Extracting excited states from lattice QCD: the Roper resonance

We present a new method [Phys. Lett. B 604 (2004) 74] for extracting excited states from a single two-point correlation function calculated on the lattice. Our method simply combines the correlation function evaluated at different time slices to give direct access to the first excited state. The method is applied to a quenched lattice study of the nucleon and our results are consistent with the identification of the extracted first excited state with the Roper resonance, N′(1440). The switching of the level ordering with respect to the negative-parity partner of the nucleon, N*(1535), is not seen at the simulated masses and, basing on crude extrapolations, is expected to occur close to the physical point.