Anomalous Chiral Symmetry Breaking above the QCD Phase Transition

We study the anomalous breaking of ${\mathrm{U}}_A\left(1\right)$ symmetry just above the QCD phase transition for zero and two flavors of quarks, using a staggered fermion, lattice discretization. The properties of the QCD phase transition are expected to depend on the degree of ${\mathrm{U}}_A\left(1\right)$ symmetry breaking in the transition region. For the physical case of two flavors, we carry out extensive simulations on a ${16}^3\times 4$ lattice, measuring a difference in susceptibilities which is sensitive to ${\mathrm{U}}_A\left(1\right)$ symmetry and which avoids many of the staggered fermion discretization difficulties. The results suggest that anomalous effects are at or below the 15% level.