Mainz 2012 – wissenschaftliches Programm

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HK: Fachverband Physik der Hadronen und Kerne

HK 55: Hadronenstruktur und -spektroskopie

HK 55.1: Gruppenbericht

Freitag, 23. März 2012, 11:00–11:30, RW 2

On the Observation of a Narrow Resonance Structure in pn-scattering and Two-Pion Production^* — •Mikhail Bashkanov — Physikalisches Institut, Univ. Tübingen

Whereas for the isovector two-pion production in NN-collisions a smooth energy dependence is observed, which is well described by t-channel meson exchange, the purely isoscalar reaction pn → dπ⁰π⁰ exhibits a pronounced narrow resonance structure at m = 2.37 GeV = 2m_Δ - 90 MeV, which is strictly correlated with the appearance of a large low-mass enhancement (ABC-Effect) in the π⁰π⁰-invariant mass spectrum. From the angular distributions we deduce I(J^P) = 0(3⁺) for the resonance structure [1].

If this structure originates from a genuine pn resonance, then it has to be sensed also in other two-pion production channels with isoscalar contributions as well as in pn scattering. Indeed, the pn total cross section exhibits a structure at the energy of interest, which is well described by inclusion of the hypothetical resonance.

Experimental results are shown for the two-pion channels d π⁰π⁰, d π⁺π⁻ and ppπ⁰π⁻. Scheduled measurements on quasifree p^→n-scattering and the p^→n → pnπ⁰π⁰ reaction are discussed.

The fact that the ABC effect is observed also for double-pionic fusion processes to heavier nuclei, suggests that this resonance structure is robust enough to survive even in nuclei.

^* supported by BMBF and COSY-FFE

[1] P. Adlarson et al., Phys. Rev. Lett. 106, 242302 (2011)