Resonance-like structure near the $ηd$ threshold in the $γ{d}${$\to$}$π^0η{d}$ reaction

Abstract

To investigate the interaction between the nucleon N and nucleon resonance N (1535)1/2 − , the ηd threshold structure, connected to the isoscalar S-wave N -N (1535)1/2 − system, has been experimentally studied in the γd→π 0 ηd reaction at incident photon energies ranging from the reaction threshold to 1.15 GeV. A strong enhancement is observed near the ηd threshold over the three-body phase-space contribution in the ηd invariant-mass distribution. An analysis incorporating the known isovector resonance D12 with a spin-parity of 2 + in the π 0 d channel has revealed the existence of a narrow isoscalar resonance-like structure D ηd with 1 − in the ηd system. The Breit-Wigner mass of D ηd is found to be 2.425 −0.006 +0.003 GeV, located just close to the ηd threshold, and the width is 0.034 +0.004 −0.003 GeV. The D ηd would be a predicted isoscalar 1 − ηN N bound state from the ηN N and πN N coupled-channel calculation, or ηd virtual state owing to strong ηd attraction.