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Kees de Jager Hall A Workshop December 14, 2011

Nucleon Electromagnetic Form Factors: An Overview. Introduction Experimental Status of EMFF Two-photon exchange effects Proton charge radius Flavor separation Analysis and Interpretation Outlook Summary. Kees de Jager Hall A Workshop December 14, 2011.

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Kees de Jager Hall A Workshop December 14, 2011

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  1. Nucleon Electromagnetic Form Factors: An Overview • Introduction • Experimental Status of EMFF • Two-photon exchange effects • Proton charge radius • Flavor separation • Analysis and Interpretation • Outlook • Summary Kees de Jager Hall A Workshop December 14, 2011 Thomas Jefferson National Accelerator Facility

  2. Historical Overview • 1910s - Rutherford discovers the positively charged core of atoms • 1932 – James Chadwick discovers the neutron • 1933 - Stern observes anomalous magnetic moment of the proton by deflecting a beam of hydrogen molecules in an inhomogeneous magnetic field • 1955 - Hofstadter et al. at Stanford discover, through elastic electron scattering, that protons have a size, quote an RMS charge radius of 0.74 ± 0.24 fm • 1968 – nucleon constituents were established from scaling in deep inelastic scattering

  3. Definitions of the EMFFs where + The Sachs FFs:

  4. Visualizing charge and magnetic distributions Vanderhaeghenand Walcher, Nucl. Phys. News 21 (2011) 14 light-front frame proton neutron rest frame neutron charge density Interpreted as pion cloud transversely polarized Fourier transforms of nucleon FFs (the neutron pictured above) have provided important insight, but suffer in that the momentum transfers are too large to ignore relativistic effects While referred to as “charge densities”, these cannot be directly related to our usual lab-frame concept of charge density

  5. Impact of EMFF on GPDs 1.GPDs allow for a unified description of form factors and parton distributions 2. Describes correlations of quarks/gluons 3. Allows for Transverse Imaging x < 0.1 x ~ 0.3 x ~ 0.8 4.EMFFsare essential to determine the high-Q2 contributions to the GPDs in order to allow access to the quark angular momentum (in model-dependent way)

  6. World Data Set on GEp by mid 1990s • Relied on Rosenbluth separation • General assumption that GEp/GMp ≈ 1 • Although data showed large scatter Rosenbluth separation (cross-section measurement at constant Q2)

  7. Alternative: Spin Transfer Reaction 1H(e,e’p) • No error contributions from • analyzing power • beam polarimetry Akhiezer et al., Sov. Phys. JETP 6, 588 (1958)

  8. World Data Set on GEp more than ten years later • Large new data set based on polarization transfer show a close to linear decrease of GEp/GMpwith Q2 • In contrast with Rosenbluth data A. Puckett et al., arXiv: 1102.5737 • Detailed reanalysis of SLAC data resulted in acceptable scatter of data • JLabRosenbluth data (open red symbols in top) in agreement with SLAC data • No reason to doubt quality of either Rosenbluth or polarization transfer data • Investigate possible theoretical sources for discrepancy

  9. Two-photon Exchange (TPE) Contributions Milner Guichon and Vanderhaeghen (PRL 91, 142303 (2003)) introduced a general formalism for two-photon exchange effects in elastic electron-nucleon scattering • The 2γ/1γ interference term δ2γ has opposite sign for e+p and e-p scattering, is expected to vary from 1 to 10%, but no data with significant accuracy exist. Experiments are ongoing at VEPP-III, CLAS and DORIS. • Other processes sensitive to TPE: • Non-linearity of ε- dependence in polarization transfer • Normal Single-Spin Asymmetries

  10. NSSA in elastic eN scattering TPE effects known to exist spin ofbeam ORtargetNORMAL to scattering plane on-shell intermediate state (MX = W) ∝ℑ(A2γ) Androic et al., PRL 107 (2011) 022501 Good agreement with theory

  11. Searching for non-linear ε-dependence in PT No non-linearity observed in PL/PT, but ε-dependence in PL Meziane et al., PRL 106 (2011) 132501 Q2=2.64 GeV2 Interpreted phenomenologically by Guttmann, Kivel and VdH, PRD 83 (2011) 094021

  12. Effect on L-T Extractions LT + BMT* PT • Arrington, Melnitchouk, Tjon • PRC 76, 035205 (2007) • full reanalysis of data, incorporating hadronic TPE calculations, adding a (small)phenomenological correction for nucleon resonances • ~1% at 2 GeV2, 2% at 5 GeV2 • Assign 100% of the extra correction as an uncertainty (affects GMp uncertainty) Arrington, Blunden, Melnitchouk PPNP 66 (2011) 782 • Corrections in full agreement with existing data on e+/e- ratio

