Five-loop epsilon expansion for U(n)xU(m) models: finite-temperature phase transition in light QCD

Abstract

We consider the U(n)xU(m) symmetric Phi^4 Lagrangian to describe the finite-temperature phase transition in QCD in the limit of vanishing quark masses with n=m=N_f flavors and unbroken anomaly at T_c. We compute the Renormalization Group functions to five-loop order in Minimal Subtraction scheme. Such higher order functions allow to describe accurately the three-dimensional fixed-point structure in the plane (n,m), and to reconstruct the line n^+(m,d) which limits the region of second-order phase transitions by an expansion in epsilon=4-d. We always find n^+(m,3)>m, thus no three-dimensional stable fixed point exists for $n=m$ and the finite temperature transition in light QCD should be first-order. This result is confirmed by the pseudo-epsilon analysis of massive six-loop three dimensional series.