First measurement of the polarization observable E and helicity-dependent cross sections in single π0 photoproduction from quasi-free nucleons

Abstract

The double-polarization observable E and the helicity-dependent cross sections σ 1/2 and σ 3/2 have been measured for the first time for single π 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 π 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 π 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 σ 1/2 and σ 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.

First measurement of helicity-dependent cross sections in π0η photoproduction from quasi-free nucleons

“…No significant effects were found for ππ and πη pairs. Also, for the helicity asymmetry E, as defined in this paper, no effects were found for single π 0 [37] and η production [33][34][35], although the effects on absolute cross sections for π 0 production are substantial. For πη production also shapes of invariant mass distributions of meson-meson and mesonnucleon pairs are basically unaffected [24].…”

“…The data analysis was based on the methods developed for the reaction identification of meson pairs (ππ and πη) described in detail in [7,23,24,27,28] for measurements with unpolarized liquid deuterium targets. The treatment of the unpolarized background from 'heavy' nuclei (carbon, oxygen), present in the butanol target, is described in [33,34,37] for photoproduction of η and π 0 mesons. For the latter, in addition to the data from the polarized butanol target, measurements with a carbon foam target and a liquid deuterium target were analyzed.…”

“…This analysis was done for the butanol, the liquid deuterium, and the carbon foam target. As discussed in [33,34,37] for other final states, the spectra from all three targets were normalized absolutely on the basis of photon flux, target density, etc. and are compared in Fig.…”

“…The experimental setup, combining the Crystal Ball and TAPS detectors with additional devices for charged particle identification, was identical to the one used for the results reported in [33,34,37], which used the same data set. The electromagnetic calorimeter combined the Crystal Ball (CB) [46] and TAPS [47] detectors.…”

Doubly-polarised pion photoproduction and the GDH sum rule on the nucleon at MAMI

“…The magenta line shows the sum from the MAID model for free-proton and free-neutron. After the identification of the proton or the neutron and the subtraction of the carbon and oxygen contribution, the deuterated butanol target allows also the extrapolation of the double-polarised observable E. In this way it is possible to expand the results from [11] at lower energy and on a larger angular coverage. Figure 3 shows the unpolarised and helicity-dependent total cross sections for the partial chan- Figure 1: Results on deuteron, on the left the helicity dependent total inclusive cross section for the γd → X, on the right the helicity dependent total cross section for the semi-exclusive channels γd → π 0 X .…”