Lattice Calculation of Baryon Masses using the Clover Fermion Action
  We present a calculation of the lowest-lying baryon masses in the quenched approximation to QCD. The calculations are performed using a non-perturbatively improved clover fermion action, and a splitting is found between the masses of the nucleon and its parity partner. An analysis of the mass of the first radial excitation of the nucleon finds a value considerably larger than that of the parity partner of the nucleon, and thus little evidence for the Roper resonance as a simple three-quark state 
LHPC Collaboration: D. G. Richards, QCDSF Collaboration: M. Gockeler,   R. Horsley, D. Pleiter, P. E. L. Rakow, G. Schierhol...