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Transcript of Two-photon Exchange John Arrington Argonne National Lab International Workshop on Positrons at...
Two-photon Exchange
John Arrington
Argonne National Lab
International Workshop on Positrons at Jefferson Lab, Mar 25-27, 2009
2
Unpolarized Elastic e-N Scattering
Early form factor measurements used Rosenbluth separation
R = d/d [(1+)/Mott] = GM2 + GE
2 in Born approx. ( = Q2/4M2)
GM2
GE2
=180o =0o
Reduced sensitivity to…
• GM if Q2 << 1
• GE if Q2 >> 1
• GE if GE2<<GM
2 (e.g. neutron)
Form factor extraction is very sensitive to angle-dependent corrections in these cases
3
New techniques: Polarization and A(e,e’N) Mid ’90s brought measurements using improved techniques
– High luminosity, highly polarized electron beams
– Polarized targets (1H, 2H, 3He) or recoil polarimeters
– Large, efficient neutron detectors for 2H, 3He(e,e’n)
– Improved models for nuclear corrections
Polarized 3He targetBLAST at MIT-Bates
Focal plane polarimeter – Jefferson Lab
LT:GM2+GE
2
PT:GE/GM
4
Polarization vs. Rosenbluth: GE/GM
pGEp/GMp from Rosenbluth measurements
I. A. Qattan, et al, PRL 94, 142301 (2005)
JLab Hall A: M. Jones, et al.; O. Gayou, et al.
New data: Recoil polarization and p(e,p) “Super-Rosenbluth”
Slope from recoil polarization
5
Two-photon exchange corrections
Clear discrepancy between LT, PT extractions
Two-photon exchange effects can explain discrepancy in GE
Requires ~3-6% -dependence, weekly dependent on Q2, roughly linear in
Guichon and Vanderhaeghen, PRL 91, 142303 (2003)
JA, PRC 69, 022201 (2004)
If this were the whole story, we would be done: L-T would give GM, PT gives GE
Still need to be careful when choosing form factors as, e.g. input to fits or data analysis
There are still issues to be addressedWhat about the constraints (~1%) from positron-electron comparisons?TPE effects on GM?TPE effects on polarization transfer?TPE effects on other measurements?
6
Tests of Two-Photon Exchange (’50s and ’60s)
Secondary beams had low luminosity; data taken at high Q2 OR large , never both.
If correction is at large (small ), it would not have been clearly seen
JA, PRC 69, 032201 (2004)
7
Aside: Rosenbluth separation for e+p
PT results LT (electron) LT (positron)
(GE/GM)2<1
Small (3-5%) -dependent TPE correction can yield large (>100%) corrections to GE, since GE contributes so little to cross section
Focus has been on how TPE impacts GEp at high Q2
Biggest issue, but not the only important one
8
Low-Q2 behavior: Unpolarized, Polarizated
0.01-0.06 GeV2
0.1-0.61.0-6.0
0.1,0.2,0.3,0.6,1.0 GeV2
TPE effect does not approach zero as Q2 0
P. Blunden, W. Melnitchouk, and J. Tjon, PRC 72, 034612 (2005)
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GMp from inclusive measurements – data extend to 30 GeV2
Impact on GMp
Proton form factor measurements from Rosenbluth separations
– TPE correction to GE is large, so are (most) LT uncertainties
– Correction to GM much smaller, but large compared to uncertainties
GMp from inclusive measurements – data extend to 30 GeV2
With TPE corrections (Blunden, et al.), GMp shifts by up to 2-3 sigma
pGEp/GMp from Rosenbluth measurementspGEp/GMp from Rosenbluth measurements
New data: Recoil polarization
pGEp/GMp from Rosenbluth measurements
New data: Recoil polarization and p(e,p) “Super-Rosenbluth”
10
“Indirect” impact: Parity Measurements Neglect TPE in calculating APV small
effect (top)
– Especially for small angles (large ) where most data is taken
– Missing -Z box, which is typically the largest correction, but is still small
Neglect TPE in extracting EM FFs much larger effect (bottom)
– Corrections largest at large – Note: form factor uncertainties typically
taken as <1%, but TPE corrections can be significantly larger (and correlated)
JA and Ingo Sick, PRC 76, 035201 (2007)
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Effect on Rosenbluth (L-T) Extractions
LTPT
LT + BMTPT
• Hadronic calculation resolves the discrepancy up to 2-3 GeV2
• Note: TPE effects of ~same size for cross section and polarizations
Effect on GE amplified in high-Q2 Rosenbluth measurements
Most polarization (and cross section) measurements at large , smaller TPE
P. Blunden, W. Melnitchouk, and J. Tjon, PRC 72, 034612 (2005)
• Note: Limited direct evidence for TPE, other RC issues to be addressed
Extraction of proton form factors not too sensitive to details, but does assume entire effect is TPE (e.g. no correction at = 1)
J. Arrington, W. Melnitchouk, and J. Tjon, PRC76, 035205 (2007)
12
TPE Beyond the Elastic Cross Section
TPE Calculations sufficient for extracting proton form factor
– Additional uncertainty at high Q2
Precise experimental tests of TPE calculations possible for the proton
– Important for validating calculations used for other reactions
– Hadronic, partonic calculations yield different sign for recoil polarization Important direct and indirect consequences on other experiments
• High-precision quasi-elastic expts.
