  We study positive and negative parity nucleons on the lattice using the
chirally improved lattice Dirac operator. Our analysis is based on a set of
three operators chi_i with the nucleon quantum numbers but in different
representations of the chiral group and with different diquark content. We use
a variational method to separate ground state and excited states and determine
the mixing coefficients for the optimal nucleon operators in terms of the
chi_i. We clearly identify the negative parity resonances N(1535) and N(1650)
and their masses agree well with experimental data. The mass of the observed
excited positive parity state is too high to be interpreted as the Roper state.
Our results for the mixing coefficients indicate that chiral symmetry is
important for N(1535) and N(1650) states. We confront our data for the mixing
coefficients with quark models and provide insights into the physics of the
nucleon system and the nature of strong decays.
