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Register a new userTransparency ratio in gamma A -> eta A and the in-medium eta width
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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.
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The low $Q^2$ slopes of the the transition form factors provide a unique method to measure the sizes of the neutral pseudo-scalar mesons, since they do not have electromagnetic form factors. From the slope one obtains the axial transition RMS radius $ R_{PS,A} = sqrt{<r^2>}$ for each PS meson. The present status of theory and experiment for these quantities are presented. A comparison of the $ R_{PS,A}$ is presented along with the electromagnetic and scalar radii of the $pi^{pm}$ mesons and the proton. We observe the striking similarity of the values of axial transition radii of all of the pseudoscalar mesons to each other and to the charge radius of the $pi^{pm}$. In the $Q^2$ = 0 limit the transition form factor is a measure of the pseudo-scalar meson radiative width (lifetime) and is a possible fourth (unexploited) method to perform such a measurement. The $pi^{0} rightarrow gamma gamma$ decay rate is a test of QCD at the confinement scale. There is a firm QCD prediction with a theoretical uncertainty of $simeq $ 1 % which calls for an experimental test at the same level of accuracy. There are three methods that have been utilized to perform this measurement and the present status of the experimental tests are outlined. The current accuracy is significantly less than the theoretical uncertainty. The efforts to improve this are briefly summarized.
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).
The total cross section of the pp -> pp eta reaction has been measured at two energies near threshold by detecting the final protons in a magnetic spectrometer. The values obtained are about a factor of 70 less than for the corresponding eta production, in good agreement with the predictions of a one-pion-exchange model.
This paper presents results from partial-wave analyses of the photoproduction reactions $gamma p rightarrow eta p$ and $gamma n rightarrow eta n$. World data for the observables DSG, $Sigma$, $T$, $P$, $F$, and $E$ were analyzed as part of this work. The dominant amplitude in the fitting range from threshold to a c.m. energy of 1900 MeV was found to be $S_{11}$ in both reactions, consistent with results of other groups. At c.m. energies above 1600 MeV, our solution deviates from published results, with this work finding higher-order partial waves becoming significant. Data off the proton suggest that the higher-order terms contributing to the reaction include $P_{11}$, $P_{13}$, and $F_{15}$. The final results also hint that $F_{17}$ is needed to fit double-polarization observables above 1900 MeV. Data off the neutron show a contribution from $P_{13}$, as well as strong contributions from $D_{13}$ and $D_{15}$.
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.
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