arXiv:nucl-th/0204061v2 3 Oct 2002 Shadowing in photo-absorption : role of in-medium hadrons Jan-e Alam a , Sanjay K. Ghosh a,∗ , Pradip Roy b , Sourav Sarkar a a Variable Energy Cyclotron Centre, 1/AF, Bidhannagar, Calcutta 700 064, INDIA b Saha Institute of Nuclear Physics, 1/AF, Bidhannagar, Calcutta 700 064, INDIA We study the effects of in-medium hadronic properties on shadowing in photon-nucleus interactions in Glauber model as well as in the multiple scattering approach. A reasonable agreement with the experimental data is obtained in a sce- nario of downward spectral shift of the hadrons. Shadowing is found to be insensitive to the broadening of the spectral func- tions. An impact parameter dependent analysis of shadowing might shed more light on the role of in-medium properties of hadrons. PACS Nos. : 25.20.Dc,25.40.Ep,24.10.Ht,12.40.Vv Theoretical studies based on various models of hadronic interaction predict a reduction in the mass of hadrons at or above nuclear matter density [1,2]. The observation of enhanced dilepton production in the low invariant mass domain in the heavy ion collision experi- ments [3] does seem to indicate a non-trivial modification of the properties of light vector mesons, particularly the ρ meson, in hot and/dense medium [4]. However, the complicated dynamics both in the initial as well as fi- nal states in these experiments inhibits a firm conclusion about nuclear medium effects at present. On the other hand, the experiments like photo- absorption on nuclei provide much cleaner systems for the study of in-medium properties of mesons [5]. With the availability of better photon beams, there has been a renewed interest in the photo-absorption processes. The phenomena of shadowing plays an important role in the photo-nuclear reactions. The photo-nuclear data at lower energies, for different nuclei, seem to indicate an early onset of shadowing [6,7]. In ref. [8] this fea- ture has been interpreted as a signature for a lighter ρ- meson in the medium, where the shadowing effect was evaluated within a Glauber- Gribov multiple scattering theory [9,10,11] and generalized vector meson dominance (VMD). In contrast the authors in ref. [12] have claimed that the early onset of shadowing can be understood within simple Glauber theory [11,13,14,15] if one takes the negative real part of the ρN scattering amplitude into account which corresponds to a higher effective in- medium ρ meson mass. In a subsequent paper [16], the authors have concluded that the enhancement of shadow- ing at low energies occurs due to lighter ρ mesons as well as intermediate π 0 produced in non-forward scattering. In the backdrop of these different inferences, we have made an attempt to understand the role of in-medium properties of hadrons in the phenomenon of shadowing in photo-nuclear reactions. The shadowing in photon- nucleus reactions can be written as, A ef f A = σ γA Aσ γN =1+ δσ γA Aσ γN (1) where σ γA = Aσ γN + δσ γA consists of the incoherent scattering of the photon from individual nucleons and a correction due to the coherent interaction with several nucleons.The later has been evaluated using both multi- ple scattering approach [10,16,17] as well as Glauber’s formula along with VMD [18]. Resonance contribution to γ − A cross-section for photon energy < ∼ 1.2 GeV has been estimated by using the prescription given in [19]. Let us first consider the in-medium effects on the kine- matics of γ − A collisions. For a photon incident on a nucleus with energy E γL , the energy in the rest frame of the nucleon is given by E γ = γ F E γL (1 − β F cos θ L ), (2) where β F = p F /E F and θ L is the angle between the incident photon and the Fermi momentum of the nucleon which now depends on the space co-ordinate through the density n(r). The square of the centre of mass energy s, of the γ − N system can then be written as, s =(p γ + p F ) 2 = m ∗ N 2 +2γ F m ∗ N E γL (1 − β F cos θ L ), (3) where m ∗ N is the effective nucleon mass inside the nucleus. The modification of vector meson masses in nuclear envi- ronment has been studied in different models [1,2,20,21]. Here, we have used two different models namely universal scaling scenario (USS) [1] and Quantum Hadrodynamical model (QHD) [22]. In USS, the effective hadronic masses (m ∗ H ) vary with nuclear density as m ∗ H m H =1 − 0.2x, (4) where x = n(r)/n 0 (r), n 0 (r) being the normal nuclear matter density. In QHD the effective masses of nucleons and vector mesons are calculated using standard tech- niques of thermal field theory [23,24] and is parametrized as : m ∗ H m H =1+  j=1 a j x j . (5) For nucleons a 1 = −0.351277 and a 2 =0.0766239; in case of ρ, a 1 = −1.30966, a 2 =1.78784, a 3 = −1.17524 and a 4 =0.294456 and finally for ω, a 1 = −0.470454, a 2 =0.313825 and a 3 = −0.0731274. No medium effect 1