• - N scattering measurements
• Proton charge radius, hyperfine splitting
• Strangeness from parity violation
• Neutron, Nuclear form factors
• Transition form factors
• Bethe-Heitler, Coulomb Distortion,…
D.Dutta, et al., PRC 68, 064603 (2003)
JA, PRC 69, 022201(R) (2004)
H.Budd, A.Bodek, and JA hep-ex/0308005
P.Blunden and I.Sick, PRC 72, 057601 (2005)
S.Brodsky, et al., PRL 94, 022001 (2005)
A.Afanasev and C.Carlson, PRL 94, 212301 (2005)
JA and I.Sick, nucl-th/0612079
P.Blunden, W.Melnitchouk, and J.Tjon, PRC72, 034612 (2005)
A.Afanasev, et al., PRD 72, 013008 (2005)
S. Kondratyuk and P. Blunden, NPA778 (2006)
V. Pasculutsa, C. Carlson, M. Vanderhaeghen, PRL96, 012301 (2006)
13
TPE measurements in e-p scattering
Precise e-p elastic cross sections (JLab,Mainz) - dependence of cross section
Polarization transfer: Pl/Pt (Jlab) - dependence of polarization ratio
Positron-electron comparisons (VEPP, JLab, DESY) - Clean extraction of two-photon terms - Map out Q2 and dependence of TPE
Can test TPE explanationMap out TPE up to Q2 ~ 2 GeV
Map out TPE for Q2 > 1-2 GeV2
Longer term (test calculations for e-p, other reactions)
Short term (verify TPE, determine proton form factors)
Born-forbidden observables in p(e,e’p) – imaginary part of TPE amplitude - Beam single-spin asymmetries (SAMPLE, A4, G0, HAPPEX) - Normal polarization transfer, normal target spin asymmetriesMeasurements to constrain TPE in other reactions - Elastic form factors for neutron or light nuclei - Other exclusive processes (e.g. N form factors)
- Experimentally, very little can be done without positron beams - Need well tested, well constrained calculations
14
Benefits of improved LT separations
Compare precise LT and PT to constrain linear part of TPE corrections
– Limiting factor in constraining TPE from PT-LT difference is precision of LT data
At high Q2, almost all -dependence comes from TPE
15
Nonlinearity Tests
Born approx R linear in ε, TPE can have nonlinearity
SLAC NE11, JLab E01-001: quadratic terms consistent with zero
Global fit, averaged over all Q2 yields P2 = 0.019±0.027
E05-007: Project P2 ≈ ±0.020 for both linearity scans, with global P2
≈ ±0.011
Set meaningful limits over a wide Q2 range
NE11: L. Andivahis, et al, PRD 50, 5491 (1994)
E01-001: I. A. Qattan, et al, PRL 94, 142301 (2005)
Global linearity limits: V.Tvaskis, et al., PRC 73, 025206 (2006)
16
E04-019: dependence in polarization transfer
Experiment ran in early 2008 Preliminary results suggest Preliminary results suggest
little or no little or no -dependence-dependence
17
Two-Photon Exchange Measurements
Comparisons of e+-p, e--p scattering [VEPP-III, Hall B, DESY-Olympus proposal]
World’s dataNovosibirsk
Previous comparisons limited to low Q2 or small scattering angle (large )
Examination of angular dependence yields evidence (3 level) for TPE in existing data
J. Arrington, PRC 69, 032201(R) (2004)
18
19
Test run at Novosibirsk
20
Two-Photon Exchange Measurements
Comparisons of e+-p, e--p scattering [VEPP-III, Hall B, DESY-Olympus]
dependence of polarization transfer and unpolarized e-p [Hall C]
World’s dataNovosibirskJLab – Hall B
Previous comparisons limited to low Q2 or small scattering angle (large )
Examination of angular dependence yields evidence (3 level) for TPE in existing data
J. Arrington, PRC 69, 032201(R) (2004)
21
Two-Photon Exchange Measurements
