K+-nucleus total cross section and polarization of nuclear matter

NUCLEAR PHYSICS A EI,SEVIER

Nuclear Physics ,4,585 (1995) 337c-338e

K + - N u c l e u s t o t a l cross section a n d p o l a r i z a t i o n of n u c l e a r m a t t e r J.C. Caillon and J. Labarsouque, Centre d'Etudes Nucl6aires de Bordeaux-Gradignan, Universit6 Bordeaux I, 33175 Gradignan Cedex, France It is well known now for a long time that the K + mesons, which are the weakest of the strong-interacting probes, penetrate deeply into the high density regions of the nucleus. In these regions, dynamical objects like the nucleons and mesons would behave differently than in vacuum. It has been shown 1'2) that the K+-nucleus cross-sections would be particularly sensitive to the in-medium properties of the a and w mesons whose exchange provides the dominant part of the medium-range K+-nucleon interaction. This sensitivity appears considering that, in the medium, the mesons behave like free ones but with densitydependent effective masses. For a better understanding of this effect, it would be interesting to go beyond such an approximation. We have performed a calculation of the K+-nucleus cross sections taking into account the coupling of the a and w mesons exchanged between the K + and the target nucleons, to the polarization of the Fermi and Dirac seas. This polarization has been calculated in the one-loop approximation but summed up to all orders in the mesons propagators (RPA-type calculation, for details see ref 3)). We have analyzed these effects on the ratio RT of K + _12 C to K + - d total cross sections which has been measured 4'5"6) from 400MeV/c to 900MeV/c.

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For the K+N interaction, we have used the full Bonn boson exchange B1 model7). For the calculation of the nuclear medium polarization, since we have made a summation up to all orders, we must use a NN interaction which provides a good description of nuclear matter and of finite nuclei. We have used here the density-dependent scalar and vector potentials obtained by Brockmann and Machleidt s) in a relativistic Brueckner~HartreeFock (RBHF) calculation starting from a nucleon-nucleon interaction in free-space where the coupling constants and form factors were obtained from NN scattering data. This interaction reproduces the saturation properties of nuclear matter (Fermi momentum, energy per nucleon, compression modulus) nearly quantitatively and leads to binding energies and mean-square radii for finite nuclei in good agreement with experiment9). Our results are presented on fig. 1. The curve (a) represents RT calculated with the free-space K+N interaction. When we use the RBHF nucleon-nucleon interaction of Brockmann and Machleidt (potential B of ref s)) in the calculation of the nuclear 0375-94741951509.50 © 1995 SSDI

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J.C. Caillon, J. Labarsouque / Nuclear Physics A585 (1995) 337c-338c

polarization, the RT ratio (curve (b)) goes up toward the experimental points. Thus, we have fonnd that the K+-nucleus total cross sections are sensitive to the

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Figure 1: Ratio RT of the K + _12 C to the K + - d total cross section (see text). dressing of the mesons exchanged between the K + and the target nucleons by the polarization of the nuclear medium. We have also found that, when the polarization is calculated with an in-medium nucleon-nucleon interaction leading to a good description of the properties of nuclear matter and finite nuclei, the agreement with experiment is considerably improved over the full energy range. To our knowledge, such an agreement has never been obtained in a fully microscopic calculation of the RT ratio without any free parameter. 1. G. E. Brown, C. B. Dover, P. B. Siegel and W. Weise, Phys. Rev. Lett. 60 (1988) 2723 2. J. C. Caillon and J. Labarsouque, Phys. Lett. B 2 9 5 (1992) 21 3. J. C. Caillon and J. Labarsouque, Nucl. Phys. A 5 7 2 (1994) 649 4. D. Bugg et al., Phys. Rev. 168 (1968) 1466 5. Y. Mardor et al., Phys. Rev. Lett. 65 (1990) 2110 6. R. A. Krauss et al., Phys. Rev. C46 (1992) 655 7. R. B/ittgen et al., Nucl. Phys. A 5 0 6 (1990) 586 8. R. Brockmann and R. Machleidt, Phys. Rev. C42 (1990) 1965 9. R. Brockmann and H. Toki, Phys. Rev. Lett. 68 (1992) 3408