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Hadrons in the Nuclear Medium

Hadronic degrees of freedom appear in an effective way in the nuclear medium. Significant modifications are predicted by QCD-inspired models for the density and temperature regimes encountered in heavy ion reactions. Sizeable mass changes ($\approx $ 10 - 20 $\%$) are predicted already at normal nuclear matter densities and should be accessible experimentally in pion- or photon-induced reactions on nuclei. These modifications of hadron properties may be associated with a decrease in the chiral condensate with increasing density and temperature. They could be a precursor phenomenon for chiral symmetry restoration.
  
Figure: The cross section for photoabsorption from a nucleon in 12C compared to the free nucleon cross section.
\begin{figure}\epsfig{file=hadron/fig4.eps, width=\columnwidth}\end{figure}
 

The properties of baryon resonances in nuclei at normal density have recently been systematically studied at Frascati, Mainz and Bonn in photoabsorption experiments which show significant medium effects. While the $\Delta $ is only slightly distorted, higher excited nucleon states, in the second and third resonance regime, appear to be washed out. Moreover, in the region above 0.6 GeV the value of the absorption cross section per nucleon is significantly reduced as compared to the free nucleon value (Fig.[*]). Fermi motion and Pauli blocking are unable to reproduce the strong damping of the total photoabsorption cross section in nuclei. A strong interaction between nucleon resonances and the nucleus is able to reproduce the data only in a limited energy range. The microscopic origin of this effect is not understood. Several explanations have been proposed such as partial deconfinement of quarks in nuclei or the onset of shadowing effects at low energy, or the decrease of vector meson masses inside nuclei. As experimentally shown in the photoproduction of $\eta $-mesons off nuclei, some resonances like the S11(1535) seem, however, to preserve their identity in the nuclear medium, probably due to their specific structure. A new approach to medium modifications of baryon resonances is provided by the excitation of the $\Delta $-resonance in reactions as e.g. 12C(e,e'p$\pi $)11Cgs reaction. Measurement of this reaction with the MAMI three-spectrometer set-up shows a cross section enhancement for high initial momenta of the struck nucleon in comparison to the 12C(e,e'p)11Bgs reaction, indicating additional reaction mechanisms beyond the usually assumed quasi-free production. A challenge in this field is to test the understanding of Quantum Chromodynamics in the non-perturbative regime, in particular the manifestations of Chiral Symmetry. Photon beams as well as high energy pion beams provide opportunities here for a rich experimental programme. Large acceptance detectors (CLAS at Jefferson Lab, Crystal Barrel at ELSA and HADES at GSI) will provide access to an extensive research programme on photon and di-lepton spectroscopy which addresses the elementary properties of hadrons as well as their possible modification within the nuclear medium. The electromagnetic structure of mesons and baryons can be studied by measuring their time-like formfactors which are important ingredients in any modelling of hadrons. Transition formfactors can be deduced from the shape of the dilepton invariant mass spectra from $\gamma e^+e^-$ or $\gamma \gamma e^+e^-$ Dalitz decays of neutral mesons. A precise knowledge of these formfactors is also important for a quantitative understanding of dilepton spectra from heavy ion and $\tau$-induced reactions. Measuring e+e--decays of vector mesons in nuclei or compressed hadronic matter, formed in nucleus-nucleus collisions, will test the prediction for meson mass shifts with increasing baryon density, associated in some model calculations with the restoration of chiral symmetry. An enhancement of dilepton yields recently observed in relativistic and nonrelativistic heavy ion reactions may be first evidence for such medium modifications of hadrons. 


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