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Bochum 1998 – scientific programme

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HK: Hadronen und Kerne

HK 40: Electromagnetic Probes VI, NN-Correlations

HK 40.1: Group Report

Tuesday, March 17, 1998, 17:00–17:30, F

Study of NN correlations by polarised photons* — •F.A. Natter, P. Grabmayr, T. Hehl, T. Lamparter, M. Mayer, R. Schneider, G.J. Wagner, and S. Wunderlich — Physikalisches Institut, Universität Tübingen, for the A2 – Collaboration


New attempts to investigate the short range nature of the NN interaction (S RC) have been made by use of polarised tagged photons in NN knockout reaction on 4He. First experiments with 6Li [1] confirmed the α-d cluster model an d the similarity of the 6Li(γ,np)4He g.s. channel to the free deuteron photodesintegration. The comparison of 12C(γ,NN) data [2] to calcula tions from the Gent and Valencia groups [3] yielded a quantitative understanding of the underlying mechanisms, e. g. one- and two body absorption, pion production, FSI and contributions fromMEC and IC. The direct two body photoabsorption on a NN pair was shown to r esult in low missing energies allowing its separation from the competing processe s. Relative momenta distributions of these nucleons before the knockout are enhanced at high momenta relative to the ones in deuterium, which might ind icate a signature of SRC. Regarding the measured angular distributions and (γ,pp)/(γ,np) ratio [4] the ρ exchange contributions and the need of medium modifications of the Δ propagat or are under discussion.


The 4He,NN) experiment was performed becaus e the photon asymmetry for the NN absorption is predicted [5] to be rather sensitiv to SRC, enhanced by the high central density and the reduced FSI influence due to the few body system. This experiment has taken place in 1996 at MAMI in the photon energy range of 100 < Eγ < 800 MeV employing a new 4He cryotarget, the Glasgow tagger, the PiP and TOF spectrometer. Data have been taken with unpolarized and linearly polarized beam, produced via coherent bremsstrahlung from a diamond radiator (Pγ ≈ 65 %) at peak energies around 220, 280 and 350 MeV. Missing mass spectra, excitation functions and asymmetries on D and 4He will be presented and compared with previous results.


[1] P. Grabmayr et al., Phys. Lett. B370 (1996) 17

[2] Th. Lamparter et al., Z. Phys. A355 (1996) 1

[3] J. Ryckebusch et al., Nucl. Phys. A568 (1994) 828
R.C. Carrasco and E. Oset, Nucl. Phys. A570 (1994) 701

[4] T. T-H. Yau PhD thesis, Glasgow university 1996

[5] Boffi et. al., Nucl. Phys. A A564 (1993) 473

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