Measurement of the vector analyzing power in elastic electron-proton scattering as a probe of the double virtual Compton amplitude

Creators: Wells, S. P.; Carr, R.; Covrig, S.; Eppstein, C.; Filippone, B. W.; Frasier, P.; Ito, T. M.; Jones, C.; Lee, P.; Maneva, E.; McCarty, K.; McKeown, R. D.; Savu, V.; Sullivan, M.

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Abstract

We report the first measurement of the vector analyzing power in inclusive transversely polarized elastic electron-proton scattering at Q^2=0.1(GeV/c)^2 and large scattering angles. This quantity must vanish in the single virtual photon exchange, plane-wave impulse approximation for this reaction, and can therefore provide information on two photon exchange amplitudes for electromagnetic interactions with hadronic systems. The observable we have measured is driven by the imaginary part of the two photon exchange amplitude, the hadronic side of which is simply the Compton amplitude for the proton with two virtual photons. We find a small but nonzero value of A=-15.4±5.4 ppm.

Additional Information

© 2001 The American Physical Society. Received 22 February 2000; published 1 May 2001. The skillful efforts of the staff at MIT/Bates to provide the high-quality beam required for these measurements, and useful conversations with T.W. Donnelly, A. Afanasev, and M. Rekalo, are gratefully acknowledged. We also thank A. Afanasev for providing us with the calculations. This work was supported by NSF Grant Nos. PHY-9870278 (Louisiana Tech), PHY-9420470 (Caltech), PHY-9420787 (Illinois),PHY-9457906/PHY-9229690 (Maryland), PHY-9733773 (VPI), and U.S. DOE Cooperative Agreement No. DE-FC02-94ER40818 (MIT/Bates) and Contract No. W-31-109-ENG-38 (ANL).

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