  The positive-parity nucleon spectrum is explored in $2 + 1$-flavour lattice
QCD in a search for new low-lying energy eigenstates near the energy regime of
the Roper resonance. In addition to conventional three-quark operators, we
consider novel, local five-quark meson-baryon type interpolating fields that
hold the promise to reveal new eigenstates that may have been missed in
previous analyses. Drawing on phenomenological insight, five-quark operators
based on $\sigma{N}$, $\pi{N}$ and $a_0{N}$ channels are constructed. Spectra
are produced in a high-statistics analysis on the PACS-CS dynamical gauge-field
configurations with $m_{\pi} = 411\textrm{ MeV}$ via variational analyses of
several operator combinations. Despite the introduction of qualitatively
different interpolating fields, no new states are observed in the energy regime
of the Roper resonance. This result provides further evidence that the
low-lying finite-volume scattering states are not localised, and strengthens
the interpretation of the Roper as a coupled-channel, dynamically-generated
meson-baryon resonance.
