Two-Pole Nature of the (1405) Resonance from Lattice QCD

This Letter presents the first lattice QCD computation of the coupled channel KE- over bar KN scattering amplitudes at energies near 1405 MeV. These amplitudes contain the resonance A(1405) with strangeness S = -1 and isospin, spin, and parity quantum numbers I(JP) = 0(1/2-). However, whether there is a single resonance or two nearby resonance poles in this region is controversial theoretically and experimentally. Using single -baryon and meson -baryon operators to extract the finite -volume stationary -state energies to obtain the scattering amplitudes at slightly unphysical quark masses corresponding to mK approximate to 200 MeV and mK approximate to 487 MeV, this study finds the amplitudes exhibit a virtual bound state below the KE threshold in addition to the established resonance pole just below the over bar KN threshold. Several parametrizations of the two -channel K matrix are employed to fit the lattice QCD results, all of which support the two -pole picture suggested by SU(3) chiral symmetry and unitarity.