Sign problem in a Z3 -symmetric effective Polyakov-line model

As an effective model corresponding to Z3-symmetric QCD (Z3-QCD), we construct a Z3-symmetric effective Polyakov-line model (Z3-EPLM) by using the logarithmic fermion effective action. Since Z3-QCD tends to QCD in the zero-temperature limit, Z3-EPLM also agrees with the ordinary effective Polyakov-line model (EPLM) there; note that (ordinary) EPLM does not possess Z3 symmetry. Our main purpose is to discuss a sign problem appearing in Z3-EPLM. The action of Z3-EPLM is real when not only the Polyakov line is real but also its Z3 images. This suggests that the sign problem becomes milder in Z3-EPLM than in EPLM. In order to confirm this suggestion, we do lattice simulations for both EPLM and Z3-EPLM by using the reweighting method with the phase quenched approximation. In the low-temperature region, the sign problem is milder in Z3-EPLM than in EPLM. We also propose a new reweighting method. This makes the sign problem very weak in Z3-EPLM.