Determination of the {eta}-nucleus optical potential

The excitation function and momentum distribution of $eta^prime$ mesons have been measured in photon induced reactions on $^{12}{}$C in the energy range of 1250-2600 MeV. The experiment was performed with tagged photon beams from the ELSA electron accelerator using the Crystal Barrel and TAPS detectors. The data are compared to model calculations to extract information on the sign and magnitude of the real part of the $eta^prime$-nucleus potential. Within the model, the comparison indicates an attractive potential of -($37 pm 10(stat)pm10(syst)$) MeV depth at normal nuclear matter density. Since the modulus of this depth is larger than the modulus of the imaginary part of the $eta^prime$-nucleus potential of -($10pm2.5$) MeV, determined by transparency ratio measurements, a search for resolved $eta^prime$-bound states appears promising.

Transparency ratio in gamma A -> eta A and the in-medium eta width

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.

Photoproduction of $omega$ mesons on nuclei near the production threshold

The photoproduction of $omega$ mesons on LH_2, C and Nb has been studied for incident photon energies from 900 to 1300 MeV using the CB/TAPS detector at ELSA. The $omega$ line shape does not show any significant difference between the LH_2 and the Nb targets. The data are compared with results of transport calculations that predict a sensitivity of the $omega$ line shape to in-medium modifications near the production threshold on a free nucleon of $E_{gamma}^{lab}$ = 1109 MeV.}

In-medium $omega$ mass from the $gamma + Nb to pi^{0}gamma + X$ reaction

Data on the photoproduction of $omega$ mesons on nuclei have been re-analyzed in a search for in-medium modifications. The data were taken with the Crystal Barrel(CB)/TAPS detector system at the ELSA accelerator facility in Bonn. First results from the analysis of the data set were published by D. Trnka et al. in Phys. Rev. Lett 94 (2005) 192303 cite{david}, claiming a lowering of the $omega$ mass in the nuclear medium by 14$%$ at normal nuclear matter density. The extracted $omega$ line shape was found to be sensitive to the background subtraction. For this reason a re-analysis of the same data set has been initiated and a new method has been developed to reduce the background and to determine the shape and absolute magnitude of the background directly from the data. Details of the re-analysis and of the background determination are described. The $omega$ signal on the $Nb$ target, extracted in the re-analysis, does not show a deviation from the corresponding line shape on a $LH_2$ target, measured as reference. The earlier claim of an in-medium mass shift is thus not confirmed. The sensitivity of the $omega$ line shape to different in-medium modification scenarios is discussed.