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Two-Photon-Exchange Effects and Ξ”(1232)\Delta(1232) Deformation

Abstract

The two-photon-exchange (TPE) contribution in epβ†’epΟ€0ep\rightarrow ep\pi ^0 with W=MΞ”W=M_{\Delta} and small Q2Q^2 is calculated and its corrections to the ratios of electromagnetic transition form factors REM=E1+(3/2)/M1+(3/2)R_{EM} = E_{1+}^{(3/2)}/M_{1+}^{(3/2)} and RSM=S1+(3/2)/M1+(3/2)R_{SM} = S_{1+}^{(3/2)}/M_{1+}^{(3/2)}, are analysed. A simple hadronic model is used to estimate the TPE amplitude. Two phenomenological models, MAID2007 and SAID, are used to approximate the full epβ†’epΟ€0ep\rightarrow ep\pi ^0 cross sections which contain both the TPE and the one-photon-exchange (OPE) contributions. The genuine the OPE amplitude is then extracted from an integral equation by iteration. We find that the TPE contribution is not sensitive to whether MAID or SAID is used as input in the region with Q2<2Q^2<2 GeV2^2. It gives small correction to REMR_{EM} while for RSMR_{SM}, the correction is about -10\% at small Ο΅\epsilon and about 1%1\% at large Ο΅\epsilon for Q2β‰ˆ2.5Q^2\approx2.5 GeV2^2. The large correction from TPE at small Ο΅\epsilon must be included in the analysis to get a reliable extraction of RSMR_{SM}.Comment: Talk given at Conference:C16-07-2

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