The two-photon-exchange (TPE) contribution in epβepΟ0 with
W=MΞβ and small Q2 is calculated and its corrections to the ratios
of electromagnetic transition form factors REMβ=E1+(3/2)β/M1+(3/2)β and RSMβ=S1+(3/2)β/M1+(3/2)β,
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 epβepΟ0 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<2 GeV2.
It gives small correction to REMβ while for RSMβ, the correction is
about -10\% at small Ο΅ and about 1% at large Ο΅ for
Q2β2.5 GeV2. The large correction from TPE at small Ο΅ must
be included in the analysis to get a reliable extraction of RSMβ.Comment: Talk given at Conference:C16-07-2