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ISSN - 0217751X, International Journal of Modern Physics A, 2007, vol. 22, p. 625-628

Low-mass ππ enhancement in baryonic ππ production: ABC effect revised by exclusive measurements

Bashkanov M., Khakimova O., 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 ABC effect - a puzzling threshold enhancement in the ππ invariant mass spectrum of double-pionic fusion to nuclear bound states - has been investigated since 40 years by inclusive measurements without providing a conclusive interpretation. First exclusive measurements to this topic have now been carried out at CELSIUS-WASA. They confirm a huge ππ threshold enhancement, prove it to be of scalar-isoscalar nature, i.e., a σ-channel phenomenon and reveal the theoretically predicted high-mass enhancement in inclusive spectra to be of πππ rather than of ππ nature - falsifying thus corresponding model predictions for the ABC effect. From the kinematically complete data samples we infer that the interaction between the two Δ particles created in this process is of crucial importance. © World Scientific Publishing Company.