Critical behavior in Nt=4 staggered fermion thermodynamics

Quantum chromodynamics with two zero mass flavors is expected to exhibit a phase transition with O(4) critical behavior. Fixing the universality class is important for phenomenology and for facilitating the extrapolation of simulation data to physical quark mass values. Other groups have reported results from lattice QCD simulations with dynamical staggered quarks at Nt=4, which suggest a departure from the expected critical behavior. We have pushed simulations to the largest volumes and smallest quark mass to date. Strong discrepancies in critical exponents and the scaling equation of state persist.