Phase structure of finite density QCD with a histogram method

We study the phase structure of QCD in the $T-mu$ plane using a histogram method and the reweighting technique by performing phase quenched simulations of two-flavor QCD with RG-improved gauge action and O($a$) improved Wilson quark action. Taking the effects of the complex phase of the quark determinant using the cumulant expansion method, we calculate the probability distribution function of plaquette and phase-quenched determinant as a function of $T$ and $mu$. We discuss the order of the QCD phase transition consulting the shape of the probability distribution function.