Non-strange Dibaryon Resonances Observed in the ${\hat{I}}^{3}d\to \ddot{I}{€}^0\ddot{I}{€}^{0}d$ Reaction

Abstract

Coherent double neutral-pion photoproduction on the deuteron, ${\hat{I}}^{3}d${$\to$}$\ddot{I}{€}^0\ddot{I}{€}^{0}d$, has been experimentally studied at incident photon energies ranging from 0.75 to 1.15 GeV. The total cross section as a function of the ${\hat{I}}^{3}d$ center-of-mass energy shows resonance-like behavior, which peaks at approximately 2.47 and 2.63 GeV. The measured angular distribution of deuteron emission is rather flat, which cannot be reproduced by the kinematics of quasi-free $\ddot{I}{€}^0\ddot{I}{€}^0$ production with deuteron coalescence. In $\ddot{I}{€}^{0}d$ invariant-mass distributions, a clear peak is observed at $2.14\pm 0.01$ GeV$/c^2$ with a width of $0.09\pm 0.01$ GeV$/c^2$. The spin-parity of this state is restricted to $1^{+}$, $2^{+}$ or $3^{-}$ from the angular distributions of the two $\ddot{I}{€}^0$s. The present work shows strong evidence for the existence of an isovector dibaryon resonance with a mass of 2.14 GeV$/c^2$. The $2^{+}$ assignment is consistent with the theoretically predicted ${\mathcal{D}}_{12}$ state, and also with the energy dependence of the $\ddot{I}€d$ partial-wave amplitude ${}^3{P}_2$ for the $\ddot{I}{€}^{\pm }d${$\to$}$\ddot{I}{€}^{\pm }d$ and $\ddot{I}{€}^{+}d${$\to$}$pp$ reactions.

12 pages, 6 figures