Selected Results from the Nucleon Spin Program at Jefferson Lab

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1 Selected Results from the Nucleon Spin Program at Jefferson Lab Xiaochao Zheng Univ. of Virginia October 17, 009 Challenges of the Standard Model and the Nucleon Spin Puzzle Thomas Jefferson National Accelerator Facility (JLab) Recent Results from JLab Spin Program Summary and Outlook

2 Standard Model of Particle Physics SU()L X U(1)Y SU(3)C

3 Standard Model of Particle Physics Success of the Standard Model in the strong interaction sector QCD tested in the high energy (perturbative, = weak ) region Major Challenges within the Standard Model Understand and test QCD in extreme conditions (RHIC, LHC) Understand and test QCD in strong interaction region (non perturbative) S Understand the nucleon structure, how quarks and gluons form the nucleon's mass, momentum, and spin energy (GeV) (~1/distance)

4 Standard Model of Particle Physics Success of the Standard Model in the strong interaction sector QCD tested in the high energy (perturbative, = weak ) region Major Challenges within the Standard Model Understand and test QCD in extreme conditions (RHIC, LHC) Understand and test QCD in strong interaction region (non perturbative) Understand the nucleon structure, how quarks and gluons form the nucleon's mass, momentum, and spin

5 Three Decades of Spin Structure Study 1980s: EMC (CERN) + early SLAC quark contribution to proton spin is very small = 1±9±14 %! spin crisis (Ellis Jaffe sum rule violated) 1990s: SLAC, SMC (CERN), HERMES (DESY) = 0~30 % the rest: quark orbital angular momentum and gluons Different decompositions: Jaffe, Ji, X. Chen et al. Bjorken Sum Rule verified to <10% level 000s: COMPASS (CERN), HERMES, RHIC Spin, JLab, : Σ ~ 30%; G probably small, quark orbital angular momentum probably significant Test of various Sum Rules Transversity, Transverse Momentum Dependent Distributions Generalized Parton Distributions

6 Medium & High Energy Physics Facilities for Lepton Scattering Facilities Accelerator FermiLab Tevatron SLAC JLab Stanford Linear Accelerator Continuous Electron Beam Accelerator Facility (CEBAF) Beam e, e. e.. Large e /e+ Collider (LEP) DESY Deutsches Elektronen Synchrotron e, e Mainz Microtron MAMI e, e MIT Bates MIT Bates Linear Accelerator polarization, CERN MAINZ Energy,,.... Luminosity (cm s 1) Time 1.96 TeV low GeV, 80% GeV, 85% GeV, 85% GeV low GeV low /1.6 GeV GeV e factor 0.03% CW (DESY II) High luminosity, and continuous polarized beam makes JLab an unique facility. duty ~ns: continuous >>ns: pulsed CW

7 Employment: ~650 User community: ~100

8 Three Experimental Halls Hall A: pair of high resolution spectrometers (HRS), E' up to 4 GeV/c, = 7 msr luminosity up to 1039 cm s 1 Hall C: High Momentum (HMS) and Short Orbit Spectrometers (SOS) luminosity up to 1039 cm s 1 Hall B: CEBAF Large Acceptance Spectrometer (CLAS) luminosity up to 1034 cm s 1

9 Hall A polarized 3He target 15 ua longitudinal, transverse and vertical Luminosity=1036 (cm s 1) (highest polarized luminosity in the world) High in beam polarization > 65% Effective polarized neutron target 13 completed experiments 6 approved with 1 GeV (A/C)

10 Hall B/C Polarized proton/deuteron targets Polarized NH3/ND3 targets Dynamical Nuclear Polarization In beam average polarization 70 90% for p 30 40% for d Luminosity up to ~ 1035 cm s 1(Hall C) ~ 1034 cm s 1(Hall B)

11 JLab Spin Experiments Results: Spin in the valence (high x) region Quark Hadron duality Moments: Spin Sum Rules and Polarizabilities Higher twists: g/d Just completed: GDH on the proton at very low Q; Transversity (n) Planned at 6 GeV gp at low Q Future: 1 GeV Inclusive: A1/d, Semi Inclusive: Transversity, TMDs, Flavor decomposition Review: Kuhn, Chen, Leader, arxiv: , PPNP 63 (009) 1

