Bochum 2009 – scientific programme
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HK: Fachverband Hadronen und Kerne
HK 74: Fundamental Symmetries
HK 74.3: Talk
Thursday, March 19, 2009, 17:15–17:30, H-ZO 70
Search for a two-photon exchange signal at HERMES — •Alejandro Lopez Ruiz — HERMES. DESY. Notkestrasse, 85. 22607 Hamburg. Germany.
Recent extractions of the elastic proton form factors have shown the importance of corrections beyond the one-photon exchange approximation or Born level in electromagnetic processes. A significative contribution that has been identified is the two-photon exchange process, and the need has been made clear to explore its role by measuring quantities sensitive to it. One such observable would be an asymmetry in the number of leptons scattered off a nucleon polarized normal to the scattering plane, which is forbidden at Born level, and therefore a non-zero measurement would give a clear indication of two-photon exchange. Such an asymmetry has been searched for in inclusive deep inelastic lepton- nucleon scattering (DIS) at HERMES, in the range 0.004 < x < 0.9, and with Q2 between 0.1 and 20 GeV2, using unpolarized electron and positron beams with an energy of 27.6 GeV and a transversely polarized proton target. Both in the DIS region and for Q2 < 1 GeV2, the asymmetries were found to be consistent with zero within the uncertainties, to the order of 10−3.