Holographic viscosity of fundamental matter

A holographic dual of a finite-temperature SU(N-c) gauge theory with a small number of flavors N-f < N-c typically contains D-branes in a black hole background. By considering the backreaction of the branes, we demonstrate that, to leading order in N-f/N-c, the viscosity to entropy ratio in these theories saturates the conjectured universal bound eta/s >= 1/4 pi. Given the known results for the entropy density, the contribution of the fundamental matter eta(fund) is therefore enhanced at strong 't Hooft coupling lambda; for example, eta(fund)similar to lambda NcNfT3 in four dimensions. Other transport coefficients are analogously enhanced. These results hold with or without a baryon number chemical potential.