Resonance-like structure near the eta d threshold in the gamma d -> pi(0) eta d reaction

To investigate the interaction between the nucleon N and nucleon resonance N(1535)1/2(-), the eta d threshold structure connected to the isoscalar S-wave N-N(1535)1/2(-) system has been experimentally studied in the gamma d -> pi(0) eta d reaction at incident photon energies ranging from the reaction threshold to 1.15 GeV. A strong enhancement is observed near the eta d threshold over the three-body phase-space contribution in the eta d invariant-mass distribution. An analysis incorporating the known isovector resonance D-12 with a spin parity of 2(+) in the pi(0)d channel reveals the existence of a narrow isoscalar resonance-like structure with 1(-) in the eta d system. Using a Flatte parametrization, the mass is found to be 2.4271(-0.006)(+0.013) GeV, close to the eta d threshold, and the width is (0.029(-0.029)(+0.006) GeV) + (0.00(-0.00)(+041))p(eta)c, where p(eta) denotes the eta momentum in the rest frame of the eta d system. The observed structure would be attributed to a predicted isoscalar 1(-) eta NN bound state from eta NN and pi NN coupled-channel calculation, or an eta d virtual state owing to strong eta d attraction.