Search for a new narrow Lambda* resonance near the Lambda eta threshold at J-PARC

  Evidence for a narrow $\Lambda^*$ resonance at a mass of about 1665 MeV (just above the combined mass of the $\Lambda$ ground state plus the $\eta$ meson) has been seen in the $M(pK^-)$ mass of the Dalitz plot for $\Lambda_c \to pK^-\pi^+$ decay at the Belle experiment. Additional evidence for this resonance is found in the partial-wave analysis (PWA) of data on $K^- p$ reactions done independently by several groups, driven by angular distributions of the $K^- p \to \Lambda\eta$ reaction by the Crystal Ball collaboration. If a narrow $\Lambda^*$ resonance exists at this mass, then the conventional quark model cannot explain it, and a possible explanation is that it is a crypto-exotic baryon with a dominant meson-baryon component to its wave function.
Therefore we proposed a definitive experiment for $K^- p \to \Lambda \eta$, using the newly built hyp-TPC detector to establish the existence of the proposed narrow $\Lambda(1665)$ and determine its spin and parity in two weeks of beamtime at J-PARC (E72), and obtained (scientific) stage-1 approval . We will discuss these evidences for the proposed $\Lambda^*$ resonance and introduce the E72 experiment.