First measurement of the polarization observable $E$ and helicity-dependent cross sections in single ${\pi }^0$ photoproduction from quasi-free nucleons

Abstract

The double-polarization observable $E$ and the helicity-dependent cross sections ${\sigma }_{1/2}$ and ${\sigma }_{3/2}$ have been measured for the first time for single ${\pi }^0$ photoproduction from protons and neutrons bound in the deuteron at the electron accelerator facility MAMI in Mainz, Germany. The experiment used a circularly polarized photon beam and a longitudinally polarized deuterated butanol target. The reaction products, recoil nucleons and decay photons from the ${\pi }^0$ meson were detected with the Crystal Ball and TAPS electromagnetic calorimeters. Effects from nuclear Fermi motion were removed by a kinematic reconstruction of the ${\pi }^{0}N$ final state. A comparison to data measured with a free proton target showed that the absolute scale of the cross sections is significantly modified by nuclear final-state interaction (FSI) effects. However, there is no significant effect on the asymmetry $E$ since the ${\sigma }_{1/2}$ and ${\sigma }_{3/2}$ components appear to be influenced in a similar way. Thus, the best approximation of the two helicity-dependent cross sections for the free neutron is obtained by combining the asymmetry $E$ measured with quasi-free neutrons and the unpolarized cross section corrected for FSI effects under the assumption that the FSI effects are similar for neutrons and protons.

arXiv commentaccepted for publication in Phys. Lett. B

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• Moravcsik's theorem on complete sets of polarization observables reexamined
• Extraction of $\gamma n\to \pi N$ observables from deuteron-target data
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• Single ${\pi }^0$ Production Off Neutrons Bound in Deuteron with Linearly Polarized Photons
• Helicity dependent cross sections for the photoproduction of ${\pi }^0{\pi }^{\pm }$ pairs from quasi-free nucleons
• Photo- and Electro-excitation of Bound Neutrons and Protons
• New graphical criterion for the selection of complete sets of polarization observables and its application to single-meson photoproduction as well as electroproduction