ar X iv : h ep - p h / 98 11 24 2 v 1 4 N ov 1 99 8 Gluon - vs . Sea quark - Shadowing

We calculate the shadowing of sea quarks and gluons and show that the shadowing of gluons is not simply given by the sea quark shadowing, especially at small x. The calculations are done in the lab frame approach by using the generalized vector meson dominance model. Here the virtual photon turns into a hadronic fluctuation long before the nucleus. The subsequent coherent interaction with more than one nucleon in the nucleus leads to the depletion σ(γ * A) < Aσ(γ * N) known as shadowing. A comparison of the shadowing of quarks to E665 data for 40 Ca and 207 P b shows good agreement. 1 1. Introduction When calculating perturbative QCD cross sections in nucleus nucleus collisions one has to take care of an additional effect not appearing on the pure nucleon nucleon level: nuclear shadowing. As a result of this depletion of the nuclear parton densities at small x one finds a strong suppression of e.g. charmonium states and minijets [1, 2]. Also for prompt photons smaller multiplicities result [3] due to the smaller number density of partons in the relevant region of the momentum fraction variable. In either model (lab frame vector meson dominance type models or infinite momentum frame parton fusion models) the shadowing of gluons is expected to be much stronger than the shadowing of sea quarks, even at small x. This seems to contradict the first naive expectation in terms of the QCD improved parton model that explains the scaling violation of the structure functions via the DGLAP splitting functions which treat sea quarks similar to gluons (in the sense that the sea quarks are produced by the gluons when Q 2 increases) in a region where essentially no momentum is carried by valence quarks, i.e. in the small x region. In the following we will focus on the lab frame interpretation which explains the shadowing phenomenon by use of the generalized vector meson dominance model (GVMD). 2. Lab frame description of shadowing In the lab frame description one essentially makes use of the hadronic structure of the virtual photon, manifesting itself in a field theoretic approach in two different time orderings assuring gauge-and Lorentz invariance to the amplitudes [4]. At small enough x (x ≪ 0.1) the handbag graph contribution gets small. The interaction then proceeds via the VMD graph where the virtual photon fluctuates into a q ¯ q pair within the …