DESY PRC May 10, 20071 Beyond the One Photon Approximation in Lepton Scattering: A Definitive...
-
date post
20-Dec-2015 -
Category
Documents
-
view
223 -
download
0
Transcript of DESY PRC May 10, 20071 Beyond the One Photon Approximation in Lepton Scattering: A Definitive...
![Page 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,](https://reader035.fdocuments.in/reader035/viewer/2022062320/56649d445503460f94a2066c/html5/thumbnails/1.jpg)
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)
![Page 2: DESY PRC May 10, 20071 Beyond the One Photon Approximation in Lepton Scattering: A Definitive Experiment at DESY for J. Arrington (Argonne) D. Hasell,](https://reader035.fdocuments.in/reader035/viewer/2022062320/56649d445503460f94a2066c/html5/thumbnails/2.jpg)
DESY PRC May 10, 2007 2
Elastic Electron Scattering from Proton
+ +…
Dirac, Pauli FF
Sachs FF
![Page 3: DESY PRC May 10, 20071 Beyond the One Photon Approximation in Lepton Scattering: A Definitive Experiment at DESY for J. Arrington (Argonne) D. Hasell,](https://reader035.fdocuments.in/reader035/viewer/2022062320/56649d445503460f94a2066c/html5/thumbnails/3.jpg)
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
![Page 4: DESY PRC May 10, 20071 Beyond the One Photon Approximation in Lepton Scattering: A Definitive Experiment at DESY for J. Arrington (Argonne) D. Hasell,](https://reader035.fdocuments.in/reader035/viewer/2022062320/56649d445503460f94a2066c/html5/thumbnails/4.jpg)
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
![Page 5: DESY PRC May 10, 20071 Beyond the One Photon Approximation in Lepton Scattering: A Definitive Experiment at DESY for J. Arrington (Argonne) D. Hasell,](https://reader035.fdocuments.in/reader035/viewer/2022062320/56649d445503460f94a2066c/html5/thumbnails/5.jpg)
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
![Page 6: DESY PRC May 10, 20071 Beyond the One Photon Approximation in Lepton Scattering: A Definitive Experiment at DESY for J. Arrington (Argonne) D. Hasell,](https://reader035.fdocuments.in/reader035/viewer/2022062320/56649d445503460f94a2066c/html5/thumbnails/6.jpg)
DESY PRC May 10, 2007 6
e+p/e-p Cross Section Ratio
P.G. Blunden et al.,Phys. Rev. C 72, 034612 (2005)
![Page 7: DESY PRC May 10, 20071 Beyond the One Photon Approximation in Lepton Scattering: A Definitive Experiment at DESY for J. Arrington (Argonne) D. Hasell,](https://reader035.fdocuments.in/reader035/viewer/2022062320/56649d445503460f94a2066c/html5/thumbnails/7.jpg)
DESY PRC May 10, 2007 7
Proton form factor ratio
Blunden et al.
![Page 8: DESY PRC May 10, 20071 Beyond the One Photon Approximation in Lepton Scattering: A Definitive Experiment at DESY for J. Arrington (Argonne) D. Hasell,](https://reader035.fdocuments.in/reader035/viewer/2022062320/56649d445503460f94a2066c/html5/thumbnails/8.jpg)
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.
![Page 9: DESY PRC May 10, 20071 Beyond the One Photon Approximation in Lepton Scattering: A Definitive Experiment at DESY for J. Arrington (Argonne) D. Hasell,](https://reader035.fdocuments.in/reader035/viewer/2022062320/56649d445503460f94a2066c/html5/thumbnails/9.jpg)
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
![Page 10: DESY PRC May 10, 20071 Beyond the One Photon Approximation in Lepton Scattering: A Definitive Experiment at DESY for J. Arrington (Argonne) D. Hasell,](https://reader035.fdocuments.in/reader035/viewer/2022062320/56649d445503460f94a2066c/html5/thumbnails/10.jpg)
DESY PRC May 10, 2007 10
The detector and target exist
![Page 11: DESY PRC May 10, 20071 Beyond the One Photon Approximation in Lepton Scattering: A Definitive Experiment at DESY for J. Arrington (Argonne) D. Hasell,](https://reader035.fdocuments.in/reader035/viewer/2022062320/56649d445503460f94a2066c/html5/thumbnails/11.jpg)
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
![Page 12: DESY PRC May 10, 20071 Beyond the One Photon Approximation in Lepton Scattering: A Definitive Experiment at DESY for J. Arrington (Argonne) D. Hasell,](https://reader035.fdocuments.in/reader035/viewer/2022062320/56649d445503460f94a2066c/html5/thumbnails/12.jpg)
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.
![Page 13: DESY PRC May 10, 20071 Beyond the One Photon Approximation in Lepton Scattering: A Definitive Experiment at DESY for J. Arrington (Argonne) D. Hasell,](https://reader035.fdocuments.in/reader035/viewer/2022062320/56649d445503460f94a2066c/html5/thumbnails/13.jpg)
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.