  13. CLAS TPE experiment • In the CLAS experimentmixed identical e+ and e- beams were directed onto a LH2 target. The scattered leptons and protons were detected in the CLAS detector. • Overdetermined kinematics allow clean identification of elastic scattering events • Results within a few months Schematic e+/e-beamline Novosibirsk Olympus CLAS

  14. OLYMPUS@DESY in DORIS • Use BLAST detector and internal target at DORIS • Max. energy 4.45 GeV • Stored current 120 mA • Installation completed • Commissioning started VEPP-III

  15. Proton cross-section data from MAMI • Bernauer et al. (PRL 105, 242001 (2010)) collected a large data set (1400 data at six beam energies), using all three A1 spectrometers • The ≤1% accuracy allowed an L/T separation in a Q2-range of 0.005 to 0.7 GeV2; error bands shown are of fits to complete data set, not representative of individual errors • Results for GEp/GMp in reasonable agreement with CODATA, but GMp data 2-3% larger than world data set (due to neglect of TPEs??) <r2>E1/2 = 0.879(8)±5±4±2±4 fm <r2>M1/2 = 0.777(18)±13±9±5±2 fm stat;syst;model CODATA (dominated by electronic Lamb Shift) <r2>E1/2 = 0.879±7 fm

  16. Polarization Transfer at low Q2-values Detailed understanding of Hall A HRS spectrometer optics and availability of BigBite spectrometer has made possible polarization transfer measurements with a ~1% accuracy in a Q2-range from 0.3 – 0.7 GeV2 Results agree with Bernauer et al., but the charge radius is extracted using the magnetic world data <r2>E1/2 = 0.875(10)±8±6fm <r2>M1/2 = 0.867(20)±9±18 fm X. Zhan et al., PLB 705, 59 (2011) These new data analyzed together with the new data set from MAMI will allow to set sensitive limits onTPE effects at low Q2

  17. Comparison of world data • Blue: MAMI results • Dashed black curve: old fit (w/o MAMI data) with TPE • Open circles: old fit w/o TPE • Red: updated Arrington fit (w/o MAMI data), including TPE corrections • Green: slope of fit at Q2 = 0 Comparison of open circles and black (dashed) curve gives quantitative indication of TPE effects Relativistic boost effects: ~1% decrease in charge radius Kelly, PRC 66 (2002) 065203

  18. The proton charge radius PSI (muonic Lamb shift) Nature 466, 213 (2010) <r2>E1/2 = 0.84184(67) fm CODATA (electronic Lamb shift) <r2>E1/2 = 0.8768(69) fm • What is reason for this >5σ discrepancy? • Electron scattering and electronic Lamb shift agree • Also, electron scattering andmuonic X-ray agree • Unknown interaction between μ and p? • Muonic hydrogen much smaller than atomic hydrogen, more sensitive to off-shell effects? • Leading theoretical uncertainty in HFS of hydrogen ground state dominated by low-Q2behaviour in Zemach radius: Zhan De Rujula, PLB 693, 555 (2010); PLB 697 26 (2010) Bernauer et al. PLB 696, 343 (2011) Cloet & Miller, PRC 83, 012201 (2011) Jentschura, EPJD 61, 7 (2011) Barger et al., arXiv: 1011.3519 Tucker-Smith and Yavin, arXiv: 1011.4922 Miller et al., arXiv: 1101.407

  19. Impact of E08-007-II • A linear fit to all Mainz data at Q2 < 0.02 GeV2yields a charge radius close to the PSI value! Clearly, the derivative at Q2 = 0 is not a good measure of the radius Carl Carlson Need independent new data at very low Q2, to be provided coming spring by E08-007-II, in order to resolve this issue

  20. Future EMFF Research @MAMI Use ISR to measure GEp down to very small Q2-values Goal: obtain a 1% accuracy in the proton charge radius FSR ISR FSR ISR Simulation of elastic scattering at three beam energies

  21. Measuring GMn • Measure (en)/(ep) ratio • Measure inclusive quasi-elastic scattering off polarized 3He • A systematic difference of several % between results from JLab and MAMI in Q2-range 0.4 – 1.0 GeV2 • Will be studied in upcoming experiment at MAMI

  22. GEnfrom polarized 3He target: 3He(e,e′n) n Target polarization ~50% Beam polarization 84% e’ • New data more than double the Q2-range of the world data set • Roberts’ dressed quark-diquark model using the Dyson-Schwinger and Faddeev equations in good agreement, better than Miller’s CQM prediction • New data allow a flavor separation of the EMFFs up to 3.4 GeV2 S. Riordan et al., PRL 105, 262302 (2010)

  23. Comparison with (Effective) Theory

  24. Status of Lattice QCD Data LQCD Significant progress in LQCD, but still limited to mπ ≥ 300 MeV and neglect of disconnected diagrams, resulting in large underestimates of e.g. isovector charge radius Bratt et al., PRD 82 (2010) 094502 experimental value

