Excited Baryon Spectrum from Lattice QCD

We report lattice calculations of the low-lying baryon mass spectrum, with an emphasis on the introduction of two new techniques. One is the constrained fitting method based on Bayesian statistics which allows reliable extraction of excited states. The other is the use of the overlap fermion action which preserves exact chiral symmetry of QCD on the lattice and allows us to reach deep down into the chiral regime (our pion mass is as low as 180 MeV) where interesting chiral dynamics are exposed. One example is the elusive Roper state N(1440)1/2⁺ which shows up naturally as the 1st-excited state of the nucleon from the standard interpolating field. Together with other baryons, our preliminary results indicate that the level-ordering of the low-lying baryon states on the lattice is largely consistent with experiment and with that expected from meson-exchange quark models.