First measurement of the helicity dependence for the gamma-p --> ppi+ pi- reaction
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Abstract
The helicity dependence of the total cross-section and the invariant-mass distributions in the
(p±) and (+−) final states for the
p → p+− reaction have been measured for the first time at
incident photon energies from 400 to 800MeV. The measurement was performed with the large-acceptance
detector DAPHNE at the tagged photon beam facility of the MAMI accelerator in Mainz. Although
this channel is found to be predominantly excited by the intermediate production of a state, both
the measured unpolarized and the helicity-dependent observables are generally not well described by the
existing theoretical models