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HK: Fachverband Physik der Hadronen und Kerne
HK 35: Elektromagnetische und Hadronische Sonden
HK 35.5: Talk
Thursday, March 13, 2008, 17:45–18:00, 1C
Two-Pion Production in Proton-Proton Collisions with Polarized Beam - Roper versus single Δ excitation * — •Arthur Erhardt, Heinz Clement, Evgueny Doroshkevich, Katharina Ehrhardt, and Gerhard J. Wagner for the COSY-TOF collaboration — Physikalisches Institut der Universität Tübingen
The pp→ ppπ+π− reaction was measured with a polarized proton beam at Tp = 750 and 800 MeV using the short version of the COSY-TOF spectrometer. The implementation of a delayed-pulse technique for Quirl and Central Calorimeter provided positive π+ identification in addition to the standard particle identification. Differential cross sections as well as vector analyzing powers have been obtained. They are compared to previous data and theoretical calculations. In contrast to predictions we find large analyzing power values up to Ay = 0.3. At these measurements the dominating reaction mechanism is the excitation of the Roper resonance and its decay into the π+π− channel either directly or via Δ excitation. From invariant Mππ mass and ππ opening angle distributions we find the direct decay into the σ channel to be the dominating decay - at variance with the PDG values, but in favor of the monopole character of the Roper excitation. Since Roper excitation and decay do not result in significant analyzing powers, the reason for the measured large values may be found in small reaction amplitudes, where single Δ excitation is connected with s-wave pion rescattering - a process not sensed so far experimentally.
*supported by BMBF, COSY-FFE, DFG(Eur. Graduate School)