Akira Ohnishi (Kyoto University) Mr Terukazu Ichihara (Kyoto University)
We study the QCD phase diagram in the strong coupling region by using the auxiliary field Monte-Carlo method. Elucidating the phase diagram structure in QCD is a big challenge. The strong coupling lattice QCD is a promising machinery, in which the effective action is obtained by integrating the link variables analytically at a given order of the strong coupling expansion, then the sign problem is weakened. We have recently obtained the QCD phase diagram with fluctuation effects in the strong coupling limit by using the auxiliary field Monte-Carlo (AFMC) method . Compared with the mean field results, hadron phase is found to be suppressed (extended) at low (high) chemical potential. These results are consistent with those obtained in the monomer-dimer-polymer (MDP) simulation . It is staightforward to include finite coupling effects in AFMC. Next-to-leading order (NLO) effective action terms proportional to $1/g^2$ appear from one plaquette configurations and are found to contain four-Fermi and eight-Fermi interactions of quarks. Sequential extended Hubbard-Stratonovich transformations lead to an effective action in the bilinear form of quarks. Preliminary results seems to suggest that finite coupling effects make the sign problem more severe than in the strong coupling limit . In the presentation, we also discuss the order of the phase transition based on the finite size scaling of the chiral susceptibility in the strong coupling limit. Several prescriptions to weaken the sign problem are also discussed.  T. Ichihara, A. Ohnishi, T. Z. Nakano, arXiv:1401.4647 [hep-lat].  W. Unger, P. de Forcrand, J. Phys. G38 (2011) 124190.  T. Ichihara, A. Ohnishi, in preparation.
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Akira Ohnishi (Kyoto University)
Mr Terukazu Ichihara (Kyoto University)