Measurement of the beam-helicity asymmetry $I^{\odot }$ in the photoproduction of $\ddot{I}{€}^0\ddot{I}{€}^{\pm }$-pairs off protons and off neutrons

Abstract

Beam-helicity asymmetries have been measured at the MAMI accelerator in Mainz for the photoproduction of mixed-charge pion pairs in the reactions ${\hat{\mathbf{I}}}^{3}p\to n\ddot{I}{€}^0\ddot{I}{€}^{+}$ off free protons and ${\hat{\mathbf{I}}}^{3}d\to (p)p\ddot{I}{€}^0\ddot{I}{€}^{-}$ and ${\hat{\mathbf{I}}}^{3}d\to (n)n\ddot{I}{€}^0\ddot{I}{€}^{+}$ off quasi-free nucleons bound in the deuteron for incident photon energies up to 1.4 GeV. Circularly polarized photons were produced from bremsstrahlung of longitudinally polarized electrons and tagged with the Glasgow-Mainz magnetic spectrometer. The charged pions, recoil protons, recoil neutrons, and decay photons from $\ddot{I}{€}^0$ mesons were detected in the 4$\ddot{I}€$ electromagnetic calorimeter composed of the Crystal Ball and TAPS detectors. Using a complete kinematic reconstruction of the final state, excellent agreement was found between the results for free and quasi-free protons, suggesting that the quasi-free neutron results are also a close approximation of the free-neutron asymmetries. A comparison of the results to the predictions of the Two-Pion-MAID reaction model shows that the reaction mechanisms are still not well understood, in particular at low incident photon energies in the second nucleon-resonance region.

accepted for publication in Eur. phys. J. A