Eta-meson photoproduction on protons, neutrons, and light nuclei in the nucleon-resonance region
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EMENTARY PARTICLES AND FIELDS Experiment
Eta-Meson Photoproduction on Protons, Neutrons, and Light Nuclei in the Nucleon-Resonance Region A. S. Ignatov, A. N. Mushkarenkov, and V. G. Nedorezov* Institute for Nuclear Research, Russian Academy of Sciences, pr. Shestidesyatiletiya Oktyabrya 7a, Moscow, 117312 Russia Received December 25, 2008
Abstract—Eta-meson photoproduction on protons, neutrons, and light nuclei is reviewed. New results obtained for photoproduction on protons and neutrons at the GRAAL facility are analyzed in detail. The experiment in question was performed in a gamma-ray beam that was obtained by means of Compton backscattering of laser photons on electrons at the European Synchrotron Radiation Facility (ESRF, Grenoble, France). A comparative analysis of data on the differential cross sections and the asymmetries for eta-meson photoproduction allowed us to reveal some special features and to study the mechanisms of eta-meson photoproduction on nucleons in detail. Data available from the literature on eta-meson photoproduction on light nuclei are discussed in detail. Results that suggest the existence of bound states (eta mesic nuclei) are presented. Particular attention is given to the new method of “tagged mesons,” which, according to the GRAAL experiment and to simulated results, makes it possible to study thoroughly the interaction of unstable mesons with intranuclear nucleons. PACS numbers: 21.65.Jk DOI: 10.1134/S1063778809100202
INTRODUCTION Investigation of properties of baryon resonances (such as the mass, width, parity, isospin, and decay modes) provides a basis for obtaining deeper insight into the internal structure of nucleons [1]. In recent years, considerable advances have been made in describing the properties of excited nucleon states on the basis of quark models [2–4], but many problems have remained unresolved. For example, some resonances allowed by selection rules are not seen in the spectra of photonucleon excitations (so-called missing resonances), and the reason for their disappearance is still not clear. At the same time, attempts are being made to find exotic resonances—in particular, dibaryons and pentaquarks—whose existence is admitted within some present-day theoretical models. In this connection, it should be noted that, since the eta-meson isospin is zero, nucleon resonances excited in eta-meson production are not delta resonances (those of isospin 3/2), which are dominant in pion-production reactions. Hence, the eta meson acts as an isospin filter that selects isospin-1/2 states. An individual problem arises in studying etameson photoproduction in nuclei. Since the lifetime of free eta mesons is rather long, so that their mean free path is commensurate with the radius of light *
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nuclei, and since the real part of their scattering length is positive (that is, eta-meson interaction with nucleons is attractive), it was assumed that there are bound states of eta mesons with nuclei (eta mesic nuclei). A series of experiments devoted to search
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