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SUMMARY:Two-photon exchange and elastic scattering of positrons/electrons 
 on the proton
DTSTART;VALUE=DATE-TIME:20100722T120000Z
DTEND;VALUE=DATE-TIME:20100722T121300Z
DTSTAMP;VALUE=DATE-TIME:20130525T110154Z
UID:indico-contribution-45@cern.ch
DESCRIPTION:Speakers: NIKOLENKO\, Dmitri (Budker Institute for Nuclear Phy
 sics)\nWe report the experiment on a precise comparison of (e+ p) and (e- 
 p) scattering cross sections\, which gives a direct experimental evidence 
 for the two-photon exchange (TPE) contribution in this reaction. Such data
  are in demand now\, because they\, most likely\, may explain the dramatic
  disagreement of proton electromagnetic form factors measurements in the p
 olarization transfer (PT) experiments at Jefferson Lab with previous unpol
 arized measurements using a Rosenbluth separation (RS) technique.\n  Commo
 n practice of the analysis of RS experiments was in approximate taking  in
 to account of TPE contribution\, believing this contribution to be small. 
 Currently there are no accurate calculations of the TPE contribution\, wha
 t is related to difficulties in accounting of the intermediate states of p
 roton. But recent calculations\, which takes particular account of proton 
 structure\, shows importance of TPE contribution for RS analysis. As far a
 s the experimental situation\, only old (in 1960th) measurements exist\, w
 here the TPE contribution were found with low precision and limited kinema
 tics coverage. \n  Reported experiment was performed recently at VEPP-3 st
 orage ring at the energy of positron/electron beams 1.6 GeV and three  reg
 ions of electron/positron scattering angle - around 10\, 18 and 64 degrees
 . The smallest angle region was used for  luminosity monitoring. Electron 
 and positron  beams were replaced each other regularly\, one cycle with tw
 o beams required 1.5 hours. Internal hydrogen gas target had a thickness 5
 *10^14 at/cm2. Experiment duration was 1500 hours with a mean luminosity o
 f 5*10^31 1/(s cm2). The preliminary results on (e+ p)/(e- p) cross sectio
 ns ratio will be presented.\n\nhttp://indico.cern.ch/contributionDisplay.p
 y?contribId=45&sessionId=46&confId=73513
LOCATION: Salle 251
URL:http://indico.cern.ch/contributionDisplay.py?contribId=45&sessionId=46
 &confId=73513
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