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

This Letter presents the first lattice QCD computation of the coupled channel $\pi \mathrm{\Sigma }\text{−}\overline{K}N$ scattering amplitudes at energies near 1405 MeV. These amplitudes contain the resonance $\mathrm{\Lambda }(1405)$ resonance with strangeness $S=-1$ and isospin, spin, and parity quantum numbers $I(J^P)=0(1/2^{-})$. 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 $m_{\pi }\approx 200\mathrm{MeV}$ and $m_K\approx 487\mathrm{MeV}$, this study finds the amplitudes exhibit a virtual bound state below the $\pi \mathrm{\Sigma }$ threshold in addition to the established resonance pole just below the $\overline{K}N$ 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.