In $2+1$ flavor lattice QCD, the spectrum of the nucleon is presented for both parities using local meson-baryon-type interpolating fields in addition to the standard three-quark nucleon interpolators. The role of local five-quark operators in extracting the nucleon excited-state spectrum via correlation matrix techniques is explored on dynamical gauge fields with $m_{\pi }=293\mathrm{MeV}$, leading to the observation of a state in the region of the noninteracting S-wave $N\pi$ scattering threshold in the negative-parity sector. Furthermore, the robustness of the variational technique is examined by studying the spectrum on a variety of operator bases. Fitting a single-state ansatz to the eigenstate-projected correlators provides robust energies for the low-lying spectrum that are essentially invariant despite being extracted from qualitatively different bases.

• Received 3 February 2015

DOI:https://doi.org/10.1103/PhysRevD.91.094509