Observation of in-medium modifications of the ω meson -: art. no. 192303

The photoproduction of omega mesons on nuclei has been investigated using the Crystal Barrel/TAPS experiment at the ELSA tagged photon facility in Bonn. The aim is to study possible in-medium modifications of the omega meson via the reaction gamma+A ->omega+X ->pi(0)gamma+X-'. Results obtained for Nb are compared to a reference measurement on a LH2 target. While for recoiling, long-lived mesons (pi(0), eta, and eta(')), which decay outside of the nucleus, a difference in the line shape for the two data samples is not observed, we find a significant enhancement towards lower masses for omega mesons produced on the Nb target. For momenta less than 500 MeV/c an in-medium omega meson mass of M-medium=[722(-4)(+4)(stat)(-5)(+35)(syst)] MeV/c(2) has been deduced at an estimated average nuclear density of 0.6 rho(0).