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Understanding Nucleon Structure and Interaction in Lepton Scattering

This seminar at the University of South Carolina explores the OLYMPUS Experiment at DESY, focusing on the effect of Two-Photon Exchange in Elastic Lepton Scattering. It covers topics such as form factors, the structure of lepton scattering, and current and future experiments to probe Two-Photon Exchange.

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Understanding Nucleon Structure and Interaction in Lepton Scattering

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  1. Seminar at University of South Carolina, Columbia, SC, Oct. 30, 2009 The OLYMPUS Experiment at DESY to Determine the Effect of Two-PhotonExchange in Elastic Lepton Scattering Michael Kohl <kohlm@jlab.org> Hampton University, VA 23668 and Jefferson Lab, VA 23606, USA

  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 OLYMPUS

  3. Nucleon Elastic Form Factors … • Fundamental quantities • Defined in context of single-photon exchange • Describe internal structure of the nucleons • Related to spatial distribution of charge and magnetism • Rigorous tests of nucleon models • Determined by quark structure of the nucleon • Role of quark angular momentum • Ultimately calculable by Lattice-QCD • Input to nuclear structure and parity violation experiments • 50 years of ever increasing activity • Tremendous progress in experiment and theory over last decade • New techniques / polarization experiments • Unexpected results

  4. (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)

  5. 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

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

  7. GpE and GpM from Unpolarized Data

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

  9. 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)

  10. 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

  11. 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

  12. 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

  13. 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

  14. Proton Form Factor Ratio Iachello 1973: Drop of the ratio alreadysuggested by VMD F. Iachello et al., PLB43 (1973) 191F. Iachello, nucl-th/0312074 1 mpGpE/GpM 0 0 2 4 6 8 10 Q2/(GeV/c)2 A.V. Belitsky et al., PRL91 (2003) 092003 G. Miller and M. Frank, PRC65 (2002) 065205 S. Brodsky et al., PRD69 (2004) 076001 Quark angular momentum Helicity non-conservation

  15. 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 • 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

  16. 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)

  17. 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≈5.75 (GeV/c)2 M.K. Jones et al., PRC74 (2006) 035201

  18. 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

  19. 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

  20. 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; modifications ~α3 New amplitudes are complex!

  21. 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

  22. 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 ? • GE/GM from 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-

  23. E04-019 (Two-gamma) • GE/GM from Pt/Pl constant vs. ε • (1–2εR/(1+ε)) Y2γ constantassuming δGE, δGM = const. • with Y2γ = const.  Y2γ = 0? • Wait for Super-Rosenbluthresults E05-017 (non-linearity) • Wait for e+/e- comparisonsOLYMPUS, VEPP-3, CLAS PRELIMINARY

  24. Lepton-proton elastic scattering 2 + … + ~α2 ~α

  25. Experiments to Verify 2g Exchange Precision comparison of positron-proton and electron-proton elastic scattering over a sizable ε range at Q2 ~ 2-3 (GeV/c)2 J. Arrington, PRC 69 (2004) 032201(R) SLAC data At low ε : <Q2> ~ 0.01 to 0.8 (GeV/c)2 At high ε : <Q2> ~ 1-5 (GeV/c)2 Θ=180o Θ=0o

  26. Two-photon exchange Elastic electron-proton to positron-proton ratio (P. Blunden)‏

  27. Two-photon exchange Elastic electron-proton to positron-proton ratio (P. Blunden)‏ BLAST @ 2.0 GeVQ2 = 0.6–2.2 (GeV/c)2

  28. Two-photon exchange

  29. 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 2009/10 – Transfer of BLAST 2011/12 – OLYMPUS Running

  30. Proposed Experiment • Electrons/positrons (100mA) in multi-GeV storage ringDORIS at DESY, Hamburg, Germany • Unpolarized internal hydrogen target (buffer system)3x1015 at/cm2 @ 100 mA → L = 2x1033 / (cm2s) • Redundant monitoring of luminositypressure, temperature, flow, current measurementssmall-angle elastic scattering at high epsilon / low Q2 • Large acceptance detector for e-p in coincidenceBLAST detector from MIT-Bates available • Measure ratio of positron-proton to electron-protonunpolarized elastic scattering to 1% stat.+sys.

  31. 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)

  32. The BLAST Detector Bates MIT UNH ASU

  33. Identification of Elastic Events e’ BLAST 1H(e,e’p) E=850 MeV e- left, p+ right p,d e- right, p+ left Charge +/- Coplanarity Kinematics Timing • Advantages of magnetic field: • suppression of background • 2-3% momentum resolution • σθ= 0.5o and σφ= 0.5o

  34. * Proton Form Factor Ratio mpGpE/GpM C.B. Crawford et al., PRL 98 (2007) 052301 Impact of BLAST data combined with cross sections on separation of GpE and GpM Errors factor ~2 smaller Reduced correlation Deviation from dipole at low Q2! *Ph.D. work of C. Crawford (MIT) and A. Sindile (UNH)

  35. * Neutron Electric Form Factor GnE E. Geis, M.K., V. Ziskin et al., PRL 101 (2008) 042501 *Ph.D. work of V. Ziskin (MIT) and E. Geis (ASU)

  36. Proposed Experiment • Electrons/positrons (100mA) in multi-GeV storage ringDORIS at DESY, Hamburg, Germany • Unpolarized internal hydrogen target (buffer system)3x1015 at/cm2 @ 100 mA → L = 2x1033 / (cm2s) • Large acceptance detector for e-p in coincidenceBLAST detector from MIT-Bates available • Measure ratio of positron-proton to electron-protonunpolarized elastic scattering to 1% stat.+sys. • Redundant monitoring of luminosity(pressure, temperature, flow, current measurements)small-angle elastic scattering at high epsilon / low Q2

  37. Luminosity Monitors: Telescopes 2 tGEM telescopes, 3 tracking planes 3.9 msr, 10o, R=160 cm, dR=10 cm Forward telescopes 10o

  38. Forward Elastic Luminosity Monitor • Forward angle electron/positron telescopes or trackers with good angular and vertex resolution • Coincidence with proton in BLAST • High rate capability • GEM technology MIT protoype: Telescope of 3 Triple GEM prototypes (10 x 10 cm2) using TechEtch foils F. Simon et al., Nucl. Instr. and Meth. A 598 (2009) 432

  39. Control of Systematics i = e+ or e- j= pos/neg polarity Geometric proton efficiency: Ratio in single polarity j Geometric lepton efficiency:

  40. Control of Systematics Super ratio: Cycle of four states ij Repeat cycle many times • Change between electrons and positrons every other day • Change BLAST polarity every other day • Left-right symmetry

  41. Projected Results for OLYMPUS 1000 hours each for e+ and e- Lumi=2x1033 cm-2s-1 500 hours each for e+ and e- Lumi=2x1033 cm-2s-1

  42. Projected Results for OLYMPUS 500 hours each for e+ and e- Lumi=2x1033 cm-2s-1

  43. 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

  44. 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

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

  46. 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

  47. 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

  48. Backup slides

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

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

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