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ISSN - 00319007, EISSN - 10797114, Physical Review Letters, 2009, vol. 102, p. -

Double-pionic fusion of nuclear systems and the "aBC" effect: Approaching a puzzle by exclusive and kinematically complete measurements

Bashkanov M., Bargholtz C., Berłowski M., Bogoslawsky D., Calén H., Clement H., Demiroers L., Doroshkevich E., Duniec D., Ekström C., Fransson K., Geren L., Gustafsson L., Höistad B., Ivanov G., Jacewicz M., Jiganov E., Johansson T., Khakimova O., Keleta S., Koch I., Kren F., Kullander S., Kupść A., Lindberg K., Marciniewski P., Meier R., Morosov B., Pauly C., Pettersson H., Petukhov Y., Povtorejko A., Pricking A., Ruber R. J M Y, Schönning K., Scobel W., Shwartz B., Skorodko T., Sopov V., Stepaniak J., Tegner P. E., Thörngren-Engblom P., Tikhomirov V., Turowiecki A., Wagner G. J., Wolke M., Zabierowski J., Zartova I., Złomanczuk J.

The ABC effect-a puzzling low-mass enhancement in the ππ invariant mass spectrum, first observed by Abashian, Booth, and Crowe-is well known from inclusive measurements of two-pion production in nuclear fusion reactions. Here we report on the first exclusive and kinematically complete measurements of the most basic double-pionic fusion reaction pn→dπ0π0 at beam energies of 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 resonancelike 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. © 2009 The American Physical Society.