1. DESY PRC May 10, 20071 Beyond the One Photon Approximation in Lepton Scattering: A Definitive Experiment at DESY for J. Arrington (Argonne) D. Hasell, M. Kohl, R. Milner (MIT)

2. DESY PRC May 10, 20072 Elastic Electron Scattering from Proton ++… Dirac, Pauli FF Sachs FF

3. DESY PRC May 10, 20073 Unpolarized Elastic e-N Scattering For ~ 50 years unpolarized cross section measurements have determined the elastic FF G p E and G p M using the Rosenbluth separation σ red = dσ/dΩ [ε(1+  )/σ Mott ] =  G M 2 + εG E 2  = Q 2 /4M 2 ε = [ 1 + 2(1+  )tan 2 θ/2 ] -1

4. DESY PRC May 10, 20074 All Rosenbluth data from SLAC and Jlab in agreement. Dramatic discrepancy between Rosenbluth and recoil polarization technique Discrepancy explained as effect of two photon exchange (TPE) Major discrepancy requiring resolution Dramatic discrepancy! JLab recoil polarization transfer data

5. DESY PRC May 10, 20075 How do we verify that the TPE contribution interpretation is correct? Precision comparison of positron-proton and electron-proton elastic scattering over a sizable ε range at Q 2 ~ 2-3 (GeV/c) 2 J. Arrington PRC 69, 032201(R) (2004) SLAC data Θ=180 o Θ=0 o At low ε : ~ 0.01 to 0.8 (GeV/c) 2 At high ε : ~ 1-5 (GeV/c) 2

6. DESY PRC May 10, 20076 e + p/e - p Cross Section Ratio P.G. Blunden et al., Phys. Rev. C 72, 034612 (2005)

7. DESY PRC May 10, 20077 Proton form factor ratio Blunden et al.

8. DESY PRC May 10, 20078 The Definitive Experiment Both electron and positron beams available Incident on unpolarized hydrogen target at luminosity of ~ 5 x 10 32 cm -2 s -1 Beam energies of 2.3 to 4.5 GeV Detector to measure elastic scattering in ε range from 0.4 to 1 at Q 2 ~ 2-3 (GeV/c) 2 i.e. scattering angles from ~ 20 o to ~ 70 o Experiment requires switching from e + beam to e - beam on timescale of order 1 week. Measure ratio of positron-proton to electron-proton unpolarized elastic scattering to 1% stat.+sys. Control of systematic uncertainties essential.

9. DESY PRC May 10, 20079 energy 2.3 to 4.5 GeV e+ and e - beams regular switching between e+ and e - lumi ~ 6 x 10 32 cm -2 s -1 with internal H 2 gas target space for detector installation and commissioning in parallel with synch. rad. operation data taking in dedicated mode ~ 1 month/year over several years Internal target experiment in DORIS

10. DESY PRC May 10, 200710 The detector and target exist

11. DESY PRC May 10, 200711 1000 hours each for e+ and e- at luminosity = 6 x 10 32 cm -2 s -1 Projected results for DORIS experiment 1000 hours each for e+ and e - Lumi = 6 x 10 32 cm -2 s -1

12. DESY PRC May 10, 200712 Other experiments JLab No positron beams. Approved experiment to compare e + p to e - p elastic scattering using secondary beams and the CEBAF Large Acceptance Spectrometer. Challenging systematics. Novosibirsk Similar experiment to DESY experiment has been considered. Positron currents are about an order of magnitude lower. No momentum measurement. Parity violating electron scattering Experiments at JLab and Mainz which measure transverse spin asymmetries are sensitive to two photon effects but not directly to the contribution which enters in G E p /G M p.

13. DESY PRC May 10, 200713 Summary The distribution of the charge and magnetism in the proton is in question. The determination of the contribution of multiple photon exchange processes is essential to resolving the discrepancy, which may have relevance for other areas of hadron structure. A precision comparison (~ 1%) of elastic electron-proton and positron- proton elastic scattering at 2.3 to 4.5 GeV at large angles (~ 60 0 ) will be definitive to resolving this discrepancy. The experiment can be carried out at DESY/DORIS using the available MIT-BLAST detector and an unpolarized hydrogen gas target. Dedicated data taking for ~ 1 month per year for several years would be sufficient to carry out the experiment.