Double-Pionic Fusion of Nuclear Systems and the ABCEffect -- Aproaching a Puzzle by Exclusive and Kinematically Complete Measurements

Bashkanov, M.; Bargholtz, C.; Berlowski, M.; Bogoslawsky, D.; Calen, H.; Clement, H.; Demiroers, L.; Doroshkevich, E.; Duniec, D.; Ekstrom, C.; Fransson, K.; Geren, L.; Gustafsson, L.; Hoistad, B.; Ivanov, G.; Jacewicz, M.; Jiganov, E.; Johansson, T.; Khakimova, O.; Keleta, S.; Koch, I.; Kren, F.; Kullander, S.; Kupsc, A.; Lindberg, K.; Marciniewski, P.; Meier, R.; Morosov, B.; Pauly, C.; Pettersson, H.; Petukhov, Y.; Povtorejko, A.; Pricking, A.; Ruber, R. J. M. Y.; Schonning, K.; Scobel, W.; Shwartz, B.; Skorodko, T.; Sopov, V.; Stepaniak, J.; Tegner, P. -E.; Thorngren-Engblom, P.; Tikhomirov, V.; Turowiecki, A.; Wagner, G. J.; Wolke, M.; Zabierowski, J.; Zartova, I.; Zlomanczuk, J.

Double-Pionic Fusion of Nuclear Systems and the ABCEffect -- Aproaching a Puzzle by Exclusive and Kinematically Complete Measurements

Authors

Description

The ABC effect - a puzzling low-mass enhancement in the $ππ$ invariant mass spectrum - is well-known from inclusive measurements of two-pion production in nuclear fusion reactions. Here we report on first exclusive and kinematically complete measurements of the most basic double pionic fusion reaction $pn \to d π^0π^0$ at 1.03 and 1.35 GeV. The measurements, which have been carried out at CELSIUS-WASA, reveal the ABC effect to be a $(ππ)_{I=L=0}$ channel phenomenon associated with both a resonance-like energy dependence in the integral cross section and the formation of a $ΔΔ$ system in the intermediate state. A corresponding simple s-channel resonance ansatz provides a surprisingly good description of the data.