Fermion-number susceptibility in lattice gauge theory

We study the response of the quark number density to infinitesimal chemical potentials in quantum chromodynamics with two flavors of light quarks. We find that both the singlet and nonsinglet susceptibilities give clear signals for the chiral-symmetry-restoration phase transition. They are large and approximately equal in the high-temperature phase, which is consistent with a plasma of light-mass quarks. The quark-number susceptibility is consistent with zero in the low-temperature phase as expected from confinement. For U(1) lattice gauge theory with four flavors of fermions we find that the singlet susceptibility is zero in both the chiral-symmetric and broken phases as expected on theoretical grounds, while the nonsinglet susceptibility jumps at the phase transition.