STUDY OF MEDIUM EFFECT IN HADRON MANY-BODY SYSTEM AT DEEP INSIDE OF NUCLEI

点击翻译按钮获取中文摘要

It is predicted theoretically in the framework of QCD and QHD (quantum hadrodynamics) that hadron masses in nuclear field is modified as a result of partial restoration of chiral symmetry or as a representation of the lower-component enhancement of the nucleon Dirac spinor in a strong scalar field. At several hundred MeV of incident energies, the interactions between nucleons are well described by theoretical models based on the meson exchange forces. Thus it is expected that those mass modifications cause modification of NN interactions which is detectable in some nuclear reactions. And the nucleon quasifree scattering is expectable one of such reactions, since this reaction can be regarded as a NN scattering in nuclear field and suited for studying the NN interaction in nuclear field. It is first found by TRIUMP people that the analyzing power for a (p,2p) reaction leading to a 1s1/2-hole state shows distinct reduction from the free p-p scattering values. At RCNP, this reduction is s … More hown to be monotonically density-dependent, which suggests the existence of some medium effect. Such reduction has also been observed at 1 GeV for a 7Li target.In the present study, we have measured polarization for the (p,2p) reactions at 1 GeV at PNPI. The two-arm magnetic spectrometer system is employed for the measurement and a focal plane polarimeter which consists of a carbon analyzer and MWPC's is used on each spectrometer in order to measure the polarization of outgoing protons. We found that the data show strong density-dependent reductions from calculated values using the free NN interaction. This result is consistent with the 392 MeV result. From the fact that the consistent results are obtained at different incident energies and from several evidences found at 392 MeV, it is concluded that the most part of the reductions are not nuclear reaction originated, such as by a multi-step effect. We have estimated the effect of nucleon-mass modification in a Schodenger-equivalent form of the relativistic DWIA and found that about a half of the reduction is explained by this model, which is again consistent with previous 392 MeV result. It is already known that a nonrelativistic calculation with the Pauli blocking effect does not reproduce the observed reduction and the present result, in conjunction with the 392 MeV results, supports the idea that the reduction is a manifestation of a medium effect in the hadron level. Less