Transcript Two-photon Exchange John Arrington Argonne National Lab International Workshop on Positrons at Jefferson Lab, Mar 25-27, 2009

Two-photon Exchange
John Arrington
Argonne National Lab
International Workshop on Positrons at
Jefferson Lab, Mar 25-27, 2009
Unpolarized Elastic e-N Scattering
 Early form factor measurements used Rosenbluth separation
sR = ds/dW [e(1+t)/sMott] = tGM2 + eGE2 in Born approx. (t = Q2/4M2)
Reduced sensitivity to…
• GM if Q2 << 1
• GE if Q2 >> 1
• GE if
GE2
GE2<<GM2 (e.g. neutron)
Form factor extraction is very
sensitive to angle-dependent
corrections in these cases
tGM2
q=180o
q=0o
2
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
LT: tGM2+eGE2
PT: GE/GM
BLAST at MIT-Bates
Polarized 3He target
Focal plane polarimeter –
Jefferson Lab
3
Polarization vs. Rosenbluth: GE/GM
mpGEp/GMp from Rosenbluth
measurements
New data: Recoil polarization
and p(e,p) “Super-Rosenbluth”
JLab Hall A: M. Jones, et al.; O. Gayou, et al.
Slope from recoil
polarization
I. A. Qattan, et al, PRL 94, 142301 (2005)
4
Two-photon exchange corrections
Clear discrepancy between LT,
PT extractions
Two-photon exchange effects
can explain discrepancy in GE
Guichon and Vanderhaeghen,
PRL 91, 142303 (2003)
Requires ~3-6% e-dependence,
weekly dependent on Q2,
roughly linear in e
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 addressed
What about the constraints (~1%) from positron-electron comparisons?
TPE effects on GM?
TPE effects on polarization transfer?
TPE effects on other measurements?
5
Tests of Two-Photon Exchange (’50s and ’60s)
Secondary beams had
low luminosity; data
taken at high Q2 OR
large q, never both.
If correction is at large q
(small e), it would not
have been clearly seen
JA, PRC 69, 032201 (2004)
6
Aside: Rosenbluth separation for e+p
Small (3-5%) e-dependent TPE correction can yield large (>100%)
corrections to GE, since GE contributes so little to cross section
PT results
LT (electron)
LT (positron)
(GE/GM)2<1
Focus has been on how
TPE impacts GEp at high Q2
Biggest issue, but not the
only important one
7
Low-Q2 behavior: Unpolarized, Polarizated
 TPE effect does not approach zero as Q2  0
0.01-0.06 GeV2
0.1-0.6
1.0-6.0
0.1,0.2,0.3,0.6,1.0 GeV2
P. Blunden, W. Melnitchouk, and J. Tjon, PRC 72, 034612 (2005)
8
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
mpGEp/GMp from Rosenbluth
measurements
GMp from inclusive measurements – data
extend to 30 GeV2
New data: Recoil polarization and
p(e,p) “Super-Rosenbluth”
With TPE corrections (Blunden, et al.), GMp
shifts by up to 2-3 sigma
9
“Indirect” impact: Parity Measurements
 Neglect TPE in calculating APV  small
effect (top)
– Especially for small angles (large e)
where most data is taken
– Missing g-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 e
– 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)
10
Effect on Rosenbluth (L-T) Extractions
•
Hadronic calculation resolves the discrepancy up to 2-3 GeV2
P. Blunden, W. Melnitchouk, and J. Tjon, PRC 72, 034612 (2005)
•
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 e, smaller TPE
•
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 e = 1)
LT
PT
LT + BMT
PT
J. Arrington, W. Melnitchouk, and J. Tjon, PRC76, 035205 (2007)
11
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.
