Spin Structure of the Nucleon: quark spin dependence R. De Vita Istituto Nazionale di Fisica Nucleare Electromagnetic Interactions with Nucleons and Nuclei EINN005 Milos September, 005
The discovery of the spin 9 O. Stern and W. Gerlach discover the electron spin. The electron magnetic moment is consistent with the Bohr magneton 933 O. Stern, I. Estermann and O.Frisch repeat the experiment for the proton. For the total magnetic moment µ p they find a value between and 3 nuclear magnetons (not as has been previously expected) The proton is not an elementary particle but has an inner structure
What did we learn since then? Understanding the nucleon spin structure and determining the contribution of the elementary constituents to the nucleon spin is a primary issue in hadronic physics First attempt to explain the nucleon spin in the naive CQM framework Σ = u + V d V Naive picture ruled out by EMC measurement of Σ=0.3±0.03 ± 0.09 = Σ + G + Σ = u V + d V L q + q S Importance of sea quarks and gluons Quark orbital angular momentum needs to be included for a full picture
Experimental Facilities First pioneering measurement of spin structure functions performed at SLAC in 975 with E80; activity continued in the 90s on polarized protons and deuterons First measurements at x<0. performed by EMC at CERN; results yielded at the breakdown of the naive quark model spin crisis Activity at CERN continued by SMC Presently study of the nucleon spin structure continues: HERMES at DESY COMPASS at CERN Hall A, B, C at JLab
HERMES @ DESY Electron/Positron beam (7 GeV) on fixed target (He,H,D) Kinematic Range: 0.0 x 0.8 at Q GeV and W GeV θ x 75 mrad, 40 mrad θ y 40 mrad Reconstruction: δp/p.0 -.0%, δθ.0mrad
COMPASS @ CERN 60 GeV Muon Beam on LiD polarized target Dual Target Cell: opposite cell polarization +54%, -50% Kinematic Range: 0.003 x 0.6 at Q = -00 GeV
CLAS Hall AC Short Orbit Spectrometer (SOS) Jefferson Lab High Momentum Spectrometer (HMS) Continuous Electron Beam high longitudinal polarization energy range 0.75 5.9 GeV current range 0. na 00µA delivered simultaneously to the three Halls large acceptance detector NH 3, ND 3 targets NH 3 /ND 3 Polarized Target P H 75 % P D 30 % Ongoing experimental program to measure spin observables in all three Halls A,B,C
Kinematical Coverage HERMES COMPASS JLAB Beam e ± µ e - Target He, H, D LiD 3 He,NH 3,ND 3 x 0.0-0.6 0.003-0.6 0.-0.7 Q 5 00 0.05-4.5 different experimental setups coverage of complementary kinematical regions Q (GeV ) 0 0 0 - COMPASS HERMES JLAB 0-3 0-0 - x
How to access the quark helicity distributions dσ de' dω = α MQ 4 E E' L µν W µν µν W µν = W + W µν W W µν spin 0 = iε µνλσ q ν λ spin S σ g ( x,q )) + ν ( p qs S qp ) g ( x,q ) σ σ the cross section depends on the spin structure functions g and g these can be separated varying the nucleon spin direction with respect to the virtual photon g ( x) x ) = + [ q ( x ) q ( x )] = e e q( x ) q q q q WW HT g ( x ) = g ( x ) + g ( x ) = g( x ) + g( y )dy / y + x g HT ( x )
Other important observables Measured asymmetries A A = = σ σ σ σ σ + σ σ + σ = D = ( A + ηa ) d Photon asymmetries A σ σ σ + σ ( A + ζa ) / 3 / = A / 3 / σ = σ The nucleon structure functions can be written in terms of photon asymmetries g g Q Mx ( x, Q ) = A + A F ( x, Q ) Q Q Q ( x, Q ) = A A F ( x, Q ) Q + 4M + 4M x x Mx Q LT T
g structure function 3 ( p n g + g ) d g = wd p d n g > g > g Significant improvement in data accuracy for both proton and deuteron Integral over the measured x range g g p d = 0. 46 ± 0. 003 ± 0. 0074 = 0. 045 ± 0. 005 ± 0. 007
g at low x new COMPASS high precision data important for extrapolation at low-x new fit of world data including COMPASS results gives improved accuracy on Σ
A at large x large x region provides information on valence quark dynamics prediction from CQM and pqcd pqcd for x SU(6): A p = 5/9 A n =0 pqcd: A p = A n = CQM with broken SU(6): A p = A n = SU(6) with different x-behavior depending on the scheme HIGH PRECISION DATA NEEDED
A at large x First precision on A n data at high x PRL 9, 0004 (004).7 GeV < Q < 4.8 GeV W > GeV Comparison with model calculations SU(6) symmetry CQM pqcd predictions PDF fits (LSS) Statistical model Chiral Soliton model Local duality model Cloudy bag model Crucial input for pqcd fit to PDF
