Study of Reaction Mechanisms and Short Range Correlations in Two Nucleon Emission off 4He Using Polarized Photons


This work presents the results of the 4He(\vec \gamma,NN) experiment which was performed in 1996 at MAMI by the A2 collaboration. The measured asymmetries exploiting linear polarized photons are a novelty besides the precise measurement of absolute cross sections over a wide angular- and photon-energy range (E_\gamma=110...600 MeV). The following detectors were employed for this 3-fold coincidence experiment: The multilayer plastic-scintillator PiP, a hodoscope with good position and energy resolution, and ToF, a large scintillator wall for detection of neutrons and protons via the time of flight method, and the Glasgow-tagger for the energy-tagging of the bremsstrahl photons. The measurement and investigation of nucleonic photo-absorption mechanisms were the intention of this experiment. Particularly it aimed towards an understanding of the genuine 2N absorption to seek access to the study of SRC. These correlations are based on the short-range nucleon-nucleon interaction which is mainly repulsive and which is a result of the quark-degrees of freedom of the nucleon.

Innovations, compared to the 12C(\gamma,NN) experiment in 1992, are the 4He cryotarget, especially developed for the measurement on helium, and the application of a diamond crystal as bremsstrahl radiator. The beam time could be held relatively low due to the high target density and stand time of the cryotarget. The large target volume resulted unfortunately in a combined energy resolution of all detectors of about 5-10 MeV in missing energy (E_2m). The usage of a diamond radiator at the tagger entrance allowed the production of linearly polarized and energy-tagged photons. An improved description of the bremsstrahl production taking into account the experimental conditions results in a very precise determination of the polarization degree with small systematic error.

The nearly complete separation of genuine 2N absorption from 3N, pion production and FSI processes succeeds via cuts in missing energy as it was established in the carbon experiments. This method works even more reliable for 4He due to the smaller probability of FSI in lighter nuclei. These reactions which have higher missing energies also contribute to the understanding of photo-absorption and were thus analysed as well. Yet, this work concentrates on the cross section and asymmetry of genuine 2N absorption assumed to be particular sensitive on effects of SRC.

Differences of certain observables compared to the expected behaviour based on the shell model, which were found in this analysis (e.g. in the asymmetry of the excitation function), presumably hints towards SRC effects. However, quantitatively statements are only possible with a comparison to a realistic model. The results of this measurement, cross sections and asymmetries for both isospin channels and a wide spectrum of observables are particular helpful for the analysis of SRC influences. Therewith many boundary conditions are given for a photo-absorption model and the correlated wave function, which should contribute to reduce possible types of correlations. The method to determine SRC, which is exploited here, is an indirect one: The comparison of theoretical calculations with different SRC models and 2N photo-absorption measurements should discriminate all but the adequate type of SRC, if the theoretical description of the photo-absorption process is complete and mathematical exactly solved. Meanwhile it is feasible to calculate the helium wave function based on the NN interaction solely. This means that the comparison of this measurement with theoretical calculations reveals the quality of the theoretical description of the (\gamma,NN) process. Moreover this comparison demonstrates the feasibility and benefit of this method to establish SRC effects.

The introduction (Chapter 1) and the description of the experimental set-up (Chapter 2) are followed by a presentation of the theory of bremsstrahlung and an improved consideration of experimental conditions (Chapter 3). Chapter 4 describes a simple model to clarify the physics of 2N photo-absorption. Therewith it may be searched for kinematical regions, relevant phase space and observables which are particular sensitive for SRC effects. The necessary steps of calibration and data-analysis are found in Chapter 5. In this work the cross section and asymmetries were studied in dependence of photon energy, missing energy and missing momentum for both, the pn and pp final states (Chapter 6). Additionally, various angular distributions with cuts on three photon energy regions were investigated. The yield of 3N emission was analysed as well, because it is a good estimate of the FSI contribution to inclusive pn and pp emission. The analysis of the data is concluded in section 6.7 and 6.8 by an attempt of a physical interpretation and a summary.