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Measurement of spin structure functions with CLAS at Jefferson Lab. Vipuli Dharmawardane. OUTLINE Formalism and experimental setup What can we learn at Jefferson Lab kinematics? Q 2 evolution of SSFs in and above the resonance region Spin physics at large x

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Measurement of spin structure functions with CLAS at Jefferson Lab


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    1. Measurement of spin structure functions with CLAS at Jefferson Lab Vipuli Dharmawardane OUTLINE • Formalism and experimental setup • What can we learn at Jefferson Lab kinematics? • Q2 evolution of SSFs in and above the resonance region • Spin physics at large x • Quark-hadron duality • Spin structure of nuclei?

    2. Inclusive electron scattering E’ E q (n,Q2) n = E-E’ W M Virtual photonNucleon  = - ½  = -1  = +1  = 0 Virtual photon asymmetries

    3. Double polarized inclusive electron scattering Pt g* Pe Nucleon A1 A2 Longitudinally polarized beam and target Unpolarized structure function F1

    4. Continuous Electron Beam Accelerator Facillity (CEBAF) Superconducting accelerator provides beams of unprecedented quality, with energies up to 6 GeV. Typical beam polarization ~80% C B A Each of the three halls offers complementary experimental capabilities and allows for large equipment installations to extend scientific reach.

    5. Continuous Electron Beam Accelerator Facillity (CEBAF) Superconducting accelerator provides beams of unprecedented quality, with energies up to 6 GeV. Typical beam polarization ~80% C B A CLAS HALL A Two high-resolution 4 GeV spectrometers HALL B Large acceptance spectrometer electron/photon beams HALL C 7 GeV spectrometer, 1.8 GeV spectrometer, large installation experiments

    6. New large acceptance spectrometer for photon-beam physics New focusing spectrometer for highest energies Jefferson Lab after the energy upgrade Double the beam energy CLAS12 Ten-fold increase in luminosity for large-acceptance electron beam measurements

    7. CEBAF LargeAcceptanceSpectrometer (CLAS) Calorimeter Time of flight counter Beam Drift Chambers Electron track (out bending) Cerenkov counter TOF counters Depending on the torus field electrons bend inward or outward Drift chambers Event in CLAS Cerenkov counters em. calorimeters • Large kinematical coverage • Detection of charged and neutral particles • Multi particle final states

    8. Polarized target Target insert • Target insert houses four cells • Each of the four cells is moved onto the electron beam using a stepping motor Ammonia beads • Polarized NH3 and ND3 • 5 Tesla Magnetic field • 1K LHe cooling bath • 12C and He targets to measure target dilution factor

    9. Target polarization • Beam was rastered over 12 mm diameter at the center →Only this portion was depolarized by beam-induced radiation damage • NMR coils are located on the outside of the cell →Primarily sensitive to the polarization of material outside the beam spot NMR coil 12 mm 15 mm proton polarization 70-80 % deuteron polarization 25-35% • Target polarization was extracted using data • For elastic scattering A|| is known • Donnelly and Raskin Ann. Phys., 169 247 (1986) deuteron For deuteron quasi-elastic peak

    10. Background subtraction He + foils + 15N Normalized counts W (GeV) Second target insert was used to collect data on solid 15N Fit to 15N data • 12C data were used to simulate 15N background • a and b were determined by fitting limited statistic 15N data with high statistic 12C data

    11. Asymmetry analysis hA2 unmeasured 0.27 < Q2(GeV2) < 0.32 • Models • A2→ Wandzura-Wilczek relation in the DIS region and the code MAID 2000 in the resonance region • F1→ Fit to world data

    12. Experimental status of SSF g1 • Existing data → Large Q2, small to moderate x • JLab → Large x precision measurements • JLab → Spin structure in non-perturbative regime JLAB @ 12 GeV JLAB JLAB @ 6 GeV Interesting physics in this relatively unmeasured region

    13. Spatial resolution of virtual photon Structure we observe by probing the nucleon by the virtual photon depends on the virtuality or Q2 of the photon λ Q2 small distances medium distances large distances elementary quarks constituent quarks Nucleon DeepInelastic Scattering Inelastic electron nucleon scattering can be viewed as the incoherent elastic scattering of the electron from free quarks withing the nucleon

    14. Inclusive Electron Scattering e’ p, h, pp γv e N*,△ N e’ e Need to measure exclusive processes in full phase space to separate resonances from each other and from non-resonant contributions. • Resonances are spin or momentum excited states of the nucleon • Three major resonance regions • About 20 known resonances N’

    15. Resonance region e’ p, h, pp γv e N*,△ N A3/2, A1/2,S1/2 For a resonance A1 and g1 can be written in terms of helicity amplitudes CLAS Preliminary N’ PhD Thesis K. Park Transition matrix elements between the ground nucleon state and the 3-quark configuration For a resonance

    16. Virtual photon asymmetry A1 : resonance region N(1520) Rapid change of helicity structure from A3/2 dominance at small Q2 to A1/2 dominance at high Q2 For a resonance

    17. Virtual photon asymmetry A1 : resonance region D(1232) SU(6) → Pure spin flip N D(1232) M 1+ Pure magnetic dipole transition A 1 = -0.5

