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Experimental Probes of Two-Photon Exchange *

3 rd Joint Meeting of the APS Division of Nuclear Physics and the Physical Society of Japan October 13-17, 2009, Waikoloa, Big Island, Hawaii, USA. Experimental Probes of Two-Photon Exchange *. Michael Kohl <kohlm@jlab.org>. Hampton University, VA 23668 and Jefferson Lab, VA 23606, USA.

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Experimental Probes of Two-Photon Exchange *

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  1. 3rd Joint Meeting of the APS Division of Nuclear Physics and the Physical Society of Japan October 13-17, 2009, Waikoloa, Big Island, Hawaii, USA Experimental Probes ofTwo-Photon Exchange* Michael Kohl <kohlm@jlab.org> Hampton University, VA 23668 and Jefferson Lab, VA 23606, USA * Supported by NSF grant PHY-0855473

  2. Outline • Form factors in the context of one-photon exchange (OPE) • The limit of OPE or: • What is GEp ? • What is the structure of lepton scattering? • Two-photon exchange (TPE): New observables • Current and future experiments to probe TPE

  3. (Hadronic) Structure and (EW) Interaction Factorization! Structure Interaction s(structured object) |Form factor|2 = s(pointlike object) → Interference! Probe Object →Utilize spin dependence of electromagnetic interaction to achieve high precision Born Approximation Inelastic Elastic Structure Hadronic object Electroweak probe Interaction Lepton scattering ~|α|2 (α=1/137)

  4. Form Factors in OPE General definition of the nucleon form factor Sachs Form Factors In one-photon exchange approximation above form factors are observables of elastic electron-nucleon scattering

  5. Form Factors from Rosenbluth Method In One-photon exchange approximation above form factors are observables of elastic electron-nucleon scattering  Determine |GE/GM| GE2 tGM2 θ=180o θ=0o

  6. Nucleon Form Factors and Polarization Double polarization in elastic ep scattering:Recoil polarization or (vector) polarized target Polarized cross section Double spin asymmetry = spin correlation Asymmetry ratio (“Super ratio”)independent of polarization or analyzing power 1H(e,e’p), 1H(e,e’p)

  7. Proton Form Factor Ratio Jefferson Lab 2000– • All Rosenbluth data from SLAC and Jlab in agreement • Dramatic discrepancy between Rosenbluth and recoil polarization technique • Multi-photon exchange considered best candidate Dramatic discrepancy! >800 citations

  8. Proton Form Factor Ratio Jefferson Lab 2000– • All Rosenbluth data from SLAC and Jlab in agreement • Dramatic discrepancy between Rosenbluth and recoil polarization technique • Multi-photon exchange considered best candidate Dramatic discrepancy! >800 citations

  9. Two-Photon Exchange: A Lot of Theory Two-photon exchange theoretically suggested Interference of one- and two-photon amplitudes • P.A.M. Guichon and M. Vanderhaeghen, PRL91 (2003) 142303; M.P. Rekalo and E. Tomasi-Gustafsson, EPJA22 (2004) 331:Formalism … TPE effect could be large • P.G. Blunden, W. Melnitchouk, and J.A. Tjon, PRC72 (2005) 034612, PRL91 (2003) 142304: Nucl. Theory … elastic ≈ half, Delta opposite • A.V. Afanasev and N.P. Merenkov, PRD70 (2004) 073002: Large logarithms in normal beam asymmetry • Y.C. Chen et al., PRL93 (2004) 122301: Partonic calculation (GPD), TPE large at high Q2 • A.V. Afanasev, S.J. Brodsky, C.E. Carlson, Y.C. Chen, M. Vanderhaeghen, PRD72 (2005) 013008: high Q2, small effect on asym., larger on x-sec., TPE on R small • M. Gorchtein, PLB644 (2007) 322: Fwd. angle, dispersion ansatz, TPE sizable • Y.C. Chen, C.W. Kao, S.N. Yang, PLB652 (2007) 269: Model-independent TPE large • D. Borisyuk, A. Kobushkin, PRC74 (2006) 065203; 78 (2008) 025208: TPE effect sizable • Yu. M. Bystritskiy, E.A. Kuraev, E. Tomasi-Gustafsson, PRC75 (2007) 015207:Importance of higher-order radiative effects, TPE effect rather small! • M. Kuhn, H. Weigel, EPJA38 (2008) 295: TPE in Skyrme Model • D.Y. Chen et al., PRC78 (2008) 045208: TPE for timelike form factors • M. Gorchtein, C.J. Horowitz, PRL102 (2009) 091806: gamma-Z box • D. Borisyuk, A. Kobushkin, PRD79 (2009) 034001: pQCD, sizable • N. Kivel, M. Vanderhaeghen, PRL103 (2009) 092004: pQCD, sizable

  10. Elastic ep Scattering Beyond OPE k’ p’ s=1/2 lepton s=1/2 proton Kinematical invariants : k p Next-to Born approximation: (me = 0) The T-matrix still factorizes, however a new response term F3 is generated by TPEBorn-amplitudes are modified in presence of TPE New amplitudes are complex!

