Chiral-symmetry breaking in lattice QCD with two and four fermion flavors

We study the thermodynamics of lattice quantum chromodynamics for two and four flavors of staggered fermions at temperature T=1/4a, where a is the lattice spacing. For four flavors with mass below 0.05/a we find evidence for a first-order chiral-symmetry-breaking phase transition. For two flavors there is an increasingly rapid crossover as the mass is lowered, but we find no conclusive evidence of a first-order transition for quark masses down to 0.0125/a. A rough estimate is made of the zero-mass transition temperature for two flavors of quarks.