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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 pnd 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(JP) = 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

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