    18. Spin structure function g1 for the proton Q2 = 0.0592 g1 Q2 = 0.707 Q2 = 0.292 Q2 = 0.0844 Q2 = 1.2 Q2 = 0.348 Q2 = 0.12 Q2 = 0.416 Q2 = 1.44 Q2 = 0.496 Q2 = 2.05 Q2 = 0.171 Q2 = 0.244 Q2 = 0.592 Q2 = 2.92 (GeV2) x

    19. Spin structure function g1 for the proton Q2 = 0.0592 g1 Q2 = 0.707 Q2 = 0.292 Q2 = 0.0844 Q2 = 1.2 Q2 = 0.348 27 Q2 bins Q2 = 0.045-5.4 GeV2 Q2 = 0.12 Q2 = 0.416 Q2 = 1.44 Q2 = 0.496 Q2 = 2.05 Q2 = 0.171 Q2 = 0.244 Q2 = 0.592 Q2 = 2.92 (GeV2) x

    20. First moment of g1(x,Q2) Elastic contribution excluded Q2→∞ Single partons pQCD Bjorken Sum Rule ? Dominated by baryon resonance excitations Closely related to the spin carried by quarks GDH slope Q2 = 0 GDH Sum rule Negative slope

    21. Generalized sum rules for G1 • Parton description • Operator Product Expansion Spin structure function g1(x,Q2) is related to the forward virtual compton scattering amplitude S1 (X. Ji et al., J. Phys. G 27, 127) Transition between parton and hadron degrees of freedom → calculable in Lattice QCD Q2 • Hadron description Inelastic part of S1 Calculable Measurable Includes the elastic contribution c calculable incPT

    22. First moment G1 for the proton and deuteron • Phenomenological Models • Burkert/Ioffe Resonance contribution pion electroproduction analysis • Soffer/Teryaev Interpolation of the integral (g1+g2)dx DIS (unmeasured) Parameterization of world data without elastic contribution DEUTERON G1 PROTON Q2 (GeV2)

    23. Γ1for the proton : low Q2 Ji and Osborne (HBcPT) G1 • Expand in chiral perturbation theory in a power series of pion mass • Calculations at next-to-leading order in momenta GDH Slope New experiment Data taken in 2006

    24. Spin structure function g1 in DIS Q2 → ∞ quarks behave as free particles Gluon radiation At finite but large Q2additional corrections Describable in pQCD Distribution functions in DGLAP equations Slow logarithmic Q2 dependence Q2 At moderate Q2higher twist effects Interactions between the struck quark and the other quarks in the nucleon Inversely proportional to Q2 → large at small Q2 +

    25. Higher twist effects Leading Twist contribution to g1 hTMC are calculable target mass corrections At JLab kinematics higher twist effects are non-negligible Q2 dependence in DGLAP equations hep-ph/0612360 • Non-perturbative effect and cannot be calculated in a model independent way • Extract h and polarized parton distributions by fitting data Analysis performed by Leader, Sidorov and Stamenov (LSS) hep-ph/0612360

    26. Higher twist effects Leading Twist contribution to g1 hTMC are calculable target mass corrections At JLab kinematics higher twist effects are non-negligible Q2 dependence in DGLAP equations Improvement in small x @ 12 GeV • Non-perturbative effect and cannot be calculated in a model independent way • Extract h and polarized parton distributions by fitting data Analysis performed by Leader, Sidorov and Stamenov (LSS) hep-ph/0612360

    27. Q2 dependence of g1/F1 • In the resonance region • different Q2 dependence • goes negative at D In DIS, both g1 and F1 show logarithmic Q2 dependence g1/F1 ratio relatively Q2 independent→ Q2 dependence of g1 at fixed x is very similar to F1 in the DIS region W = 2 GeV No DIS data at large x !

    28. Why study large x ? DIS HEP data base: CTEQ6M parameterization Q2 = 8 GeV2 Highest x accessible (DIS) with 6 GeV beam ≈ 0.6 At 12 GeV can go upto x ≈ 0.8 Proton dv uv Deuteron Valence quarks AAC parametrization Valence quarks dominate at large x

    29. Virtual photon asymmetry A1 at large x : Theory • x → 1 in pQCD Minimal gluon exchanges Spectator pair have opposite helicities dominant A1→ 1 Farrar and Jackson PRL 35, 1416 (1975) • SU(6) quark model S=1 and S=0 equi-probable S1/2 • Hyperfine perturbed quark model • makes S=1 pairs more energetic than S=0 pairs • At large x struck quark carry the spin of the nucleon • N. Isgur, Phys. Rev. D 59, 34013 s1/2 Symmetric WF HFP quark model • Duality • suppress transitions to specific resonances in the final state • Close and Melnitchouk, Phys. Rev. C 68, 035210

    30. Virtual photon asymmetry A1 Jlab/ Hall B Duality Hyperfine perturbed QM World data parameterized at Q2=10 GeV2 Proton and deuteron results are in better agreement with the HFP quark model

