The isospin structure of photoproduction of pi-eta pairs from the nucleon in the threshold region

Abstract

Photoproduction of $\ddot{I}€\hat{I}\cdot$-pairs from nucleons has been investigated from threshold up to incident photon energies of $\approx$~1.4~GeV. The quasi-free reactions ${\hat{I}}^{3}p\to p\ddot{I}{€}^0\hat{I}\cdot$, ${\hat{I}}^{3}n\to n\ddot{I}{€}^0\hat{I}\cdot$, ${\hat{I}}^{3}p\to n\ddot{I}{€}^{+}\hat{I}\cdot$, and ${\hat{I}}^{3}n\to p\ddot{I}{€}^{-}\hat{I}\cdot$ were for the first time measured from nucleons bound in the deuteron. The corresponding reactions from a free-proton target were also studied to investigate final-state interaction effects (for neutral pions the free-proton results could be compared to previous measurements; the ${\hat{I}}^{3}p\to n\ddot{I}{€}^{+}\hat{I}\cdot$ reaction was measured for the first time). For the $\ddot{I}{€}^0\hat{I}\cdot$ final state coherent production via the ${\hat{I}}^{3}d\to d\ddot{I}{€}^0\hat{I}\cdot$ reaction was also investigated. The experiments were performed at the tagged photon beam of the Mainz MAMI accelerator using an almost $4\ddot{I}€$ coverage electromagnetic calorimeter composed of the Crystal Ball and TAPS detectors. The total cross sections for the four different final states obey the relation $\ddot{I}ƒ(p\ddot{I}{€}^0\hat{I}\cdot )$ $\approx$ $\ddot{I}ƒ(n\ddot{I}{€}^0\hat{I}\cdot )$ $\approx$ $2\ddot{I}ƒ(p\ddot{I}{€}^{-}\hat{I}\cdot )$ $\approx$ $2\ddot{I}ƒ(n\ddot{I}{€}^{+}\hat{I}\cdot )$ as expected for a dominant contribution from a $\hat{I}{”}^{\star }\to \hat{I}\cdot \hat{I}”(1232)\to \ddot{I}€\hat{I}\cdot N$ reaction chain, which is also supported by the shapes of the invariant-mass distributions of nucleon-meson and $\ddot{I}€$-$\hat{I}\cdot$ pairs. The experimental results are compared to the predictions from an isobar reaction model.

accepted for publication in Phys. Lett. B