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Using the Crystal Barrel(CB)/TAPS detector system at the ELSA accelerator facility in Bonn the eta photoproduction off nuclei (C, Ca, Nb and Pb) was studied via the hadronic decay channel eta -> pi^0pi^0eta . Recent results on the in-medium properties of the eta-meson, derived from the transparency ratio measurements, are presented. The absorption of the eta-meson in nuclear matter is compared to the properties of other mesons (eta, omega).
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We shed light upon the eta mass in nuclear matter in the context of partial restoration of chiral symmetry, pointing out that the U_{A}(1) anomaly effects causes the eta-eta mass difference necessarily through the chiral symmetry breaking. As a consequence, it is expected that the eta mass is reduced by order of 100 MeV in nuclear matter where partial restoration of chiral symmetry takes place. The discussion given here is based on Ref. [1].
In-medium modification of the eta mass is discussed in the context of partial restoration of chiral symmetry in nuclear medium. We emphasize that the U_A(1) anomaly effects causes the eta-eta mass difference necessarily through the chiral symmetry breaking. As a consequence, the eta mass is expected to be reduced by order of 100 MeV in nuclear matter where about 30% reduction of chiral symmetry takes place. The strong attraction relating to the eta mass generation eventually implies that there should be also a strong attractive interaction in the scalar channel of the eta-N two-body system. We find that the attraction can be strong enough to form a bound state.
The photoproduction of eta-mesons off different nuclei has been measured with the CBELSA/TAPS detector system for incident photon energies between 1500 - 2200 MeV. The transparency ratio has been deduced and compared to theoretical calculations describing the propagation of eta-mesons in nuclei. The comparison indicates a width of the eta -meson of the order of Gamma= 15-25 MeV at rho =rho_0 for an average momentum p_{eta} = 1050 MeV/c, at which the eta-meson is produced in the nuclear rest frame. The inelastic eta N cross section is estimated to be 3 - 10 mb. Parameterizing the photoproduction cross section of eta-mesons by sigma(A) = sigma_0 A^{alpha}, a value of alpha = 0.84pm0.03 has been deduced.
Taking advantage of both the low-emittance proton-beam of the Cooler Synchrotron COSY and the high momentum precision of the COSY-11 detector system, the mass distribution of the eta meson was measured with a resolution of 0.33 MeV/c^2 (FWHM), improving the experimental mass resolution by almost an order of magnitude with respect to previous results. Based on the sample of more than 2300 reconstructed pp --> pp eta events the total width of the eta meson was determined to be 0.226 +- 0.017(stat.) +- 0.014(syst.) MeV/c^2.
Properties of $rho$-meson in symmetric nuclear matter are investigated in a light-front constituent quark model (LFCQM), using the in-medium inputs calculated by the quark-meson coupling (QMC) model. The LFCQM used in this study was already applied for the studies of the electromagnetic properties of $rho$-meson in vacuum, namely, the charge~$G_0$, magnetic~$G_1$, and quadrupole~$G_2$ form factors, electromagnetic charge radius, and electromagnetic decay constant. We predict that the electromagnetic decay constant, charge radius, and quadrupole moment are enhanced as increasing the nuclear matter density, while the magnetic moment is slightly quenched. Furthermore, we predict that the value $Q^2_{rm zero}$, which crosses zero of the charge form factor, $G_0(Q^2_{rm zero})=0$ ($Q^2 = -q^2 > 0$ with $q$ being the four-momentum transfer), decreases as increasing the nuclear matter density.
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