Beam-helicity asymmetry I^⊙ in the photoproduction of π⁰–pairs off the deuteron

Markus Oberle^a

Department of Physics, University of Basel, Ch-4056 Basel, Switzerland

^a e-mail: markus.oberle@unibas.ch

Abstract

Beam-helicity asymmetries have been measured at the MAMI accelerator in Mainz for the photoproduction of neutral pion pairs in the reactions γp → pπ⁰π⁰ and γd → (n)pπ⁰π⁰, γd → (p)nπ⁰π⁰ off free protons and off quasi-free nucleons bound in the deuteron for incident photon energies up to 1.4 GeV. A circularly polarized photon beam was produced off a longitudinally polarized electron beam using bremsstrahlung processes. The photons were tagged with the Glasgow magnetic spectrometer. Decay photons from the π⁰ mesons, recoil protons, and recoil neutrons were detected in the 4π covering detector system composed of the Crystal Ball and TAPS electromagnetic calorimeters. The free and quasi-free results are in almost perfect agreement. The measured asymmetries for reactions off protons and neutrons are very similar, in contrary to expectations. The results are compared to the predictions from the Two-Pion-MAID reaction model.