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

M. Bashkanov , C. Bargholtz , M. Berlowski , D. Bogoslawsky , H. Calen , H. Clement , L. Demiroers , E. Doroshkevich

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

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classification ⚛️ nucl-ex

keywords measurementsfusioncompletedeltaeffectexclusivekinematicallynuclear

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The ABC effect - a puzzling low-mass enhancement in the $\pi\pi$ 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 $pn \to d \pi^0\pi^0$ at 1.03 and 1.35 GeV. The measurements, which have been carried out at CELSIUS-WASA, reveal the ABC effect to be a $(\pi\pi)_{I=L=0}$ channel phenomenon associated with both a resonance-like energy dependence in the integral cross section and the formation of a $\Delta\Delta$ system in the intermediate state. A corresponding simple s-channel resonance ansatz provides a surprisingly good description of the data.

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

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