D.Dutta, et al., PRC 68, 064603 (2003)
JA, PRC 69, 022201(R) (2004)
•  - N scattering measurements
H.Budd, A.Bodek, and JA hep-ex/0308005
• Proton charge radius, hyperfine splitting
S.Brodsky, et al., PRL 94, 022001 (2005)
• Strangeness from parity violation
• Neutron, Nuclear form factors
P.Blunden and I.Sick, PRC 72, 057601 (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)
• Transition form factors
• Bethe-Heitler, Coulomb Distortion,…
S. Kondratyuk and P. Blunden, NPA778 (2006)
V. Pasculutsa, C. Carlson, M. Vanderhaeghen,
PRL96, 012301 (2006)
12
TPE measurements in e-p scattering
Short term (verify TPE, determine proton form factors)
Precise e-p elastic cross sections (JLab,Mainz)
- e dependence of cross section
Map out TPE for Q2 > 1-2 GeV2
Polarization transfer: Pl/Pt (Jlab)
- e dependence of polarization ratio
Positron-electron comparisons (VEPP, JLab, DESY)
- Clean extraction of two-photon terms
- Map out Q2 and e dependence of DsTPE
Can test TPE explanation
Map out TPE up to Q2 ~ 2 GeV
Longer term (test calculations for e-p, other reactions)
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 asymmetries
Measurements to constrain TPE in other reactions
- Elastic form factors for neutron or light nuclei
- Other exclusive processes (e.g. N  D form factors)
- Experimentally, very little can be done without positron beams
- Need well tested, well constrained calculations
13
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 e-dependence comes from TPE
14
Nonlinearity Tests
 Born approx  sR 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 dP2 ≈ ±0.020
for both linearity scans, with
global dP2 ≈ ±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)
15
E04-019: e dependence in polarization transfer
Experiment ran in early 2008
Preliminary results suggest
little or no e-dependence
16
Two-Photon Exchange Measurements
 Comparisons of e+-p, e--p scattering [VEPP-III, Hall B, DESY-Olympus proposal]
Previous comparisons limited to low Q2 or
small scattering angle (large e)
Examination of angular dependence yields
evidence (3s level) for TPE in existing data
J. Arrington, PRC 69, 032201(R) (2004)
World’s data
Novosibirsk
17
18
Test run at Novosibirsk
19
Two-Photon Exchange Measurements
 Comparisons of e+-p, e--p scattering [VEPP-III, Hall B, DESY-Olympus]
Previous comparisons limited to low Q2 or
small scattering angle (large e)
Examination of angular dependence yields
evidence (3s level) for TPE in existing data
J. Arrington, PRC 69, 032201(R) (2004)
World’s data
Novosibirsk
JLab – Hall B
 e dependence of polarization transfer and unpolarized se-p [Hall C]
20
Two-Photon Exchange Measurements
 Comparisons of e+-p, e--p scattering [VEPP-III, Hall B, DESY-Olympus]
Previous comparisons limited to low Q2 or
small scattering angle (large e)
Examination of angular dependence yields
evidence (3s level) for TPE in existing data
J. Arrington, PRC 69, 032201(R) (2004)
World’s data
Novosibirsk
JLab – Hall B
 e dependence of polarization transfer and unpolarized se-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)
21
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 e, 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. 1mA, would be a huge improvement
– Great coverage for elastic, many other reaction channels
– Higher current or polarization  TPE in polarization observables
22
23
Fin…
24
Radiative Corrections: Beyond the Born Approximation
Additional two-photon contributions expected to be small (~aEM)
Theoretical estimates generally indicated ~1% corrections
Linearity of Rosenbluth plot taken as evidence of small TPE
Comparison of positron to electron scattering was the “definitive” test
25
JLab E01-001: Test of Radiative Corrections
 RC terms largely e-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)
26
E05-017: Extended “Super-Rosenbluth”
 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 e dependence is TPE for Q2 > 5 GeV2
102 Kinematics
points
Q2 0.40-5.76 GeV2
13 points at Q2=0.983
10 points at Q2=2.284
27
Positron-electron comparison in CLAS@JLab
28
Positron-electron comparison in CLAS@JLab
1%
29
CLAS TPE Test Run
 Focus was on background issues
 Clearly identified e-p and e+p elastic events
ratio ~1
• Leptons in sector 5
only: 1/6 of data
• Red for negative
torus polarity, Black
for positive.
• Only crude CLAS
calibrations
lepton scattering lab angle
Q2<0.5 GeV2
0.4<e<0.95
30
OLYMPUS: BLAST@DORIS
31
The BLAST Detector
Left-right symmetric
BEAM
Large acceptance:
0.1 < Q2/(GeV/c)2 < 0.8
20o < q < 80o, -15o <  < 15o
COILS
Bmax = 3.8 kG
DRIFT CHAMBERS
Tracking, PID (charge)
dp/p=3%, dq = 0.5o
DRIFT CHAMBERS
TARGET
COILS
CERENKOV COUNTERS
e/p separation
SCINTILLATORS
Trigger, ToF, PID (p/p)
NEUTRON COUNTERS
Neutron tracking (ToF)
CERENKOV
COUNTERS
 All the advantages of VEPP-3 expt.
– Pure beam, well defined energy
 Q2, e coverage close to CLAS
 Coincidence measurement
– Could do (e,e’n), other reactions
BEAM
NEUTRON COUNTERS
SCINTILLATORS
32