Pion-nucleon scattering in the Roper channel from lattice QCD

We present a lattice QCD study of N pi scattering in the positive-parity nucleon channel, where the puzzling Roper resonance N*(1440) resides in experiment. The study is based on the PACS-CS ensemble of gauge configurations with N-f = 2 + 1 Wilson-clover dynamical fermions, m pi similar or equal to 156 MeV and L similar or equal to 2.9 fm. In addition to a number of qqq interpolating fields, we implement operators for N pi in p-wave and N sigma in s-wave. In the center-of-momentum frame we find three eigenstates below 1.65 GeV. They are dominated by N(0), N(0) pi(0)pi(0) [mixed with N(0)sigma(0)] and N(p)pi(-p) with p similar or equal to 2 pi/L, where momenta are given in parentheses. This is the first simulation where the expected multi-hadron states are found in this channel. The experimental Np phase shift would-in the approximation of purely elastic Np scattering-imply an additional eigenstate near the Roper mass m(R) similar or equal to 1.43 GeV for our lattice size. We do not observe any such additional eigenstate, which indicates that N pi elastic scattering alone does not render a low-lying Roper. Coupling with other channels, most notably with N pi pi, seems to be important for generating the Roper resonance, reinforcing the notion that this state could be a dynamically generated resonance. Our results are in line with most of the previous lattice studies based just on qqq interpolators, which did not find a Roper eigenstate below 1.65 GeV. The study of the coupled-channel scattering including a three-particle decay N pi pi remains a challenge.Slovenian Research Agency ARRS; Austrian Science Fund [FWF:I1313-N27]; Deutsche Forschungsgemeinschaft [SFB/TRR 55]; U.S. Department of Energy [DE-AC05-06OR23177]This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at repository@u.library.arizona.edu