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ISSN - 0217751X, International Journal of Modern Physics A, 2007, vol. 22, p. 617-620

Observation of the ABC effect in the first exclusive measurements of pn → dπ0π0

Khakimova O., Bashkanov M., Skorodko T., Bargholtz C., Bogoslowsky D., Calén H., Cappellaro F., Clement H., Ekström C., Fransson K., Geren L., Gustafsson L., Höistad B., Ivanov G., Jacewicz M., Jiganov E., Johansson T., Kaskulov M. M., Keleta S., Koch I., Kren F., Kullander S., Kupść A., Kuznetsov A., Lindberg K., Marciniewski P., Meier R., Morosov B., Oelert W., Pauly C., Petukhov Y., Povtoreyko A., Ruber R. J M Y, Schwick A., Scobel W., Shafigullin R., Shwartz B., Sopov V., Stepaniak J., Tegner P. E., Tchernyshev V., Thörngren-Engblom P., Tikhomirov V., Turowiecki A., Wagner G. J., Wolke M., Yamamoto A., Zabierowski J., Zartova I., Złomanćzuk J.

The reaction pn → dπ0π0 constitutes the most fundamental system for the study of the ABC effect - a puzzling threshold enhancement in the ππ invariant mass spectrum of the ππ system in double-pionic fusion to nuclear bound states. The first exclusive measurements of this reaction have now been carried out at CELSIUS-WASA. They exhibit a, striking low-mass enhancement in the ππ invariant mass spectrum, prove it to be a σ-channel phenomenon and falsify conventional theoretical models, which predict an associated high-mass enhancement not present in the new exclusive data. In the kinematically complete data sample we observe a ΔΔ excitation, where the interaction between the two Δ obviously plays a crucial role for the generation of the ππ low-mass enhancement. © World Scientific Publishing Company.