A at large x Isgur, PRD 59, 03403 (003) Close and Melnitchouk, PRC 68, 0350 (003) } Proton Deuteron
Polarized Quark Distributions Combined analysis of A n and A p results Valence quark dominating at high x u quark spin as expected d quark spin stays negative! disagree with pqcd model calculations assuming hadron helicity conservation indication for quark orbital angular momentum ( u+ u )/(u+u ) ( d+ d )/(d+d ) 0.5 0 E997 HERMES CQM pqcd with HHC PDF fits (LSS) Statistical model Chiral Soliton model Consistent with valence quark models,statistical model or pqcd PDF fits 0.5 0 0. 0.4 0.6 0.8 x
Quark distributions q Preliminary results from CLAS/Jlab based on proton and deuteron target data Assumptions: no sea quarks naïve parton model correction for deuteron D state and Fermi motion CLAS/JLab Preliminary
Semi-Inclusive DIS Correlation between detected hadron and struck q f Flavor - Separation Inclusive DIS: _ Σ= ( u + u + d + d + s + s) Semi-inclusive DIS: _ u; u; d; d; s; s ( q f ) q f : D fh (z): A h (Polarized) quark distributions fragmentation functions giving the probability that a quark of flavor f fragments into a hadron of type h ( x,q ) σ = σ h / h / σ + σ h 3/ h 3/ + R(x,Q = + γ ) f e f f e q f q f f (x,q (x,q ) ) dzd dzd h f h f (z,q (z,q ) )
Semi-Inclusive DIS Deuteron Proton non zero A asymmetry for π +, π -, and K + production K - asymmetry compatible with zero ( sea quarks) fit to extract q/q
Semi-Inclusive DIS qf (x) q (x) f = q q + f f (x) (x) q q f f (x) (x) u(x)/u(x) > 0 u quark polarized parallel to the nucleon spin d(x)/d(x) < 0 d quark polarized antiparallel to the nucleon spin u(x)/u(x) d(x)/d(x) 0 s(x)/s(x) 0 sea quarks unpolarized
Hadron-Parton Transition Region Lepton scattering allows the study of the transition between the partonic and hadronic regime by varying the wavelenght (Q ) of the probe Q evolution of spin structure functions higher twist contribution to spin structure functions duality in spin structure functions baryon resonance physics talk by A. Fantoni test of Chiral perturbation theory predictions...
Q evolution of structure functions Γ ( ) ( Q g x,q ) = dx high precision data from 0.05 GeV up to.5 GeV in Q strong transition from positive values at high Q to negative values for low Q contribution from baryon resonances comparison to χpt predictions preliminary
Bjorken Sum Rule Γ p Γ n = ( p n g g ) dx Q g 6 a combined analysis of Hall A and Hall B measurements consistent with previous SLAC data and other isospin 3/ contribution cancel out better agreement with χpt than for separated proton and neutron integrals preliminary
g n (x) and HT contribution JLab-HallA Precision measurement of g n, 0.57 < Q <.34 GeV,W > GeV,at x ~ 0. Direct comparison to twist- g ww prediction using world g n data. Quantitative measurement of higher twist effects provides information on nucleon structure beyond simple parton model (e.g. quark-gluon correlations).
Future Perspectives Experimental activity continues... More data expected in the near future from COMPASS, HERMES, and Jefferson Lab Completion and extension of already ongoing programs Projects for new facilities and upgrade of existing ones being developed RHIC SPIN Jefferson Lab GeV upgrade PAX @ GSI
Quark Contribution to Proton Spin Structure at RHIC Parity violating single spin asymmetry in W productions W physics requires upgrades in PHENIX and STAR, planned to be complete by 009 and 00 respectively
JLab @ GeV Anticipated upgrade of Jefferson Lab to () GeV beam with new spectrometer JLab at GeV, 5 µa Q 0 (GeV/c ) W GeV Definitive measurement of A n at high-x with polarized 3 He
Summary and Outlook precise measurements of all components of the nucleon spin are necessary to complete our understanding of the nucleon structure high precision data on quark helicity distributions now available over a large kinematical domain flavor separation achieved through semi-inclusive scattering and measurement on different targets mapping of hadron-parton transition region new data coming in the near future new projects being developed after 30 years of dedicated experiments we have learned a lot, but there is still a lot to learn...