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HK: Hadronen und Kerne

HK 48: Electromagnetic Probes VII, Electro-, Photoproduction

HK 48.4: Gruppenbericht

Mittwoch, 18. März 1998, 15:30–16:00, B

Pion and Eta Photoproduction on the Nucleon — •L. Tiator, D. Drechsel, O. Hanstein, and G. Kn"ochlein — Institut f"ur Kernphysik, Universit"at Mainz, Becherweg 45, D-55099 Mainz

A new partial wave analysis of pion photoproduction has been obtained in the framework of fixed-t dispersion relations valid form threshold up to 500 MeV. It is based on new Mainz data for π^∘ and π⁺ production off the proton and both older and more recent data from Bonn, Frascati and TRIUMF for π⁺ and π⁻. At threshold we obtain a good agreement with the existing data for both charged and neutral pion production. In the resonance region we have precisely determined the electromagnetic properties of the Δ(1232) resonance, in particular the E2/M1 ratio R_EM=−2.5 ± 0.1 %. We show that a model independent resonance background subtraction is possible with the speed-plot technique and obtain the Δ pole at W =(1211 − 50i) MeV and the E2/M1 ratio of the residues as R_Δ = − 0.035 − 0.046 i.[1]

Furthermore we have analysed new polarization data on eta photoproduction on the proton recently measured in Bonn and at GRAAL in Grenoble. In a combined analysis with the differential cross section measurement at MAMI in Mainz it is possible to extract the s- and d-wave multipoles and determine the properties of the D₁₃(1520) resonance.[2]

[1] O. Hanstein, D. Drechsel and L. Tiator, Phys. Lett. B 385 (1996) 45; Phys. Lett. B 399 (1997) 13 and nucl-th/9709067 (submitted to Nucl. Phys. A)

[2] L. Tiator, Proc. of the GW/TJNAF Workshop on N^* Physics, to be published in π N Newsletters 1998.