    31. Projected errors : 12 GeV upgrade

    32. Polarized parton distributions at large x Not well known at large x naïve LO analysis • JLab Hall A and Hall B results for Dd/d show no indication of a sign change • Disagree with pQCD predictions (assumes hadron helicity conservation)

    33. Polarized parton distributions at large x Not well known at large x 1 pQCD -⅓ Hyperfine-perturbed Quark model

    34. NLO analysis of data positively polarized negatively polarized x x Spin of the nucleon Quark polarization NLO analysis (AAC06) → DS ≈ (27±7) % Phys. Rev. D74, 14015 (2006)

    35. Gluon polarization as + 03 Dg through Q2 evolution (AAC06) fit All DIS data up to 2003 M. Hirai etal., Phys. Rev. D74, 14015 (2006) AAC03 DG = 0.5±1.27 All DIS data up to 2006 + recent RHIC data (PHENIX) AAC06 DG = 0.31±0.32

    36. NLO analysis of CLAS data Central values of distributions agree with existing world data After including CLAS data • Large increase in the Q2-lever arm allows for a great improvement on the precision of DG xDg hep-ph/0612360 At JLab kinematics higher twists are non negligible

    37. Projected errors : 12 GeV upgrade • Large increase in the Q2-lever arm allows for a great improvement on the precision of DG 12 GeV projected errors xDg At JLab kinematics higher twists are non negligible

    38. The quark structure of nuclei e’ e’ γv ? e γv = e F1p, g1p F1p|A, g1p|A Is the proportion of the spin contributed by its constituents change as the environment around nucleons change ? Spin of the nucleon Inclusive electron scattering

    39. Observation that structure functions are altered in nuclei stunned much of the HEP community 23 years ago Origin of the EMC effect A F 2 D F 2 Demonstrate the change in the quark gluon structure of the nucleon in medium x > 0.3: A valence quark in a bound nucleon has less momentum than in a free one valence quarks dominate x

    40. Is the EMC effect a valence quark phenomenon or are sea quarks involved? E772 Best probe of the sea : Drell-Yan experiments no clear excess of anti-quarks in nuclei

    41. EMC Effect - Theoretical Explanations What is it that alters the quark momentum in the nucleus? More than 1000 papers Multi-quark clusters dynamical rescaling combination of Fermi motion + nucleonic and pionic d.o.f No unambiguously accepted explanation of the EMC effect !

    42. The polarized EMC effect : Theory sensitive to quark polarization degrees of freedom Nuclear matter Valence + Sea Unpolarized Polarized g1p|A/g1p Valence only Valence only Quark-Meson Coupling Model Chiral Quark-Soliton model Cloet, Bentz and Thomas Phys. Rev. Lett.95, 052302 (2005) Smith and Miller Nucl-th/0505048

    43. The polarized EMC effect : Theory Valence + Sea Valence only • Valence only calculations consistent with Cloet, Bentz, Thomas calculations • Same model shows small effects due to sea quarks for the unpolarized case (consistent with data) Large enhancement for x < 0.3 due to sea quarks Sea is not much modified

    44. Can we measure it? For J > ½ new kinds of structure functions appear In the Bjorken limit 2J+1 quark distributions and structure functions Can scale nuclear matter results • Spin-independent case • Spin-dependent case : • Only few (valence) nucleons contribute to nuclear polarization • Have to do calculations for finite nuclei

    45. Quark distributions in 11B The nucleus is described using a relativistic shell model • Higher multipole distributions are greatly suppressed relative to the leading results multipole structure functions Cloet et al., Phys. Lett. B642:210 (2006) K = 1 spin-dependent distributions K = 3 spin-dependent distributions

    46. EMC effect in nuclei : Theory Cloet et al., Phys. Lett. B642:210 (2006) unpolarized A dependence of polarized quark distributions Polarized K=1 multipole Helicity 3/2 of the nucleus Medium modifications lead to a decrease of the fraction of the spin carried by quarks

    47. 7LiH as a polarized target P3/2 S1/2 • Both Li and H are polarized Not a problem: use existing free proton data How to go from g17Li to g1p|7Li ? Need to know the Polarization of the proton in 7Li Shell model combination of 1 unpaired proton + 2 paired neutrons and a closed S1/2 shell Cluster model Net 57% polarization of the proton in 7Li S = 1/2 triton orbiting in an L=1 state about the a cluster Green’s function Monte Carlo algorithm ≈ 59%

    48. Projected errors Assumes 11 GeV beam, 40% target polarization, 80% beam polarization and running for 70 days Systematic error target is < 5%

    49. SUMMARY • A broad physics program to study the spin structure of the proton, neutron and their excited states in progress at Jefferson Lab • Unprecedented quality of recent data pushing theory to new frontiers • Glimpse into unexplored large x region of valence quarks • No sign of hadron helicity conservation in x→1 dependence • JLab@12GeV will resolve 30 year old question about x→1 behavior of parton distributions • With 12 GeV, poised to make a brilliant contribution to our understanding of the Physics of nucleon and Nuclei: • Ideally equipped to study the large x region and solve the 23-year-old problem of the EMC effect

    50. ADDITIONAL SLIDES