  11. Observables involving real part of TPE E04-019(Two-gamma) e+/e- x-section ratioCLAS,VEPP3,OLYMPUS Rosenbluth non-linearityE05-017 Need positrons to identify Y2γ Born Approximation Beyond Born Approximation P.A.M. Guichon and M.Vanderhaeghen, Phys.Rev.Lett. 91, 142303 (2003) M.P. Rekalo and E. Tomasi-Gustafsson, E.P.J. A 22, 331 (2004) Slide idea: L. Pentchev

  12. Some remarks ~ • Presence of TPE modifies GE and GM, AND generates new structure F3 • Measurement of one type of observable (double polarization or Rosenbluth cross sections is insufficient to separately determine both GE/GMAND Y2γ. • Without positrons, it is possible to use double polarization observables AND Rosenbluth cross sections as functions of Q2 and ε to extract both GE/GM and Y2γ(Q2, ε) ASSUMING that TPE is the accepted picture. • Any change in the ε dependence of Pl or Pt/Pl is an indicator of non-zero Y2γ, however its absence is no disproof, as Y2γ can also be ε-independent. Small. • Any non-linear ε dependence of cross section is an indicator of non-zero Y2γ. Absence is no disproof, as Y2γ can also be ε-independent. Small effect. • RB plots ARE very linear in ε  Y2γ constant vs. ε ? • Pt/Pl constant vs. ε  (1–2εR/(1+ε)) Y2γ constant Y2γ = 0 ? • Positrons are needed to definitively establish TPE. The Y2γ terms change sign with the charge of the lepton, so the ONLY definitive test of the picture is to compare observables probed with e+ and e-

  13. E04-019 (Two-gamma) GE/GM from Pt/Pl constant vs. ε  (1–2εR/(1+ε)) Y2γ constant  with Y2γ = const.  Y2γ = 0? PRELIMINARY M. Meziane, BF-05 (Wed.)

  14. OLYMPUS pOsitron-proton and eLectron-proton elastic scattering to test the hYpothesis of Multi- Photon exchange Using DoriS OLYMPUS Hera ZEUS Hermes 2008 – Full proposal 2009 – Funding approval 2010/11 – Transfer of BLAST 2012 – OLYMPUS Running

  15. OLYMPUS: BLAST@DESY/DORIS 500 hours each for e+ and e- Lumi=2x1033 cm-2s-1

  16. e+/e- cross section ratio to verify TPE VEPP3 CLAS Experiment proposals to verify TPE hypothesis: e+/e- ratio: CLAS/PR04-116 secondary e+/e- beam – 2011/12 Novosibirsk/VEPP-3 storage ring / intern. target – 2009 OLYMPUS@DESY storage ring / intern. target – 2012

  17. New Proton Measurements at High Q2 • High-Q2 measurements at Jefferson Lab • Hall C E05-017: Super-Rosenbluth Q2 = 0.9 – 6.6 (GeV/c)2 Completed in summer 2007 • GEp-III /Hall C: E04-108/E04-019 Q2 = 2.5, 5.2, 6.8, 8.5 (GeV/c)2Completed in spring 2008 • SANE /Hall C E05-017: Polarized Target Q2 = 5 – 6 (GeV/c)2 Completed in spring 2009 BF-04 (Wed.) BF-05 (Wed.) LJ-06 (Sat.) • Proposed experiments • PAC32: PR12-07-109 /Hall A (GEp-IV)L. Pentchev, C.F. Perdrisat, E. Cisbani, V. Punjabi, B. Wojtskhowski, M. Khandaker et al.Q2=13,15 (GeV/c)2: Approved • PAC32: PR12-07-108 /Hall A (high-Q2 x-sec.)S. Gilad, B. Moffit, B. Wojtsekhowski, J. Arrington et al.Q2 =7-17.5 (GeV/c)2: Approved • PAC34: PR12-09-001 /Hall C (GEp-V)E.J. Brash, M. Jones, C.F. Perdrisat, V. Punjabi et al.Q2=6,10.5,13 (GeV/c)2: Conditionally approved CF-06 (Thu.)

