Developments in Nucleon Structure Experiments
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Developments in Nucleon Structure Experiments
Transcript of Developments in Nucleon Structure Experiments
Rutherford - atoms have nuclei
Stern - μp = (1+κp)(e/2M)
Hofstadter - nuclei are not point-like, rather high density core and sparse surrounding
Mott - light point-like fermion off point-like heavy fermion;
History
2
e(k’)
Manipulate Q and scattering angle to select which G
For constant Q, fit to epsilon: slope gives GE²/τ, intercept gives GM²
Strategy
7
Disadvantages
Rosenbluth Separation more vulnerable to higher order diagrams in cross-section.
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Proton GE
We see that for high Q², measurements have high errors and utter inconsistency
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Scattering, like Rosenbluth, but no longer scanning Q and epsilon. Measure polarizations of proton:
GM known better, so GE/GM determines GE
Polarization Transfer Another aspect: look at asymmetry
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+, - refer to electron beam helicity
For right proton polarization angles θ* and φ*, A approximately proportional to GE/GM
Very recent asymmetry example from BLAST
Example Polarimeter Proton polarimeters rely on secondary scattering, in which spin interactions induce known angular dependences in cross section. In this CEBAF polarimeter, wire chambers before and after track proton trajectory; carbon target provides scattering medium. Energy is not determined.
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A Complication We need polarization at target not at polarimeter. Spin precesses in HRS, so needs to apply spin- rotation matrix, which comes from Monte Carlo
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Neutron Difficulties There are no free neutrons in these experiments, so compromise is necessary. Two methods: • Use D– relatively insensitive to wavefunction FSI, MEC, IC effects • Use ³He– Dominated by S- state wavefunction, so neutron dominates spin. Bad for low Q²
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Left: Polarization and Rosenbluth Separation data Right: Polarization data. 17
GMP/μPGD GEP/GD
Neutron GE, GM
Left: D and ³He Asymmetry + D recoil Right: Polarization data
GMP/μPGDGEP/GD
Comparison • Both have trouble with free neutrons • Rosenbluth is easier and offer more statistics • Polarized experiments require one measurement per Q², which reduces systematic uncertainties • By studying ratio of GE/GM, can cancel dependence on systematics of scale: i.e., beam polarization and polarimeter properties • Uncertainty due to two-photon processes impact both cross section and polarization, but the polarization methods rely on ratios.
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Prevailing interpretation: neutron is positive core surrounded by negative pion cloud. Proton may virtual strange pair contribution
Expected to be similar
In the infinite momentum frame, F1→ Transverse distributions without model dependence, but, use GDP that are model dependent
Conclusion
• Rosenbluth older, easier and still useful • Polarization complimentary, better in many ways • Electric form factors harder to measure • Neutron properties of special interest • No details of quark momentum, energy, spin contribution; nothing on gluons • Future FF measurements: RHIC, CEBAF, • Future 2γ contribution measurements: JLAB, VEPP3, DESY
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References
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• R. Gilman,et al. CEBAF.1995 • Close, Donnachie, Shaw; 2007 • Predrisat, Punjabi, Vanderhaeghen; 2007 • Arrington, Roberts, Zanotti; May 2007 • Long Range Nuclear Plan; Dec 2007 • BLAST; July 2008 • Miller, Arrington; Sep 2008 • Ji, McKeown; unpublished • Hiren; two weeks ago
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Stern - μp = (1+κp)(e/2M)
Hofstadter - nuclei are not point-like, rather high density core and sparse surrounding
Mott - light point-like fermion off point-like heavy fermion;
History
2
e(k’)
Manipulate Q and scattering angle to select which G
For constant Q, fit to epsilon: slope gives GE²/τ, intercept gives GM²
Strategy
7
Disadvantages
Rosenbluth Separation more vulnerable to higher order diagrams in cross-section.
9
Proton GE
We see that for high Q², measurements have high errors and utter inconsistency
10
11
Scattering, like Rosenbluth, but no longer scanning Q and epsilon. Measure polarizations of proton:
GM known better, so GE/GM determines GE
Polarization Transfer Another aspect: look at asymmetry
12
+, - refer to electron beam helicity
For right proton polarization angles θ* and φ*, A approximately proportional to GE/GM
Very recent asymmetry example from BLAST
Example Polarimeter Proton polarimeters rely on secondary scattering, in which spin interactions induce known angular dependences in cross section. In this CEBAF polarimeter, wire chambers before and after track proton trajectory; carbon target provides scattering medium. Energy is not determined.
14
A Complication We need polarization at target not at polarimeter. Spin precesses in HRS, so needs to apply spin- rotation matrix, which comes from Monte Carlo
15
Neutron Difficulties There are no free neutrons in these experiments, so compromise is necessary. Two methods: • Use D– relatively insensitive to wavefunction FSI, MEC, IC effects • Use ³He– Dominated by S- state wavefunction, so neutron dominates spin. Bad for low Q²
16
Left: Polarization and Rosenbluth Separation data Right: Polarization data. 17
GMP/μPGD GEP/GD
Neutron GE, GM
Left: D and ³He Asymmetry + D recoil Right: Polarization data
GMP/μPGDGEP/GD
Comparison • Both have trouble with free neutrons • Rosenbluth is easier and offer more statistics • Polarized experiments require one measurement per Q², which reduces systematic uncertainties • By studying ratio of GE/GM, can cancel dependence on systematics of scale: i.e., beam polarization and polarimeter properties • Uncertainty due to two-photon processes impact both cross section and polarization, but the polarization methods rely on ratios.
19
20
Prevailing interpretation: neutron is positive core surrounded by negative pion cloud. Proton may virtual strange pair contribution
Expected to be similar
In the infinite momentum frame, F1→ Transverse distributions without model dependence, but, use GDP that are model dependent
Conclusion
• Rosenbluth older, easier and still useful • Polarization complimentary, better in many ways • Electric form factors harder to measure • Neutron properties of special interest • No details of quark momentum, energy, spin contribution; nothing on gluons • Future FF measurements: RHIC, CEBAF, • Future 2γ contribution measurements: JLAB, VEPP3, DESY
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References
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• R. Gilman,et al. CEBAF.1995 • Close, Donnachie, Shaw; 2007 • Predrisat, Punjabi, Vanderhaeghen; 2007 • Arrington, Roberts, Zanotti; May 2007 • Long Range Nuclear Plan; Dec 2007 • BLAST; July 2008 • Miller, Arrington; Sep 2008 • Ji, McKeown; unpublished • Hiren; two weeks ago
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