Roper Resonance and S_{11}(1535) from Lattice QCD

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  • 22 June 2003
Abstract
Using the constrained curve fitting method and overlap fermions with the lowest pion mass at 180 MeV, we observe that the masses of the first positive and negative parity excited states of the nucleon cross over as they approach the chiral limit. Both results at the physical pion mass agree with the experimental values of the Roper resonance (N^{1/2+}(1440)) and S_{11} (N^{1/2-}(1535)). This is seen for the first time in a lattice QCD calculation and it verifies that the Roper state is a radial excitation of the nucleon with three valence quarks. These results are obtained on a quenched 16^3 x 28 lattice with a = 0.2 fm. We also extract the ghost \eta' N states (a quenched artifact) which are shown to decouple from the nucleon interpolation field above m_{\pi} ~ 300 MeV. From the quark mass dependence of these states in the chiral region, we conclude that spontaneously broken chiral symmetry dictates the dynamics of light quarks in the nucleon.

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