12 Longitudinal Spin (I) Spin in Valence (high x) Region (where we can test pqcd and models such as RCQM)

13 Valence (high x) A1p and A1n results pqcd with HHC RCQM RCQM Hall B CLAS, Phys.Lett. B641, 11 (006) Hall A E99 117, PRL 9, (004) PRC 70, (004)

14 Before JLab Data for q/q pqcd with HHC With E99117 Data pqcd with HHC (HERMES data at large x not shown)

15 pqcd with Quark Orbital Angular Momentum PRL99, (007) Inclusive Hall A and B and Semi-Inclusive Hermes BBS (pqcd w/ HHC) BBS+OAM

16 Projections for JLab at 11 GeV A1p at 11 GeV

17 Longitudinal Spin (II) Quark Hadron Duality in Spin Structure Function <resonances> = <DIS>?

18 Duality in Spin Structure: CLAS EG1b Results Phys.Rev.C75:03503,007

19 Duality in Spin Structure: Hall A E01 01 Results A13He (resonance vs DIS) PRL 101, 1850 (008) Γ1 resonance comparison with pdfs integrated over resonances covered by the data, from the pion threshold to an xmin corresponding to W=1.905 GeV

20 Parton Distributions (CTEQ6 and DSSV) Unpolarized PDFs Polarized PDFs CTEQ6, JHEP 007, 01 (00) DSSV, PRL101, (008)

21 Spin Sum Rules for First Moments Sum Rules Moments of Spin Structure Functions Global Property

22 Bjørken Sum Rule 1 Q Q = [ g x, Q g x, Q ] d x= g A C NS 6 p 1 ga: n 1 p 1 n 1 axial vector coupling constant from neutron decay CNS: Q dependent QCD corrections (for flavor non singlet) A fundamental relation relating an integration of spin structure functions to axial vector coupling constant Based on Operator Product Expansion within QCD or Current Algebra, plus isospin invariance. Valid at large Q (where higher twist effects negligible) Data are consistent with the Bjørken Sum Rule at 5 10% level

23 Gerasimov Drell Hearn Sum Rule Circularly polarized photon on longitudinally polarized nucleon 0 d 1/ 3 / = EM M A fundamental relation between the nucleon spin structure and its anomalous magnetic moment Based on general physics principles Lorentz invariance, gauge invariance low energy theorem unitarity optical theorem causality unsubtracted dispersion relation applied to forward Compton amplitude First measurement on proton up to 800 MeV (Mainz) and up to 3 GeV (Bonn) agree with GDH with assumptions for contributions from un measured regions. New measurements from LEGS provided complimentary results on the proton, more precise results on the deuteron.

24 Generalized GDH Sum Rule Many approaches: Anselmino, Ioffe, Burkert, Drechsel, Ji and Osborne (J. Phys. G7, 17, 001): Forward Virtual Virtual Compton Scattering Amplitudes: S1(Q, ), S(Q, ) Same assumptions: no subtraction dispersion relation optical theorem (low energy theorem) S 1 Q =4 el G1 Q, d

25 Connecting GDH and Bjorken Sum Rules Q evolution of GDH Sum Rule provides a bridge linking strong QCD to pqcd Bjorken and GDH sum rules are two limiting cases High Q, Operator Product Expansion : S1(p n) ~ ga Bjorken Q~0, Low Energy Theorem: GDH S1 ~ High Q (> ~1 GeV): Operator Product Expansion Intermediate Q region: Lattice QCD calculations Low Q region (< ~0.1 GeV): Chiral Perturbation Theory Calculations: HBχPT: Ji, Kao, Osborne, Spitzenberg, Vanderhaeghen RBχPT: Bernard, Hemmert, Meissner Reviews: Theory: Drechsel, Pasquini, Vanderhaeghen, Phys. Rep. 378,99 (003) Experiments: Chen, Deur, Meziani, Mod. Phy. Lett. A 0, 745 (005)

26 JLab E (Hall A) Neutron spin structure moments and sum rules GDH integral on neutron Q evolution of neutron spin structure moments (sum rules) with pol.3he transition from quark gluon to hadron Test χpt calculations Results published in several PRL/PLB s PRL 89 (00) 4301 Q

