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We investigate the phase transitions of (2+Nf)-flavor QCD, where two lightflavors and Nf massive flavors exist, aiming to understand the phase structureof (2+1)-flavor QCD. Performing simulations of 2-flavor QCD with improvedstaggered and Wilson fermions and using the reweighting method, we calculateprobability distribution functions in the many-flavor QCD. Through the shape ofdistribution functions, we determine the critical surface terminating firstorder phase transitions in the parameter space of the light quark mass, heavyquark mass and the chemical potential, and find that the first order regionbecomes larger with Nf. We then study the critical surface at finite densityfor large Nf and the first order region is found to become wider with theincreasing chemical potential. On the other hand, the light quark massdependence of the critical mass of heavy quarks seems weak in the region weinvestigated. The result of this weak dependence suggests that the criticalmass of heavy quark remains finite in the chiral limit of 2-flavors and thereexists a second order transition region on the line of the 2-flavor masslesslimit above the tri-critical point. Moreover, we extend the study of 2-flavorQCD at finite density to the case of a complex chemical potential andinvestigate the singularities where the partition function vanishes, so-calledLee-Yang zeros. The plaquette effective potential is computed in the complexplane. We find that the shape of the effective potential changes fromsingle-well on the real axis to double-well at large imaginary chemicalpotential and the double-well potential causes the singularities.

First order transition regions in the quark masses and chemical potential parameter space of QCD