Baryonic response of dense holographic QCD

The response function of a homogeneous and dense hadronic system to a time-dependent (baryon) vector potential is discussed for holographic dense QCD (D4/D8 embedding) both in the confined and deconfined phases. Confined holographic QCD is an uncompressible and static baryonic insulator at large N_c and large λ, with a gapped vector spectrum and a massless pion. Deconfined holographic QCD is a diffusive conductor with restored chiral symmetry and a gapped transverse baryonic current. Similarly, dense D3/D7 is diffusive for any non-zero temperature at large N_c and large λ. At zero temperature dense D3/D7 exhibits a baryonic longitudinal visco-elastic mode with a first sound speed 1/√3 and a small width due to a shear viscosity to baryon ratio η/nB = ħ/4. This mode is turned diffusive by arbitrarily small temperatures, a hallmark of holography.