Comparisons of e+-p, e--p scattering [VEPP-III, Hall B, DESY-Olympus]
dependence of polarization transfer and unpolarized e-p [Hall C]
– More quantitative measure of the discrepancy
– Test against models of TPE at both low and high Q2
TPE effects in Born-forbidden observables [Hall A, Hall C, Mainz]
– Target single spin asymmetry, Ay in e-n scattering
– Induced polarization, py, in e-p scattering
– Vector analyzing power, AN, in e-p scattering (beam normal spin asymmetry)
World’s dataNovosibirskJLab – Hall B
Previous comparisons limited to low Q2 or small scattering angle (large )
Examination of angular dependence yields evidence (3 level) for TPE in existing data
J. Arrington, PRC 69, 032201(R) (2004)
22
Why do we need more?
The proposed e+/e- experiments are very limited (but very important)
– Verify TPE as source of discrepancy
– First quantitative measure of TPE effect on cross section
– Begin to test , Q2 dependence of calculations
No plans to study polarization observables Nothing proposed to look at other reactions
– Very limited tests might be possible with CLAS or Olympus
To thoroughly test calculations, need other measurements:
– Polarization, Born-forbidden observables
– Range of positron/electron comparisons (polarization, other reactions…)
A “real” positron beam, e.g. 1A, would be a huge improvement
– Great coverage for elastic, many other reaction channels
– Higher current or polarization TPE in polarization observables
23
24
Fin…
25
Radiative Corrections: Beyond the Born Approximation
Additional two-photon contributions expected to be small (~EM)Theoretical estimates generally indicated ~1% correctionsLinearity of Rosenbluth plot taken as evidence of small TPEComparison of positron to electron scattering was the “definitive” test
26
JLab E01-001: Test of Radiative Corrections RC terms largely -independent except for electron brem.
Form factor ratios for Q2 of 2.5-2.64 GeV2, before and after RC - Andivahis, Qattan, and Walker (solid = after RC)
27
E05-017: Extended “Super-Rosenbluth”
102 Kinematics points
Q2 0.40-5.76 GeV2
13 points at Q2=0.983
10 points at Q2=2.284
Ran summer 2007 in Hall C at Jefferson Lab Extremely high precision LT separations over large kinematic range
– Improved measurement of TPE effects over large Q2 range
– Very precise linearity tests at Q2= 0.983, 2.284 GeV2
– Nearly all dependence is TPE for Q2 > 5 GeV2
28
Positron-electron comparison in CLAS@JLab
29
Positron-electron comparison in CLAS@JLab
1%
30
CLAS TPE Test Run
Focus was on background issues Clearly identified e-p and e+p elastic events
rati
o of
yie
ld e
- p/e
+p
lepton scattering lab angle
• Leptons in sector 5 only: 1/6 of data• Red for negative torus polarity, Black for positive.
• Only crude CLAS calibrations
Q2<0.5 GeV2
0.4<<0.95
ratio ~1
31
OLYMPUS: BLAST@DORIS
32
The BLAST Detector
Left-right symmetric
Large acceptance:0.1 < Q2/(GeV/c)2 < 0.820o < < 80o, -15o < < 15o
COILS Bmax = 3.8 kG
DRIFT CHAMBERSTracking, PID (charge)p/p=3%, = 0.5o
CERENKOV COUNTERSe/ separation
SCINTILLATORSTrigger, ToF, PID (/p)
NEUTRON COUNTERSNeutron tracking (ToF)
DRIFT CHAMBERS
CERENKOVCOUNTERS
SCINTILLATORS
NEUTRON COUNTERS
TARGET
BEAM
BEAM
COILS
All the advantages of VEPP-3 expt.– Pure beam, well defined energy
Q2, coverage close to CLAS
Coincidence measurement– Could do (e,e’n), other reactions