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Two-photon Exchange John Arrington Argonne National Lab

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

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Two-photon Exchange John Arrington Argonne National Lab

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  1. Two-photon Exchange John ArringtonArgonne 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 sR = ds/dW [e(1+t)/sMott] = tGM2+ eGE2in Born approx. (t = Q2/4M2) • Reduced sensitivity to… • GM if Q2 << 1 • GE if Q2 >> 1 • GE if GE2<<GM2(e.g. neutron) • Form factor extraction is very sensitive to angle-dependent corrections in these cases GE2 tGM2 q=180o q=0o

  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 LT:tGM2+eGE2 PT:GE/GM BLAST at MIT-Bates Polarized 3He target Focal plane polarimeter – Jefferson Lab

  4. Polarization vs. Rosenbluth: GE/GM mpGEp/GMp from Rosenbluth measurements New data: Recoil polarization and p(e,p) “Super-Rosenbluth” Slope from recoil polarization JLab Hall A: M. Jones, et al.; O. Gayou, et al. I. A. Qattan, et al, PRL 94, 142301 (2005)

  5. Two-photon exchange corrections Clear discrepancy between LT, PT extractions Two-photon exchange effects can explain discrepancy in GE Requires ~3-6% e-dependence, weekly dependent on Q2, roughly linear in e 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 addressed • What 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 Q2OR large q, never both. If correction is at large q (small e), it would not have been clearly seen JA, PRC 69, 032201 (2004)

  7. 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 resultsLT (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

  8. 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)

  9. 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 GMp from inclusive measurements – data extend to 30 GeV2 With TPE corrections (Blunden, et al.), GMp shifts by up to 2-3 sigma mpGEp/GMp from Rosenbluth measurements mpGEp/GMp from Rosenbluth measurements New data: Recoil polarization mpGEp/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 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)

  11. Effect on Rosenbluth (L-T) Extractions • Hadronic calculation resolves the discrepancy up to 2-3 GeV2 • Note: TPE effects of ~same size for cross section and polarizations • Effect on GEamplified in high-Q2 Rosenbluth measurements • Most polarization (and cross section) measurements at large e, 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 e= 1) LT + BMT PT LT PT 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 Short term (verify TPE, determine proton form factors) Precise e-p elastic cross sections (JLab,Mainz) - e dependence of cross section Polarization transfer: Pl/Pt(Jlab) - e dependence of polarization ratio Map out TPE for Q2 > 1-2 GeV2 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

  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 e-dependence comes from TPE

  15. Nonlinearity Tests • Born approx  sRlinear 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)

  16. E04-019: e dependence in polarization transfer Experiment ran in early 2008Preliminary results suggest little or no e-dependence

  17. 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

  18. Test run at Novosibirsk

  19. Two-Photon Exchange Measurements • Comparisons of e+-p, e--p scattering [VEPP-III, Hall B, DESY-Olympus] • e dependence of polarization transfer and unpolarized se-p [Hall C] 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

  20. Two-Photon Exchange Measurements • Comparisons of e+-p, e--p scattering [VEPP-III, Hall B, DESY-Olympus] • 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, Ayin e-n scattering • Induced polarization, py,in e-p scattering • Vector analyzing power, AN, in e-p scattering (beam normal spin asymmetry) 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

  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. Fin…

  23. 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

  24. 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, andWalker (solid = after RC)

  25. 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 alle 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

  26. Positron-electron comparison in CLAS@JLab

  27. 1% Positron-electron comparison in CLAS@JLab

  28. ratio ~1 CLAS TPE Test Run • Focus was on background issues • Clearly identified e-p and e+p elastic events • Leptons in sector 5 only: 1/6 of data • Red for negative torus polarity, Black for positive. • Only crude CLAS calibrations ratio of yield e-p/e+p lepton scattering lab angle Q2<0.5 GeV2 0.4<<0.95

  29. OLYMPUS: BLAST@DORIS

  30. The BLAST Detector BEAM DRIFT CHAMBERS TARGET COILS CERENKOV COUNTERS BEAM NEUTRON COUNTERS SCINTILLATORS • Left-right symmetric • Large acceptance:0.1 < Q2/(GeV/c)2 < 0.820o < q< 80o, -15o <  < 15o • COILS Bmax = 3.8 kG • DRIFT CHAMBERSTracking, PID (charge)dp/p=3%, dq = 0.5o • CERENKOV COUNTERSe/p separation • SCINTILLATORSTrigger, ToF, PID (p/p) • NEUTRONCOUNTERSNeutron tracking (ToF) • 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

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