27 JLab CLAS Eg1a/Eg1b (Hall B) Proton spin structure moments and sum rules Γ1 p EG1b, PLB67, 1 (009) EG1a, PRL 91, 00 (003)

28 GDH Sum and Spin Structure Function Moments at very low Q Test fundamental understanding Test PT at very low Q Γ1 n E94 010, from 3He, PRL 9, 0301(004) E97 110, from 3He, preliminary very low Q! EG1a, EG1b: from d p

29 JLab CLAS EG4 (Hall B) Proton and deuteron spin structure moments and sum rules at very Low Q Expected statistical accuracy from EG4 Ran in 006 Data being analyzed

30 Γ 1 of p n Bjorken Sum agree well with PT pqcd w/o HT corrections agree with data surprisingly well down to Q=1 GeV. EG1b, PRD 78, (008) E EG1a: PRL 93 (004) 1001

31 Effective Strong Coupling Constant A new attempt at low Q Experimental Extraction of S from Bjorken Sum

32 The strong coupling constant from pqcd α s (Q) is well defined in pqcd at large Q. Can be extracted from data (e.g. Bjorken Sum Rule). g 1 g 1 dx= 1 p n p n = ga 6 1 s 3.58 s Not well defined at low Q, diverges at QCD

33 Definition of effective QCD couplings PLB B95 70 (1980); PRD (1984); PRD (1989). Prescription: Define effective couplings from a perturbative series truncated to the first term in α s. Generalized Bjorken sum rule: g Use p 1 g dx= n 1 p n 1 p n 1 ga s 3.58 = 1 s HigherTwists 6 g1 ga s = 1 6 to define an effective α sg1. Process dependent. But can be related through Commensurate scale relations S.J. Brodsky & H.J Lu, PRD (1995) S.J. Brodsky, G.T. Gabadadze, A.L. Kataev, H.J Lu, PLB (1996) Extend it to low Q down to 0: include all higher twists.

34 Effective Coupling Extracted from Bjorken Sum A. Deur, V. Burkert, J. P. Chen and W. Korsch PLB 650, 44 (007) and PLB 665, 349 (008) α s/π first attempt of effective S extraction at low Q no strong Q dependence of strong force at large distances

35 Comparison with theory Fisher et al. Bloch et al. Maris Tandy Bhagwat et al. Cornwall Schwinger Dyson Godfrey Isgur: Constituant Quark Model Furui & Nakajima: Lattice de Teramond et al: AdS/CFT (preliminary) the conformality (no Q dependence) may imply that it's possible to use AdS/CFT correspondance to calculate strong interaction at low Q.

36 Transverse Spin (I): Inclusive g Structure Function and Moments Burkhardt Cottingham Sum Rule

37 g: twist 3, q g correlations Experiments: transversely polarized target SLAC E155x, (p/d) JLab Hall A (n), Hall C (p/d) g leading twist related to g1 by Wandzura Wilczek relation g WW 1 x, Q = g 1 x, Q x dy g1 y, Q y g x, Q =g WW x, Q g x, Q g gww: a clean way to access twist 3 contribution, quantify q g correlations.

38 Precision Measurement of gn(x,q): Search for Higher Twist Effects Measure higher twist, study quark gluon correlations. PRL 95, 1400 (005)

39 BC Sum Rule 0<X<1 :Total Integral P N ry p y r e v 3 He Q = 0 g x, Q dx=0 na i m reli Brown: SLAC E155x Red: Hall C RSS Black: Hall A E Green: Hall A E (preliminary) Blue: Hall A E01-01 (preliminary) BC = Meas+low_x+Elastic Meas : Measured x-range low-x : unmeasured low x part of the integral. Assume Leading Twist behaviour Elastic: From well known form factors (<5%)

40 BC Sum Rule BC satisfied w/in errors for JLab Proton,.8 violation seen in SLAC data P N v 3 He p y r e m reli in ary BC satisfied w/in errors for Neutron (though just barely in vicinity of Q=1) BC satisfied w/in errors for 3He

