapping from strongly correlated quantum field theories to weakly curved higher dimensional classical gravity. This review explores the connection

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Strongly correlated quantum fluids: ultracold quantum gases, quantum chromodynamic plasmas and holographic duality

We revisit and extend previous calculations of glueball decay rates in the Sakai-Sugimoto model, a holographic top-down approach for QCD with chiral quarks based on $\mathrm{D}8\text{\ensuremath{-}}\overline{\mathrm{D}8}$ probe branes in Witten's holographic model of nonsupersymmetric Yang-Mills theory. The rates for decays into two pions, two vector mesons, four pions, and the strongly suppressed decay into four ${\ensuremath{\pi}}^{0}$ are worked out quantitatively, using a range of the 't Hooft coupling which closely reproduces the decay rate of $\ensuremath{\rho}$ and $\ensuremath{\omega}$ mesons and also leads to a gluon condensate consistent with QCD sum rule calculations. The lowest holographic glueball, which arises from a rather exotic polarization of gravitons in the supergravity background, turns out to have a significantly lower mass and larger width than the two widely discussed glueball candidates ${f}_{0}(1500)$ and ${f}_{0}(1710)$. The lowest nonexotic and predominantly dilatonic scalar mode, which has a mass of 1487 MeV in the Witten-Sakai-Sugimoto model, instead provides a narrow glueball state, and we conjecture that only this nonexotic mode should be identified with a scalar glueball component of ${f}_{0}(1500)$ or ${f}_{0}(1710)$. Moreover the decay pattern of the tensor glueball is determined, which is found to have a comparatively broad total width when its mass is adjusted to around or above 2 GeV.

Glueball decay rates in the Witten-Sakai-Sugimoto model

Topological insulators and superconductors from D-brane

We revisit the model of mesons as rotating strings with massive endpoints and confront it with meson spectra. We look at Regge trajectories both in the (J, M
 2) and (n, M
 2) planes, where J and n are the angular momentum and radial excitation number respectively. We start from states comprised of u and d quarks alone, move on to trajectories involving s and c quarks, and finally analyze the trajectories of the heaviest observed $$ b\overline{b} $$
 mesons. The endpoint masses provide the needed transition between the linear Regge trajectories of the light mesons to the deviations from linear behavior encountered for the heavier mesons, all in the confines of the same simple model. From our fits we extract the values of the quark endpoint masses, the Regge slope (string tension) and quantum intercept. The model also allows for a universal fit where with a single value of the Regge slope we fit all the (J, M
 2) trajectories involving u, d, s, and c quarks. We include a list of predictions for higher mesons in both J and n.

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Rotating strings confronting PDG mesons

We consider renormalization group flows between conformal field theories in five (six) dimensions with a string (M-theory) dual. By compactifying on a circle (torus) with appropriate boundary conditions, we obtain continuous families of confining fourdimensional theories parametrized by the ratio Λflow/ΛQCD, with Λflow the scale at which the flow between fixed points takes place and ΛQCD the confinement scale. We construct the dual geometries explicitly and compute the spectrum of scalar bound states (glueballs). We find a ‘universal’ subset of states common to all the models. We comment on the modifications of these models, and the corresponding fine-tuning, required for a parametrically light ‘dilaton’ state to be present. We also comment on some aspects of these theories as probed by extended objects such as strings and branes.

https://link.springer.com/content/pdf/10.1007%2FJHEP05%282014%29003.pdf

Multiscale confining dynamics from holographic RG flows

We study meson spectrum obtained from massive open string modes in a holographic dual of QCD constructed on the basis of a D4/D8-brane configuration in type IIA string theory. The spectrum includes mesons with spin higher than one. Taking into account the effect of curved background perturbatively, we obtain a mass formula for these mesons. The mass formula exhibits the linear Regge behavior at the leading order with subleading nonlinear corrections. We argue that the string spectrum captures some features of the observed meson spectrum. For example, a2(1320), b1(1235), π(1300), a0(1450), etc., are identified as the first excited massive open string states and ρ3(1690), π2(1670), etc., are identified as the second excited states. Subject Index: 129, 168

Mesons as Open Strings in a Holographic Dual of QCD

We propose a holographic dual of large Nc quantum chromodynamics (QCD) with the gauge groups O(Nc )a ndUSp(Nc )a ndNf flavors of massless quarks. This is constructed by adding O6-planes to an intersecting D4-D8 system in type IIA superstring theory. The holographic dual description is formulated in Witten’s D4-brane background with D8-branes and O6-planes embedded in it as probes. The D4-brane background gives rise to a smooth interpolation of D8-D8 pairs and an O6-O6 pair. We show that the resultant brane configuration explains geometrically the flavor symmetry breaking patterns in O(Nc )a ndUSp(Nc) QCD, which are caused by quark bilinear condensates. We next discuss that baryons can be realized as D4-D4 pairs wrapped on S 4 , which intersect with the O6-plane. By analyzing the tachyons on it, we reproduce the stability conditions of the baryons that are expected from the gauge theory viewpoint. The stable baryon configurations are classified systematically using K-theory. We also give a similar analysis of the flux tubes and again reproduce the results that are consistent with QCD. Subject Index: 101, 121

O(Nc) and USp(Nc) QCD from String Theory

We propose a holographic dual of large N_c QCD with the gauge group O(N_c) and USp(N_c) and N_f flavors of massless quarks. This is constructed by adding O6-planes to an intersecting D4-D8 system in type IIA superstring theory. The holographic dual description is formulated in the Witten's D4-brane background with D8-branes and O6-planes embedded in it as probes. The D4-brane background gives rise to a smooth interpolation of D8-anti-D8 pairs and an O6-anti-O6 pair. We show that the resultant brane configuration explains geometrically the flavor symmetry breaking patterns in O(N_c) and USp(N_c) QCD that are caused by quark bilinear condensates. 
We next discuss that baryons can be realized as D4-anti-D4 pairs wrapped on S^4 that intersect with the O6-plane. By analyzing the tachyons on it, we reproduce the stability conditions of the baryons that are expected from the gauge theory viewpoint. The stable baryon configurations are classified systematically by using K-theory. We also give a similar analysis for the flux tubes and again reproduce the results that are consistent with QCD.

O(N) and USp(N) QCD from String Theory

We study meson spectrum obtained from massive open string modes in a holographic dual of QCD constructed on the basis of a D4/D8-brane configuration in type IIA string theory. The spectrum includes mesons with spin higher than one. Taking into account the effect of curved background perturbatively, we obtain a mass formula for these mesons, which exhibits the linear Regge behavior at the leading order with subleading non-linear corrections. We argue that the string spectrum captures some features of the observed meson spectrum. For example, a_2(1320), b_1(1235), \pi(1300), a_0(1450), etc., are identified as the first excited massive open string states and \rho_3(1690), \pi_2(1670), etc., are identified as the second excited states.

Toshiya Imoto

Papers

Mesons as Open Strings in a Holographic Dual of QCD

O(Nc) and USp(Nc) QCD from String Theory

O(N) and USp(N) QCD from String Theory

Mesons as Open Strings in a Holographic Dual of QCD