The Silence of the Little Strings

TL;DR

This paper investigates the hydrodynamic properties of Little String Theory at high energy, revealing universal viscosity ratios, the vanishing speed of sound at the Hagedorn temperature, and detailed pole structures in stress tensor correlators.

Contribution

It provides the first detailed calculation of stress tensor correlators in Little String Theory, including viscosity ratios and pole structures, using dual string theory and supergravity methods.

Findings

Shear viscosity to entropy density ratio is 1/4π, universal and correction-free.

Speed of sound vanishes at the Hagedorn temperature.

Bulk viscosity to entropy density ratio is 1/10π.

Abstract

We study the hydrodynamics of the high-energy phase of Little String Theory. The poles of the retarded two-point function of the stress energy tensor contain information about the speed of sound and the kinetic coefficients, such as shear and bulk viscosity. We compute this two-point function in the dual string theory and analytically continue it to Lorentzian signature. We perform an independent check of our results by the Lorentzian supergravity calculation in the background of non-extremal NS5-branes. The speed of sound vanishes at the Hagedorn temperature. The ratio of shear viscosity to entropy density is equal to the universal value 1/4\pi and does not receive \alpha' corrections. The ratio of bulk viscosity to entropy density equals 1/10\pi. We also compute the R-charge diffusion constant. In addition to the hydrodynamic singularities, the correlators have an infinite series of finite-gap poles, and a massless pole with zero attenuation.