Modification of the nucleon-nucleon scattering amplitude by nuclear medium effects

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Nuclear medium effect has been investigated experimentally through nucleon quasifree scattering. Exclusive measurements of the quasifree scattering give a direct way to study the nucleon-nucleon interaction in nuclear medium and is is expected to extract information on the properties of nucleons and mesons in nuclear field. For this puroose, defferential cross sections and analyzing powers (Ay) have been measured for the proton knockout reaction from s_<1/2> orbits of several target nuclei by using the ring cyclotron at Research Center for Nuclear Physics, Osaka University. The Ay values are reduce from those of free proton-proton scattering and the reduction rate is found to depend on target. We found that an averaged density seen through this reaction can be estimated in a special case where the recoil momentum, which corresponds to the experimental setting employed in this study, is zero and it reaches almost half of the nuclear saturation density in some cases. The reduction of Ay shows a monotomically decreasing function of the averaged density, which strongly suggests the existence of a nuclear medium effect on the nucleon-nucleon interaction. This result has been compared with the prediction by the Dirac approach. In this approach, , modification of the nucleon spinor in nuclear field is described as a reduction of the effective nucleon mass. We estimate the effective nucleon mass by using the averaged density defined above and calculate the modefication of the nucleon-nucleon observables by following the procedure proposed by a theoretical group in Indiana university. The calculation gives a qualitatively consistent result with our experimental result.