  18. Imaginary part of TPE: SSA’s spin of beam OR target NORMAL to scattering plane on-shell intermediate state (MX = W) E.g. target normal spin asymmetry Beam: PVES at Bates, MAMI and Jlab; Target: PR05-015, PR08-005 BF-06 (Wed.) BF-07 (Wed.)

  19. Transverse Beam Asymmetry Plot: Courtesy of J. Mammei

  20. Summary • The limits of OPE have been reached with available today’s precision Nucleon elastic form factors, particularly GEp under doubt • The TPE hypothesis is suited to remove form factor discrepancy,however calculations of TPE are model-dependent • Experimental probes: Real part of TPE: Y2γ – Imaginary part: SSA’s • Need both positron and electron beams for a definitive test of TPEOLYMPUS, CLAS, VEPP-3 • ε dependence of polarization transfer, ε-nonlinearity of cross sectionstransverse beam symmetries • Improved precision and extension of “standard” methods to high Q2 • A comprehensive and rich program underway and/or proposedis expected to be conclusive within a few years • Broader Impact: gamma-Z box in PVES; TPE effects in DIS

  21. Interpreting Electron Scattering … “[…] most of what we know and everything we believe about hadron structure [… is based on electron scattering]” (W. Turchinetz) “The electromagnetic probe is well understood, hence …” (a common phrase in many articles) We have made big investments in lepton scattering facilitiesto explore hadron structure The elastic form factors characterize the simplest process in nuclear physics, namely elastic scattering (straightforward, one should think) We have to understand the elastic form factors beforewe can claim to have understood anything else

  22. Backup slides

  23. GpE and GpM from Unpolarized Data

  24. GpE and GpM from Unpolarized Data charge and magnetization density (Breit fr.) Dipole form factor within 10% for Q2 < 10 (GeV/c)2

  25. Recoil Polarization Technique • Pioneered at MIT-Bates • Pursued in Halls A and C, and MAMI A1 • In preparation for Jlab @ 12 GeV V. Punjabi et al., Phys. Rev. C71 (2005) 05520 Focal-plane polarimeter Secondary scattering of polarizedproton from unpolarized analyzer Spin transfer formalism to account for spin precession through spectrometer

  26. Polarized Targets BLAST Internal Target: Atomic Beam Source UVA / “SLAC”-Target: Dynamic Nuclear Polarization Limited luminosity for polarized hydrogen/deuterium targets, Very precise at low to moderately high Q2 from W. Meyer, SPIN2008

  27. Nucleon Form Factors: Last Ten Years J. Arrington PANIC08 Magenta: underway or approved

  28. Extensions with Jlab 12 GeV Upgrade • BLUE = CDR or PAC30 approved, GREEN = new ideas under development J. Arrington PANIC08 ~8 GeV2

  29. Two-Photon Exchange: Exp. Evidence Two-photon exchange theoretically suggested TPE can explain form factor discrepancy J. Arrington, W. Melnitchouk, J.A. Tjon, Phys. Rev. C 76 (2007) 035205 Rosenbluth data withtwo-photon exchangecorrection Polarization transfer data

  30. Polarized Target Experiments at High Q2 Polarized Target: Independent verification of recoil polarization result is crucial Polarized internal target / low Q2: BLASTQ2<0.65 (GeV/c)2 not high enough tosee deviation from scaling RSS /Hall C: Q2 ≈ 1.5 (GeV/c)2 SANE/Hall C: completed March 2009 BigCal electron detector Recoil protons in HMS parasitically Extract GE/GM to <5% at Q2≈5-6 (GeV/c)2 M.K. Jones et al., PRC74 (2006) 035201

  31. New Proton Measurements at High Q2 • Extension to higher Q2 at Jefferson Lab • GEp-III /Hall C: PR04-108/PR04-019 Completed in spring 2008 • Sign change of GE/GM observed (preliminary, C. Perdrisat @ PANIC08) PRELIMINARY • Or maybe not (preliminary, CIPANP09)

  32. Imaginary part of TPE: SSA’s spin of beam OR target NORMAL to scattering plane on-shell intermediate state (MX = W) lepton hadron Beam: PVES at Bates, MAMI and Jlab; Target: PR05-015, PR08-005 BF-06 (Wed.) BF-07 (Wed.)

  33. Target normal spin asymmetry general formula, of order e2 involves the imaginary part of two-photon exchange amplitudes

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