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Helicity-Dependent Angular Correlations in Double-Charged-Pion Photoproduction on Nuclear Targets (Strauch, Berman) Polarization observables can provide valuable information on the dynamics of nuclear reactions. Section A5 describes in more detail our analysis of the double-charged-pion production on the proton with circularly polarized photons. In that analysis lies the main thrust of our present efforts; however, we also have performed a similar, exploratory analysis for the double-charged-pion production on 3He and 4He. Figure 16 shows the helicity-dependent cross-section asymmetry as a function of the relative laboratory azimuthal angle DF between the two final-state p+ and p- mesons. Shown are preliminary data from 3He (triangles) and 4He (circles), along with a fit of a sin(nDF) series to the data. These distributions for 3He and 4He show distinct differences which might be related to differences in nuclear structure, nuclear density, or final-state effects.
Figure 16. Helicity-dependent cross-section asymmetry as a function of the relative laboratory azimuthal angle between the final-state p+ and p- mesons. Shown are preliminary data from 3He (triangles) and 4He (circles) along with a fit to the data. While still sizable, the asymmetries for 4He are smaller than those for 3He (and still smaller than those for the proton [see Section A4]).The energy dependence of the Fourier coefficients of the helicity-dependent angular distributions is shown in Fig. 17. The upper and lower panels show different Fourier components of the observed angular distributions. The asymmetry has been corrected for the energy-dependent polarization of the photon beam. For a better comparison, the smaller asymmetries for 3He were scaled to match the asymmetries for the proton. We notice not only that these asymmetries are large, but that the dependence on incident photon energy is strikingly different for 3He and the proton, showing the existence of nuclear effects not seen on the nucleon.
Figure 17. Preliminary, scaled Fourier components of the cross-section asymmetry in the photoproduction of p+p- events from 3He (open triangles) for circularly polarized photons, as a function of the gN center-of-mass energy, compared with corresponding data for the proton (filled circles).After finalizing the analysis of the data on the proton, we plan to return to the nuclear targets and to extend our analysis to the double-charged-pion photoproduction on 2H (g2 data), 3He (g3 data), and 4He (g3 data). Not only will a comparison of the data for the proton and the deuteron give information on the case for the neutron, but a study of the evolution of the effect for 2H, 3He, and 4He will quantify the density dependence of this new observable. |
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