Spectral analysis of excited nucleons in lattice QCD with maximum entropy method

We study the mass spectra of excited baryons with the use of the lattice QCD simulations. We focus our attention on the problem of the level ordering between the positive-parity excited state N′(1440) (the Roper resonance) and the negative-parity excited state N∗(1535). Nearly perfect parity projection is accomplished by combining the quark propagators with periodic and anti-periodic boundary conditions in the temporal direction. Then we extract the spectral functions from the lattice data by utilizing the maximum entropy method. We observe that the masses of the N′ and N∗ states are close for wide range of the quark masses (Mπ=0.61-1.22GeV) and for the physical point after the extrapolation in terms of the quark mass. The latter aspect is in contrast to the phenomenological prediction of the phenomenological quark models. The role of the Wilson doublers in the baryonic spectral functions is also studied.