Nuclear Shadowing and In-medium Properties of the Ρ

We explain the early onset of shadowing in nuclear photoabsorption within a multiple scattering approach and discuss its relation to in-medium modifications of the ρ 0. The nuclear photoabsorption cross section is known to be shadowed at large energies, i.e. σ γA < Aσ γN. This was at first interpreted as a confirmation of the vector meson dominance (VMD) model, which assumes that the photon might fluctuate into vector meson states with a probability of order α em. To give rise to shadowing, these hadronic fluctuations must travel at least a distance l V that is larger than their mean free path inside the nucleus. This so called coherence length l V can be estimated from the uncertainty principle l V ≈ |k γ − k V | −1 = kγ − k 2 γ − m 2 V −1 (1) where k γ and k V denote the momentum of the photon and the vector meson respectively and m V is the vector meson mass. Recent photoabsorption data 1,2 indicate an early onset of shadowing at E γ ≈1 GeV. From (1) one sees that the lightest vector meson, e.g. the ρ 0 , has the largest coherence length and therefore its properties determine the onset of the shadowing effect. This lead to the interpretation 3 of the low energy onset of shadowing as a signature of a decreasing ρ 0 mass in medium since a decrease of m ρ increases the coherence length l ρ. A quantitative description of the shadowing effect is possible within the Glauber model 4. The nuclear photoabsorption cross section can be related via the optical theorem to the nuclear forward scattering amplitude. In order α em one finds the two contributions shown in Fig. 1. The left amplitude stems from forward scattering of the photon from one nucleon inside the nucleus. Summing over all nucleons this amplitude alone leads to the unshadowed cross section σ γA = Aσ γN. Shadowing arises from the interference with the * WORK SUPPORTED BY DFG AND BMBF.