desy prc may 10, 20071 beyond the one photon approximation in lepton scattering: a definitive...
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DESY PRC May 10, 2007 1
Beyond the One Photon Approximation in Lepton
Scattering: A Definitive Experiment at
DESY
for
J. Arrington (Argonne) D. Hasell, M. Kohl, R. Milner (MIT)
DESY PRC May 10, 2007 2
Elastic Electron Scattering from Proton
+ +…
Dirac, Pauli FF
Sachs FF
DESY PRC May 10, 2007 3
Unpolarized Elastic e-N Scattering
For ~ 50 years unpolarized cross section measurements have determined the elastic FF Gp
E and GpM using the
Rosenbluth separation
σred = dσ/dΩ [ε(1+)/σMott] = GM2 + εGE
2
= Q2/4M2 ε = [ 1 + 2(1+)tan2 θ/2 ] -1
DESY PRC May 10, 2007 4
• 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
DESY PRC May 10, 2007 5
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 Q2 ~ 2-3 (GeV/c)2
J. Arrington PRC 69, 032201(R) (2004)
SLAC data
Θ=180o Θ=0o
At low ε : <Q2> ~ 0.01 to 0.8 (GeV/c)2
At high ε : <Q2> ~ 1-5 (GeV/c)2
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e+p/e-p Cross Section Ratio
P.G. Blunden et al.,Phys. Rev. C 72, 034612 (2005)
DESY PRC May 10, 2007 7
Proton form factor ratio
Blunden et al.
DESY PRC May 10, 2007 8
The Definitive Experiment• Both electron and positron beams available
• Incident on unpolarized hydrogen target at luminosity of
~ 5 x 1032 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 Q2 ~ 2-3 (GeV/c)2 i.e. scattering angles from ~ 20o to ~ 70o
• Experiment requires switching from e+ beam to e- beam on timescale of order 1 week.
• Measure ratio of positron-proton to electron-protonunpolarized elastic scattering to 1% stat.+sys.
• Control of systematic uncertainties essential.
DESY PRC May 10, 2007 9
energy 2.3 to 4.5 GeV e+ and e- beams regular switching between e+ and e-
lumi ~ 6 x 1032 cm-2 s-1 with internal H2 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
DESY PRC May 10, 2007 10
The detector and target exist
DESY PRC May 10, 2007 11
1000 hours each for e+
and e- at luminosity =
6 x 10 32 cm-2 s-
1
Projected results for DORIS experiment
1000 hours eachfor e+ and e-
Lumi = 6 x 1032 cm-2 s-1
DESY PRC May 10, 2007 12
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.
• NovosibirskSimilar 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 GE
p/GMp.
DESY PRC May 10, 2007 13
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
(~ 600) 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.