Experimental Overview

Transcription

1 Int. Workshop MAMI and Beyond, Mainz, March 30 April 3, 2009 Nucleon Form Factors: Experimental Overview Michael Kohl Hampton University, VA and Jefferson Lab, VA 23606, USA

2 Outline Proton & Neutron electric and magnetic form factors Introduction, motivation and formalism Traditional and new techniques Overview of experimental data Energy frontier High Q 2 : Proton form factor ratio and two-photon exchange Precision frontier Low Q 2 : Pion cloud effect A. Thomas, W. Weise, The Structure of the Nucleon (2001) 2

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 t of the nucleon 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 3

4 (Hadronic) Structure and (EW) Interaction Structure Interaction Factorization! σ(structured object) Form factor 2 = σ(pointlike object) Probe Object Interference! Utilize spin dependence of electromagnetic interaction to achieve high precision Lepton scattering Born Approximation Electroweak probe Interaction Structure Inelastic Elastic Hadronic object 4

5 The Beginnings ep-elastic Finite size of the proton Robert Hofstadter Nobel prize 1961 R. Hofstadter, Rev. Mod. Phys. 56 (1956) 214 ed-elastic Finite size + nuclear structure 5

6 Nucleon Elastic Form Factors 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

7 Rosenbluth Separation G 2 E τg M 2 θ=180 o θ=0 o 7

8 G p E and G p M from Unpolarized Data 8

9 G p E and G p M from Unpolarized Data Dipole form factor charge and magnetization density (Breit fr.) within 10% for Q 2 < 10 (GeV/c) 2 9

10 Nucleon Form Factors and Polarization Double polarization in elastic/quasielastic ep or en scattering: Recoil polarization or (vector) polarized target 1,2 H(e,e p), 1,2 H(e,e p), 2 H(e,e n), 2 H(e,e n), 3 He(e,e n), Polarized cross section Double spin asymmetry = spin correlation Asymmetry ratio ( Super ratio ) independent of polarization or analyzing power 10

11 Recoil Polarization Technique Pioneered at MIT-Bates V. Punjabi et al., Pursued in Halls A and C, and MAMI A1 Phys. Rev. C71 (2005) In preparation for 12 GeV Focal-plane polarimeter Secondary scattering of polarized proton from unpolarized analyzer Spin transfer formalism to account for spin precession through spectrometer 11

12 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 Q 2 from W. Meyer, SPIN

13 Proton Form Factor Ratio Jefferson Lab 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

14 Proton Form Factor Ratio Jefferson Lab 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

15 Proton Form Factor Ratio p E /G p M μ p G p 1 F. Iachello et al., PLB43 (1973) 191 F. Iachello, nucl-th/ Iachello 1973: Drop of the ratio already suggested by VMD Q 2 /(GeV/c) 2 A.V. Belitsky et al., PRL91 (2003) G. Miller and M. Frank, PRC65 (2002) S. Brodsky et al., PRD69 (2004) Quark angular momentum Helicity non-conservation 15

16 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) : Formalism TPE effect could be large P.G. Blunden, W. Melnitchouk, and J.A. Tjon, PRC72 (2005) , PRL91 (2003) : Nucl. Theory elastic half, Delta opposite M.P. Rekalo and E. Tomasi-Gustafsson, EPJA22 (2004) 331: Importance of higher-order radiative effects Y.C. Chen et al., PRL93 (2004) : Partonic calculation, TPE large at high Q 2 A.V. Afanasev and N.P. Merenkov, PRD70 (2004) : Large logarithms in normal beam asymmetry A.V. Afanasev, S.J. Brodsky, C.E. Carlson, Y.C. Chen, M. Vanderhaeghen, PRD72 (2005) : high Q 2, small effect on asym., larger on x-sec., TPE on R small 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) : TPE effect rather small! M. Kuhn, H. Weigel, EPJA38 (2008) 295: TPE in Skyrme Model D.Y. Chen et al., PRC78 (2008) : TPE for timelike form factors M. Gorchtein, C.J. Horowitz, arxiv: [hep-ph]: gamma-z box 16

17 Two-Photon Exchange: Exp. Evidence Two-photon exchange theoretically suggested Rosenbluth data with two-photon exchange correction Polarization transfer data TPE can explain form factor discrepancy J. Arrington, W. Melnitchouk, J.A. Tjon, Phys. Rev. C 76 (2007) OLYMPUS Experiments to verify hypothesis: e+/e-: CLAS/PR Novosibirsk/VEPP-3 OLYMPUS@DESY (=BLAST@DORIS) SSA: PR05-15 (pol. He-3) ε-dep.: PR (pol.), PR (unp.) 17

