Two-Pion Production in Nucleon-Nucleon Collisions and the ABC-Effect - Approaching a Puzzleby exclusive and Kinematically complete Measurements
T. Skorodko, M. Bashkanov, C. Bargholtz, D. Bogoslawsky, H. Calen, F. Cappellaro, H. Clement, L. Demiroers, E. Doroshkevich, D. Duniec, C. Ekstrom, K. Fransson, L. Geren, L. Gustafsson. B. Hoistad, G. Ivanov, l M. Jacewicz, E. Jiganov, T. Johansson, M. Kaskulov, O. Khakimova, S. Keleta, I. Koch, F. Kren, S. Kullander, A. Kupsc, A. Kuznetsov, K. Lindberg, P. Marciniewski, B. Martemyanov, R. Meier, B. Morosov, W. Oelert, C. Pauly, H. Pettersson, Y. Petukhov, A. Povtorejko, A. Pricking, R. J. M. Y. Ruber, K. Schonning, W. Scobel, B. Shwartz, V. Sopov, J. Stepaniak, P. -E. Tegner, P. Thorngren-Engholm, V. Tikhomirov, A. Turowiecki, G. J. Wagner, M. Wolke, A. Yamamoto, J. Zabierowski, I. Zartova, J. Zlomanczuk
Abstract
The ABC effect - a puzzling low-mass enhancement in the ππ invariant mass spectrum - is known from inclusive measurements of two-pion production in nuclear collisions, where it always showed up, if the participating nucleons fused to a bound nuclear system in the final state. The first exclusive measurements on the ABC effect have been carried out very recently at CELSIUS-WASA for the fusion reactions leading to d, 3He and 4He nuclei in the final state. The data analyzed so far for the fusion processes to d and 3He reveal this effect to be a σ channel phenomenon associated with the formation of a strongly attractive ΔΔ system. The data for the strictly isospin-selective double-pionic fusion to 4He, where we expect the largest effect, are currently still analyzed. All inclusive data on this system are well described by our model, too. This case also constitutes the heaviest nuclear system, where exclusive measurements of double-pionic fusion can be carried out with present-day instruments. Surprisingly, the pp ppπ0π0 reaction in the ΔΔ region is observed to also show a ABC-like low-ππ mass enhancement, a phenomenon, which deserves special attention.
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