The curvature of the chiral QCD phase transition line in a finite volume

Using a bosonized NJL model at finite temperature and baryon chemical potential, we investigate effects of a finite volume on the curvature of the chiral phase transition line in QCD in the plane of temperature and chemical potential. We use a functional Renormalization Group method to capture the effects of bosonic as well as fermionic fluctuations which are essential for the physics of the finite-volume system. We find a significant change of the curvature with the volume size, depending on the choice of spatial boundary conditions for the quark fields and the amount of explicit chiral symmetry breaking by a finite bare quark mass. In particular, we observe a flattening of the line in intermediate volume sizes. In the framework of the model, this result can be interpreted in terms of finite-volume effects on the chiral condensate and the constituent quark mass. The presence of such effects in lattice QCD could account for differences in results for the curvature from lattice simulations in different volumes.