18 Polarized Target Experiments at high Q2 Polarized Target: Independent verification of recoil polarization result is crucial Polarized internal target / low Q2: BLAST Q2<0.65 (GeV/c)2 not high enough to see 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 E t Extract t GE/GM to t <5% att Q (G (GeV/c) V/ )2 M.K. Jones et al., PRC74 (2006)

19 New Measurements at high Q 2 Extension to higher Q 2 at Jefferson Lab GEp-III /Hall C: PR04-108/PR Completed in spring 2008 Sign change of G E /G M observed (preliminary, C. PANIC08) 19

20 New Measurements at high Q 2 Extension to higher Q 2 at Jefferson Lab GEp-III /Hall C: PR04-108/PR Completed in spring 2008 Sign change of G E /G M observed (preliminary, C. PANIC08) Hall C PR Super-Rosenbluth Q 2 = (GeV/c) 2 completed in summer 2007 Proposed experiments PAC32: PR /Hall A (GEp-IV) L. Pentchev, C.F. Perdrisat, E. Cisbani, V. Punjabi, B. Wojtskhowski, M. Khandaker et al. Q 2 =13,15 (GeV/c) 2 : Approved PAC32: PR /Hall A (high-q 2 x-sec.) S. Gilad, B. Moffit, B. Wojtsekhowski, J. Arrington et al. Q 2 = (GeV/c) 2 : Approved PAC34: PR /Hall C (GEp-V) E.J. Brash, M. Jones, C.F. Perdrisat, V. Punjabi et al. Q 2 =6,10.5,13 (GeV/c) 2 : Conditionally approved 20

21 BLAST at MIT-Bates Bates Large Acceptance Spectrometer Toroid Symmetric, large acceptance, general purpose detector Detection of e ±, π ±, p, d, n Longitudinally polarized electrons in SHR 850 MeV, 200 ma, P e = 65% Highly polarized internal gas target of pure H and D (Atomic Beam Source) 6 x atoms/cm 2, L = 6 x /(cm 2 s), P H/D = 80% 21

22 Proton Form-Factor Ratio μ p G p E/G p * M C.B. Crawford et al., PRL98 (2007) Impact of BLAST data combined with cross sections on separation of G p E and G p M Errors factor ~2 smaller Reduced correlation Deviation from dipole at low Q 2! *Ph.D. work of C. Crawford (MIT) and A. Sindile (UNH) 22

23 New Measurements on Proton at low Q 2 Hall A PR Hall A PR07-004, Recoil polarization (PAC31/33) Recoil polarization, LEDEX PR08-007, to be released at APS April to meeting be relea Polarized target, cond. approved LEDEX PR05-004, G. Ron et al., PRL99 (2007)

24 New Measurements on Proton at low Q2 E = 450 MeV Hall A PR Recoil polarization LEDEX PR08-007, t be to b relea l E = 720 MeV Rosenbluth separation at low Q2 Precise charge and magnetic radii

25 G n E in absence of free neutron target No free neutron target elastic and quasi-elastic scattering Nuclear corrections (FSI, MEC, ) Smallness of G n E does not allow L-T sep. of d(e,e n) or d(e,e ) d(e,e p) ) 3 He(e,e n) quasielastic Polarized Helium-3 G n EG n M interference MAMI A3, A1, Hall A 2 H(e,e n) quasielastic Vector-polarized deuterium G n EG n M interference Nikhef, Bates/BLAST, Hall C G 2 Q from A+T 20 / H(e,e d) G n EG p E interference Schiavilla+Sick G n E 2 H(e,e d) elastic, A(Q 2 ) G n EGE p E interference Galster, Platchkov, 2 H(e,e n) quasielastic Neutron recoil polarization G n EG n M interference Bates, MAMI A3, A1, Hall C

26 Neutron Electric Form Factor G n E 26

27 Neutron Electric Form Factor G n * E E. Geis et al., PRL101 (2008) *Ph.D. work of V. Ziskin (MIT) and E. Geis (ASU) 27

28 New Measurements of G n E E PRELIMINARY Polarized He-3 B. Wojtsekhowski Q 2 =1.2, 1.7, 2.5, 3.5 MAMI-A1 PRELIMINARY Polarized He-3 P. Grabmayr, y, Erice 2007 Q 2 =0.25, 1.5 (GeV/c) 2 28

29 Future Measurements of G n E PR H(e,e n) X MAMI 3 He(e,e n) PAC34 (2009): PR B.D. Anderson, J. Arrington, S. Kowalski, R. Madey, B. Plaster, A. Yu. Semenov et al. Hall C: SHMS + NPOL Up to Q 2 = 7 (GeV/c) 2 Superseding PR x x PR He(e,e n) X MAMI 3 He(e,e n) PAC34 (2009): PR B. Wojtsekhowski, G. Cates, S. Riordan et al. Hall A: Polarized He-3 Up to Q 2 = 10 (GeV/c) 2 MAMI-A1 Polarized He-3 Q 2 =0.25, 1.50 (GeV/c) 2 x x 29

