2 Meson production on complex nuclei by π − with the Crystal Ball detector ∗

We report on preliminary results for the production of neutral mesons by 750 MeV/c π− on complex nuclear targets (C, Al, Cu) and hydrogen. Our simultaneous measurement of π, 2π, and η final states allows an investigation of the nuclear final-state-interaction effects. The data show that nuclear absorption is mainly responsible for an observed change in the shape of the 2π invariant mass spectra. Our preliminary result for the σ → γγ branching ratio is BR(σ → γγ) < 3.6× 10 × φ at 90% C.L. on a carbon target, where φ is the fraction of 2π produced via the σ intermediate state. Contribution to the International Workshop on Chiral Fluctuations in Hadronic Matter, Orsay, France, September 2001. [email protected] Supported in part by US DOE, NSF, NSERC, RMS and VS. The Crystal Ball Collaboration: M. Clajus, A. Marus̆ić, S. McDonald, B. M. K. Nefkens, N. Phaisangittisakul, S. Prakhov, J. W. Price, A. Starostin, and W. B. Tippens, UCLA, D. Isenhower and M. Sadler, ACU, C. Allgower and H. Spinka, ANL, J. Comfort, K. Craig, and T. Ramirez, ASU, T. Kycia, BNL, J. Peterson, UCo, W. Briscoe and A. Shafi, GWU, H. M. Staudenmaier, UKa, D. M. Manley and J. Olmsted, KSU, D. Peaslee, UMd, V. Bekrenev, A. Koulbardis, N. Kozlenko, S. Kruglov, and I. Lopatin, PNPI, G. M. Huber, G. J. Lolos, and Z. Papandreou, UReg, I. Slaus and I. Supek, RBI, D. Grosnick, D. Koetke, R. Manweiler, and S. Stanislaus, ValU.