  25. (Logarithmic) Scaling • Basic pQCD scaling predicts F1∝1/Q4 ; F2∝1/Q6→ F2/F1∝1/Q2 • Data clearly do not follow this trend (yet?) • The introduction of a quark orbital angular momentum component results in • F2/F1∝1/Q • Belitsky et al. have included logarithmic corrections in pQCD limit • Proton data appear to follow this scaling behaviour to some extent, but new neutron data do not

  26. This disagrees with a generally accepted expectation that dates to Schwinger in the 1950’s that: Flavor separation of F1,2 The ratios F2q/F1q become constant for Q2 > ~1 GeV2 ! F2/F1∝1/Q2 Assuming that the s-quark contribution is negligible (based on the PVe results) u d Cates, de Jager, Riordan and Wojtsekhowski, PRL vol. 106, pg 252003 (2010) Note that the corresponding ratio F2p/F1p shows no particular change in behavior for Q2 > ~1 GeV2

  27. Scaling of the up and down quarks Cates, de Jager, Riordan and Wojtsekhowski, PRL vol. 106, pg 252003 (2010) Fd seems to scale like 1/Q4 whereas Fu seems to scale more like 1/Q2

  28. Scaling difference caused by diquarks Suggested by Jerry Miller u-quark scattering amplitude is dominated by scattering from the lone “outside” quark. Two constituents implies 1/Q2 d-quark scattering amplitude is necessarily probing inside the diquark. Two gluons need to be exchanged (or the diquark would fall apart), so scaling goes like 1/Q4 While at present this idea is at a conceptual stage, it is an intriguingly simple interpretation for the very different behaviors (and in agreement with the DSE calculation of Cloët, Roberts and Wilson).

  29. Impact parameter bis defined relative to the transverse center ofthe quark’s longitudinal momentum fractions • in proton • Why is the d-quark so much wider? Mapping of nucleon constituents (in the proton) Miller, Strikman, Weiss, arXiv: 1105.6364 • ρd/ρu → -1 for b → ∞ due to dominance of chiral 2π exchange near threshold • ρd/ρu≈ 0.5 for 0.2 < b < 1.6 fm (mean-field value of number of quarks) • ρd/ρu ≈ 0.3 for b → 0 due to large x-behavior • need accurate GEn data at small Q2

  30. The SuperBigbite program GEMs • The Super Bigbiteprogram: • large dipole magnet • GEM trackers (~100,000 channels) • hadron and EM calorimeter • Trigger and DAQ • operating in open geometry at a luminosity of 1038 cm-2s-1 • will extend measurements of EMFFs to double the existing Q2-range • included in the JLab long-term capital funding PIs: Gordon Cates BogdanWojtsekhowski

  31. Projected EMFF data with SBS @ 12 GeV

  32. Further neutron FF measurements at lower Q2-values • MAMI will implement a new large neutron detector, to be used to improve data set on GMn (through (e,n)/(e,p) ratio) and on GEn through polarization transfer, both on LD2 target • Large acceptance of BigBite and high neutron efficiency of hadron calorimeter yield large FOM for (n,n’) polarimeter with SBS SBS in Hall A MAMI Complementary data set of high accuracy

  33. Time-Like Region _ • Can be probed through e+e- -> NN or inverse reaction • Large influx of data through analysis of ISR channel at BABAR • Effective proton FF (GE = GM) • Increase of FF towards threshold • Agreement with pQCD at high Q2 • Measure GE/GM by fitting distribution of helicity angle • Significant disagreement between new BABAR results and those from LEAR • GE ≈ GM at Q2 > 6 GeV2

  34. Time-like Region Prospects Simulation: BABAR: √s = 10.6 GeV, 230 fb-1 BES-III: √s = 3.77 GeV, 10 fb-1 3-4 times higher statistics at BES-III ! protons neutrons • Expect large expansion of data, especially for the neutron FF at BES-III

  35. Summary and Outlook • In the decade since the first observation of the surprising GEp/GMp ratio important developments in the study of EMFFs have yielded: • an extension of the data to much higher Q2-values • surprising insights in the constituent behavior from a flavor decomposition of the FFs • a broad ongoing program to obtain quantitative information on TPE, but looking forward to the data from CLAS-2γ and OLYMPUS • accurate data on the proton FFs at low Q2, waiting for a combined analysis • However, a broad effort is required to resolve the strong discrepancy with the muonic result on the proton charge radius • The JLab 12 GeV upgrade will extend the space-like EMFFs to double the present Q2-range, with an equally impressive improvement in the time-like region from BES-III. Highly accurate data are also needed at low to intermediate Q2-values • It is imperative that this experimental program is accompanied by a similar progress in our theoretical understanding of the nucleon

  36. THANK YOU ! acknowledging detailed discussions with John Arrington, Carl Carlson, Gordon Cates,AchimDenig, Wally Melnitchouk, Harald Merkel, Seamus Riordan and BogdanWojtsekhowski

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