30 G n M in absence of free neutron target No free neutron target elastic and quasi-elastic scattering Nuclear corrections (FSI, MEC, ) Neutron efficiency 3 He(e,e ) quasielastic, inclusive i d(e,e )-p(e,e ) difference Polarized He-3 d(e,e!p), d(e,e n) Bates, Hall A Issues: large nucl. corr. Issues: P n n, FSI Need to know n-efficiency a>>1, b>c G n M 2 H(e,e ) quasielastic, inclusive Vector-polarized deuterium Bates/BLAST IncAs (LOI ) Issues: Know G p E/G p M, G p M 2 H(e,e n) ratio quasielastic 2 H(e,e p) SLAC, Bates, Nikhef, MAMI, Hall B Issues: Know G p M Need to know n-efficiency

31 Neutron Magnetic Form Factor G n M Pre-polarization era G n M world data from unpolarized experiments Cross section ratio quasielastic d(e,e n) d(e,e p)

32 Neutron Magnetic Form Factor G n M Pre-polarization era G n M world data from unpolarized experiments Cross section ratio quasielastic d(e,e n) d(e,e p) + CLAS preliminary

33 Neutron Magnetic Form Factor G n M Pre-polarization era G n M world data from unpolarized experiments Cross section ratio quasielastic d(e,e n) d(e,e p) + CLAS preliminary (final)

34 Neutron Magnetic Form Factor G n M Pre-polarization era G n M world data from unpolarized experiments Cross section ratio quasielastic d(e,e n) d(e,e p) + CLAS preliminary (final) Polarization era G n M world data + 3 He + BLAST preliminary

35 Neutron Magnetic Form Factor G n M Pre-polarization era G n M world data from unpolarized experiments Cross section ratio quasielastic d(e,e n) d(e,e p) + CLAS preliminary (final) Polarization era G n M world data + 3 He + BLAST preliminary No pol. data >0.6 (GeV/c) 2 Discrep./jump of several σ at Q 2 ~ (GeV/c) 2 IncAs/LOI case deemed uncompelling by PAC34

36 Future Measurements of G n M PAC32 (2007): PR G.P. Gilfoyle, W.K. Brooks, M.F. Vineyard, JD J.D. Lachniet, LB L.B. Weinstein i et al. Hall B /CLAS12 Up to Q 2 = 14 (GeV/c) 2 Both approved experiments based on ratio method PAC34 (2009): PR B. Wojtsekhowski, R. Gilman, B. Quinn et al. Hall A /BigFamily Proposed up to Q 2 = 18 (GeV/c) 2 Approved up to Q 2 = 12 (GeV/c) 2

37 Nucleon Form Factors: Last Ten Years J. Arrington PANIC08 Magenta: underway or approved 37

38 Extensions with Jlab 12 GeV Upgrade J. Arrington PANIC08 38 ~8 GeV 2 BLUE = CDR or PAC30 approved, GREEN = new ideas under development

39 Summary Nucleon electromagnetic elastic form factors Tremendous progress during last decade High precision, low systematic uncertainties through polarization experiments Worldwide activity at its peak Progress in past decade: High-Q 2 surprise in G p E/G p M; strong impact on theoretical picture Evidence for two-photon t exchange effects New precise picture of G n E for Q 2 <1.5 (GeV/c) 2, G n M <5 (GeV/c) 2 Evidence for structure beyond G Dipole at low Q 2 in all form factors Dipole Many new experiments underway or proposed 39

40 Comment on e-scattering at MAMI Consider effort to implement polarized target for H and D for electron scattering in A1 With luminosities of /(cm 2 s), can explore polarized response in the Q 2 < 2 (GeV/c) 2 domain with high precision o o o o Elastic form factors of p, n, d Low-Q 2 spin structure of p, n, d Exclusive production (e,e π), (e,e K) etc. off polarized p, n, d Deuteron structure (elastic, breakup, pion prod.), vector+tensor Technology at UVA (Dynamical nuclear polarization), and at Jlab and Spring-8 (HD-Ice) to be developed further and maintained May take several years to fully establish, but effort will be rewarding 40

41 Backup slides 41

42 IncAs: A precision G n M measurement LOI /PAC34 42

43 Neutron Electric Form Factor G n E Charge form factor Charge distribution Breit frame Forbidden interpretation as rest charge distribution ib ti r (fm) Transverse size from GPD G. Miller, nucl-th/