Bochum 2009 – scientific programme

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HK: Fachverband Hadronen und Kerne

HK 4: Hadron Structure and Spectroscopy I

HK 4.6: Talk

Monday, March 16, 2009, 15:45–16:00, H-ZO 20

From the ABC Effect to the ABC-Resonance – the First Genuine Dibaryon State?^* — •Mikhail Bashkanov — Physikalisches Institut der Universität Tübingen

The ABC effect – an intriguing low-mass enhancement in the ππ invariant mass spectrum – is known from inclusive measurements of two-pion production in nuclear fusion reactions. Its explanation has been a long-standing problem since 50 years.

New exclusive and kinematically complete measurements of the most basic fusion reaction pn → d π⁰π⁰ have been carried out with WASA at COSY covering the full energy region, where the ABC effect can be observed. These measurements with a two orders of magnitude higher statistics than the previous CELSIUS-WASA measurements reveal the ABC effect to be the consequence of a narrow resonance in the pn and d π⁰π⁰ systems with a mass 90 MeV below the ΔΔ mass and a width of 50 MeV. The latter is 5 times smaller than what is expected from a conventional t-channel ΔΔ excitation. According to the angular distributions the quantum numbers should be I(J^P) = 0(1⁺) or 0(3⁺). Such a dibaryon resonance has been predicted by various theoretical calculations, some of which even predict this resonance to be a member of a dibaryon multiplet.

From the fact that the ABC effect is observed also for double-pionic fusion processes to heavier nuclei, we conclude that this resonance is robust enough to survive even in nuclei. ^* supported by BMBF, COSY-FFE, DFG (Eur. Graduate School) and Wallenberg Foundation