Modification of the $��$-Meson Lifetime in Nuclear Matter

The photo production of $��$ mesons on the nuclei C, Ca, Nb and Pb has been measured using the Crystal Barrel/TAPS detector at the ELSA tagged photon facility in Bonn. The dependence of the $��$ meson cross section on the nuclear mass number has been compared with three different types of models, a Glauber analysis, a BUU analysis of the Giessen theory group and a calculation by the Valencia theory group. In all three cases, the inelastic $��$ width is found to be $130-150 \rm{MeV/c^2}$ at normal nuclear matter density for an average 3-momentum of 1.1 GeV/c. In the restframe of the $��$ meson, this inelastic $��$ width corresponds to a reduction of the $��$ lifetime by a factor $\approx 30$. For the first time, the momentum dependent $��$N cross section has been extracted from the experiment and is in the range of 70 mb.
5 pages, 4 figures