Experimental constraints on the $omega$-nucleus real potential

In a search for $omega$ mesic states, the production of $omega$-mesons in coincidence with forward going protons has been studied in photon induced reactions on $^{12}$C for incident photon energies of 1250 - 3100 MeV. The $pi^0 gamma$ pairs from decays of bound or quasi-free $omega$-mesons have been measured with the CBELSA/TAPS detector system in coincidence with protons registered in the MiniTAPS forward array. Structures in the total energy distribution of the $pi^0 gamma$ pairs, which would indicate the population and decay of bound $omega~^{11}$B states, are not observed. The $pi^0 gamma$ cross section of 0.3 nb/MeV/sr observed in the bound state energy regime between -100 and 0 MeV may be accounted for by yield leaking into the bound state regime because of the large in-medium width of the $omega$-meson. A comparison of the measured total energy distribution with calculations suggests the real part $V_0$ of the $omega~^{11}$B potential to be small and only weakly attractive with $V_0(rho=rho_0) = -15pm$ 35(stat) $pm$20(syst) MeV in contrast to some theoretical predictions of attractive potentials with a depth of 100 - 150 MeV.

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.}