41 Spin Polarizabilities Higher Moments of Spin Structure Functions at Low Q

42 Higher Moments: Generalized Spin Polarizabilities (how nucleons respond to virtual photons) generalized forward spin polarizability γ Q = 0, Q TT, Q d 3 [ ] 16 M x 4 M x Q 0 Q 0 Q = x g x,q g x, Q dx Q Q 0 generalized longitudinal transverse spin polarizability δ LT 0 Q L T Q = 1 0,Q, Q LT d Q 16 M x = g x,q g x, Q ] d x [ Q 0

43 Neutron Spin Polarizabilities δ LT insensitive to resonance Significant disagreement between data and both PT calculations for δ LT Good agreement with MAID model predictions γ0 δ LT E94 010, PRL 93 (004) 15301

44 Proton Spin Polarizability γ 0p Only longitudinal data, model for transverse (g) γ 0 sensitive to resonance Large discrepancies with PT! PLB67, 1 (009) γ 0p Q6

45 Summary of Comparison with χpt Results on GDH sum, 1p, 1n, 1p n in general agree well with at least one of the PT calculations; δ LT puzzle: δ LT not sensitive to, one of the best quantities to test PT, data disagree with all calculations (HB PT, RB PT/ ) by several hundred %! A challenge to PT theorists. Very low Q data g1/g on n(3he) (E97 110), also g1 on p and D available soon (EG4) Recently approved: g on proton E08 07

46 Color Polarizabilities and Higher Twists Higher Moments of Spin Structure Functions at High Q d = X E X B /8 f = X E X B /

47 Color Polarizabilities and Higher Twists g p 1 g dx= n 1 p n 1 p n 1 ga s 3.58 = 1 s HigherTwists 6 ga s = 1 s 4 6 Q Q leading twist (twist ) M = a 4d f 9 Q 4 twist-4 leading twist, can be obtained from moments of g1 twist-3, can be obtained from moments of g higher twists

48 d(q) 1 d Q =3 0 x [ g x,q g x, Q ] d x PROTON WW Existing World Data on d: BROWN : E155, PLB. 553 (003) 18 BLACK : E94010, PRL. 9 (004) 0301 NEUTRON RED : RSS. PRL 98(007) Magenta: E99 117, PRC 70(004)06507 X. Zheng, October 17, 009

49 d(q) Some preliminary data MAID Model RED : RSS. (Hall C, NH3,ND3) arxiv: BLUE: E (Hall A, 3He) preliminary GREEN: E (Hall A, 3He) courtesy of V. Sulkosky very preliminary NEUTRON stat only other ongoing analysis: Hall C SANE for the proton Hall A dn X. Zheng, October 17, 009

50 Color Polarizabilities and Higher Twists Proton: nucl ex/05080 fit Q= GeV, p f = 0.160± pe = 0.08± p /M = 0.064± B =0.06± p 0.05 Neutron Phys.Lett.B613: ,005 f n =0.033±0.043 n6 / M 4 = 0.019±0.017 Proton Neutron f p n = 0.18± ne =0.033±0.09 n B= 0.001±0.016 fit Q= GeV, p n 6 Phys.Rev.Lett.93:1001,004 4 / M =0.1±0.0±0.01 For both proton and neutron, the value indicates the 4 term roughly cancel with 6 term, i.e. the total higher twist effect is small, down to Q=1 GeV. EG1b result in preparation, higher precision data are expected. 50

51 1 116 GeV CEBAF Add new hall Upgrade magnets and power supplies CHL- Enhance equipment in existing halls

52 Solenoid spectrometer for SIDIS at 11 GeV Proposed for PVDIS at 11 GeV GEMs

53 Polarized 3He Target Performance figure credit: C. Dutta

54 Polarized 3He Target Performance Several Target Groups: JLab, UVa, W&M, Temple, Kentucky, UNH,...

55 Summary Spin structure study full of surprises and puzzles A decade of experiments from JLab: exciting results valence spin structure, quark hadron duality spin sum rules, polarizabilities, and extraction of effective S test χpt calculations, δ LT puzzle precision measurements of g/d: higher twists first quasi elastic target SSA: photon to probe GPDs JLab plays a major role in recent experimental efforts shed light on our understanding of strong + QCD Bright future complete a chapter in spin structure study with 6 GeV 1 GeV Upgrade will greatly enhance our capability Goal: a full understanding of nucleon structure and strong interaction