Quark-number susceptibility of high-temperature QCD

We measure the response of the quark number to an infinitesimal chemical potential in high-temperature QCD with two light flavors of dynamical fermions. In the chirally symmetric phase the susceptibilities for quark number density and for the density of the third component of isospin are large and equal within statistics, which is consistent with a plasma of light quarks. In the broken-symmetry phase the susceptibility for quark number density is small, as expected from quark confinement.