Chiral phase structure of three flavor QCD at vanishing baryon number density

We investigate the phase structure of QCD with three degenerate quark flavors as a function of the degenerate quark masses at vanishing baryon number density. We use the highly improved staggered quarks on lattices with temporal extent N-tau = 6 and perform calculations for six values of quark masses, which in the continuum limit correspond to pion masses in the range 80 MeV less than or similar to m(pi) less than or similar to 230 MeV. By analyzing the volume and temperature dependence of the chiral condensate and chiral susceptibility, we find no direct evidence for a first-order phase transition in this range of pion mass values. Relying on the universal scaling behaviors of the chiral observables near an anticipated chiral critical point, we estimate an upper bound for the critical pion mass, m(pi)(c) less than or similar to 50 MeV, below which a region of first-order chiral phase transition is favored.