Bonn 2010 – wissenschaftliches Programm
HK 61.1: Hauptvortrag
Freitag, 19. März 2010, 08:30–09:00, HG X
Two-Pion Production in Nucleon-Nucleon Collisions — ABC-Resonance* — •Mikhail Bashkanov — Physikalisches Institut, Univ. Tübingen
In a long-term research program the two-pion production in nucleon-nucleon collisions has been studied by exclusive experiments from threshold up to 1.4 GeV at CELSIUS and COSY. Aside from the threshold region the production is dominated by the mutual excitation of two Δ resonances, in general described by a t-channel ΔΔ process. However, in the case of double-pionic fusion, i. e., when the participating nucleons merge into a bound nuclear state, another process takes over. It is of isoscalar nature, proceeds via a reaction chain pn → ΔΔ → NNππ and produces a strong and intriguing low-mass enhancement in the ππ invariant mass spectrum – known as ABC puzzle since 50 years. New measurements of the most basic fusion reaction pn → d π0π0 at WASA reveal this ABC effect to be associated with a narrow resonance in pn and d π0π0 systems with a mass 90 MeV below the ΔΔ mass and a width of only 50 MeV. The latter is 5 times smaller than expected from a conventional t-channel ΔΔ excitation. From the angular distributions we tentatively assign the quantum numbers I(JP) = 0(3+) for this resonance. Such a resonance has in fact been predicted by various quark model calculations.
From the fact that the ABC effect is observed also for double-pionic fusion processes to heavier nuclei, we conclude that this resonance is obviously robust enough to survive even in the nuclear medium.
* supported by BMBF, COSY-FFE and DFG (Eur. Grad. School)