Benchmark solutions for n-d scattering using a single model interaction have been produced by five different groups using different methods. These s-wave results have been obtained at three energies for both quartet and doublet scattering.
Results for do. /dQ and Az in the reaction pp -+~m are predicted by a simple quark model. They are compared to recent experimental data from the CERN LEAR, as well as to previous predictions from nucleon-exchange models. At low energy the quark model does better than the nucleon-exchange models, but the overall comparison to experiment remains poor. In particular, the double-dip structure of the experimental A& data is only partly represented. This shortcoming of the simple quark model is traced back to a too small J =2 amplitude. This has interesting implications for the range of this specific annihilation process.PACS number(s): 25.43.+ t, 21.30.+ y, 13.75.Cs, 12.40. Qq
The large set of accurate data on differential cross section and analyzing power from the CERN LEAR experiment onpp → π − π + in the range from 360 to 1550 MeV/c is well reproduced within a distorted wave approximation approach. The initialpp scattering wave functions originate from a recentN N model. The transition operator is obtained from a combination of the 3 P0 and 3 S1 quarkantiquark annihilation mechanisms. A good fit to the data, in particular the reproduction of the double dip structure observed in the analyzing powers, requires quark wave functions for proton, antiproton, and pions with radii slightly larger than the respective measured charge radii. This corresponds to an increase in range of the annihilation mechanisms and consequently the amplitudes for total angular momentum J = 2 and higher are much larger than in previous approaches. The final state ππ wave functions, parameterized in terms of ππ phase shifts and inelasticities, are also a very important ingredient for the fine tuning of the fit to the observables.
For the photoproduction of vector mesons, all single and double spin observables involving vector meson two-body decays are defined consistently in the γN center of mass. These definitions yield a procedure for extracting physically meaningful single and double spin observables that are subject to known rules concerning their angle and energy evolution. As part of this analysis, we show that measuring the two-meson decay of a photoproduced ρ or φ does not determine the vector meson's vector polarization, but only its tensor polarization. The vector meson decay into lepton pairs is also insensitive to the vector meson's vector polarization, unless one measures the spin of one of the leptons. Similar results are found for all double spin observables which involve observation of vector meson decay. To access the vector meson's vector polarization, one therefore needs to either measure the spin of the decay leptons, make an analysis of the background interference effects or relate the vector meson's vector polarization to other accessible spin observables.
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