l - ex / 0 50 90 22 v 2 1 6 Se p 20 05 Photoproduction of η - mesons off C and Cu nuclei for photon energies below 1 . 1 GeV

The η-meson photoproduction cross sections have been measured on the C and Cu targets for the photon energies between 600 and 1100 MeV to investigate the behavior of the S11(1535) resonance in a nucleus. The excitation functions of the cross section as well as the angular and momentum distributions of η-mesons are in quantitative agreement with the QuantumMolecular Dynamics (QMD) model calculations in which the η-meson emission processes other than the S11(1535) resonance are also incorporated as proposed in the η-MAID model. It is shown that the excitation of the D15(1675) resonance might play an important role for Eγ > 900 MeV. Preprint submitted to Elsevier Science 8 February 2008 The behavior of hadrons in the nuclear medium is one of the most intriguing topics in hadron and nuclear physics. Photon induced reactions are advantageous to producing hadrons deeply inside a nucleus because photons are hardly absorbed. Modifications in appearances may always be observed. Most of them originate simply from the basic effects of the nuclear medium, such as the Fermi motion of nucleons, Pauli blocking of the final state and collisions with nucleons. In addition, an interesting possibility has been proposed; i.e., mass modification arising from partial restoration of chiral symmetry in the nuclear medium [1,2]. The effects of the mass change of the ρ-meson have been studied in ρ photoproduction on nuclear targets [3,4] as well as in the hadron reactions [5]. However, mass change of baryons has not been studied well except for the ∆ resonance [6]. The S11(1535) resonance is proposed to be a candidate of the chiral partner of the ground state nucleon, and its resonance energy is expected to shift down by about 100 MeV in the nuclear medium where chiral symmetry is partially restored [2]. The S11(1535) resonance is known to decay into the Nη channel with a large branching ratio of 30–55% [7], while other nucleon resonances in this energy region hardly decay to the Nη channel. Therefore, the excitation and decay of the S11(1535) resonance is a dominant feature of η photoproduction off the nucleon in the region of photon energies below 1000 MeV [8,9]. It is, thus, expected that the properties of the S11(1535) resonance in the nuclear medium can be studied through η photoproduction off nuclei. The measurements of the A(γ, η) reactions have been reported by RöbigLandau et al. on C, Ca, Nb and Pb for Eγ < 800 MeV [10], and by Yorita et al. on C, Al and Cu for Eγ < 1000 MeV [11]. In both measurements, the S11(1535) resonance is clearly observed in the excitation function, which can be reproduced by a calculation taking into account the basic effects of the nuclear medium with parameters deduced from the total cross section of the γp → ηp reaction. It seems, however, that the success of the interpretation of the A(γ,η) reaction with the S11(1535) resonance alone is partially due to the lack of the quality in the previous data for Eγ > 800 MeV [11] as well as those of the γp → ηp reaction. In the last several years, there were essential progresses in experimental and theoretical work on the γp → ηp reaction. For the experimental side, precise Email address: [email protected] (T. Kinoshita). 1 Present address: RIKEN, Wako 351-0198, Japan 2 Present address: Japan Cycle Development Institute, Tokaimura 319-1194, Japan 3 Present address: Center for Nuclear Study (CNS), University of Tokyo, Wako 3510198, Japan 4 Present address: High Energy Accelerator Research Organization (KEK), Tsukuba 305-0801, Japan