Roberts, D.Kamleh, W.Leinweber, D.2013-09-172013-09-172013Physics Letters B: Nuclear Physics and Particle Physics, 2013; 725(1):164-1690370-2693http://hdl.handle.net/2440/80052We apply the eigenvectors from a variational analysis in lattice QCD to successfully extract the wave function of the Roper state, and a higher mass P<inf>11</inf> state of the nucleon. We use the 2+1 flavour 32³×64 PACS-CS configurations at a near-physical pion mass of 156 MeV. We find that both states exhibit a structure consistent with a constituent quark model. The Roper d-quark wave function contains a single node consistent with a 2S state, and the third state wave function contains two, consistent with a 3S state. A detailed comparison with constituent quark model wave functions is carried out, obtained from a Coulomb plus ramp potential. These results validate the approach of accessing these states by constructing a variational basis composed of different levels of fermion source and sink smearing. Furthermore, significant finite volume effects are apparent for these excited states which mix with multi-particle states, driving their masses away from physical values and enabling the extraction of resonance parameters from lattice QCD simulations. © 2013 .en© 2013 Published by Elsevier B.V.Roper resonanceWave functionsLattice QCDJournal article002013095610.1016/j.physletb.2013.06.0560003226068000222-s2.0-8488069843818520Kamleh, W. [0000-0002-6177-5366]Leinweber, D. [0000-0002-4745-6027]