Final results are presented for the spin-spin correlation parameters CsL and CLL for np elastic scattering with a polarized neutron beam incident on a polarized proton target. The beam kinetic energies are 484, 634, and 788 MeV, and the c.m. angular range is 80"-180". These data will contribute significantly to the determination of the isospin-0 amplitudes in the energy range from 500 to 800 MeV. PACS numberk): 13.75.Q 13.88.+e, 14.20.Pt, 21.30.+ y
Elastic cross sections have been measured for the scattering of m+ and n from ' 0 at 114 MeV in the angular range of 115 '-175' (c.m. ). Large cross sections and large m+ jm differences have been observed beyond 140' (c.m. ). First-order optical model calculations fail to reproduce the large-angle data. Secondorder effects generally improve the fits, but the details of the present data together with the results of earlier measurements at smaller angles have not been satisfactorily explained.The pion is the only currently used probe of nuclei that exists in both positive and negative charge states within the same isospin multiplet. This means, for example, that the pion can be used to study details of Coulomb effects in hadronic scattering, as well as differences in neutron and proton distributions in nuclei. This latter sensitivity arises because of the difference in the isospin structure of the m+p(m n) and m+n(m p) systems. In the region of the A(1232) resonance this results in the well-known 9:1 ratio for the total elastic cross section. With measurements of m -scattering on self-conjugate nuclei, it has been possible to investigate various charge symmetry breaking effects in nuclei. ' Results of such studies are reliable only if the pionnucleus interaction is adequately understood. This interaction has been the subject of intensive investigation for several years, with work that has resulted in good fits by several models to scattering data which extend generally over the forward hemisphere.For scattering at backward angles, however, these models give strongly divergent predictions, indicating that higher-order effects are r.ceded to explain the difference. These considerations have led us to begin a study of pion-nucleus scattering at large angles, using both m+ and vr, with the aim of improving our understanding of the pion-nucleus interaction.To date, only two sets of m -elastic scattering data on a self-conjugate nucleus which extend out to 180' have been published, i.e. , for ' C at 162 (Ref. 4) and 100 MeV. 5 At 100 MeV, the sr+/vr cross section ratio was found to be less than 1.5, and at 162 MeV it was smaller. In both cases the shapes of m+ and~angular distributions were very similar at backward angles.The 100 MeV data were analyzed5 within the delta-hole formalism, 6 but the mdifference could not be explained simultaneously for both elastic and inelastic scattering. We note that both of these data sets indicate a relative maximum near 180'. In this paper, we present data on ' 0 at 114 MeV which indicate a minimum near 180, with a m+/m ratio of about 2.5. This is the largest such ratio that has been reported for a selfconjugate nucleus.The experiment was performed at the Clinton P. Anderson Meson Physics Facility, using the Energetic Pion Channel and Spectrometer (EPICS). The EPICS spectrometer and the experimental setup for the back-angle measurements are described elsewhere. ' The target used was in the form of beryllium oxide, with areal density 481 mg/cm . Typical energy resolution was of the orde...
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