Exploring Nucleon Spin Structure in Deep-Inelastic Scattering at HERMES

1. Measurement of the spin structure functions g1p,d,n and latest results on quark helicity distributions from Deep-Inelastic Scattering at HERMES Polina Kravchenko (DESY) for the collaboration 10 September MENU 2007

2. Introduction The Spin Structure of theNucleon • Quark spin • Gluon spin • Orbital angular momentum

3. Introduction The Spin Structure of the Nucleon Quark Parton Model Formalism γ Partons Nucleon Selects quarks with spin anti-parallel to the nucleon • Virtual photon can only couple to quarks of opposite helicity • Select q-(x) or q+(x) by changing the orientation of target nucleon spin • or helicity of incident lepton beam

4. Introduction The Spin Structure of the Nucleon Quark Parton Model Formalism γ Partons Nucleon γ Partons Nucleon Selects quarks with spin parallel to the nucleon • Virtual photon can only couple to quarks of opposite helicity • Select q-(x) or q+(x) by changing the orientation of target nucleon spin • or helicity of incident lepton beam Spin Independent Structure Function F1 Spin Dependent Structure Function g1 Δq

5. Introduction The Spin Structure of the Nucleon • HERMES result : • g1 for the p,d,n from inclusive longitudinal DSA • -> result on • Δq form 5-flavor purity extraction • Δs from isoscalar method

6. Deep Inelastic Scattering Inclusive scattering: detected scattered lepton Virtual-photon kinematics: Fraction of nucleon momentum carried by struck quark: Semi-inclusive scattering: detected scattered lepton and produced hadrons Fraction of virtual-photon energy carried by produced hadron h:

7. HERMES experiment Polarized lepton beam of The HERA ring <Pb> ~ 54% Hermes spectrometer Resolution: Δp/p~2%, ΔΘ<1 mrad PID: 98% lepton identification with <1% hadron contamination Excellent separation ofπ, K and p via RICH Polarized gas target <Pt> ~ 85(-84)%

8. leptonicpart of the xsection  independent of proton structure  can be calculated explicitely hadronic part of the xsection  contains info on proton structure + strong interaction effects DIS cross section

9. Hadronic tensor • parametrised by structure functions F1 and F2 are unpolarized structure functions g1and g2are polarized structure functions b1 ..b4are polarized structure functions for spin 1

10. kin. fact. param. measured param. How to measure g1: inclusive DIS Measured inclusive asymmetries: Relation to structure function g1: kinematic factors Unfolding of radiative corrections: • Measured events have to be corrected for: Background tail (radiation from (quasi)-elastic) Radiation from DIS and detector smearing • The smearing of events is simulated through a Monte Carlo which includes a full detector description and a model for the cross-section • The approach is independenton the model for the asymmetry in the measured region

11. D data is the most precise so far • Statistical uncertainties are diagonal elements of covariance matrix • Systematic unc. are dominated by target and beam polarization g1 result for p and d

12. g1 result for neutron ωD = 0.05 ±0.01 • g1nnegative everywhere except at very high-x • Low-Q2 data tends to zero at low-x Contrary to SMC data at high Q2

13. Q2=5 GeV2, NNLO in MS scheme g1 Integrals. Assuming saturation in the deuteron integral theory theory from hyperon beta decay (a8=0.586±0.031) x SU(3) symmetry from neutron beta decay (a3=1.269±0.003)

14. Measurement of Δq : Semi-Inclusive DIS Use correlation between detected hadron and struck quark -> ‘Flavor separation’ Dqh is a measure of the probability that a quark of flavor q will fragment into a hadron of type h Purity is a conditional probability that a hadron of type h observed in the final state originated from a struck quark of flavor q

15. Method of Δq extraction

16. Method of Δq extraction Measured Asymmetry vector contains an asymmetry for each hadron from each target

17. Method of Δq extraction Simulated Purities extracted from LUND string model MC simulation (LEPTO+JETSET) tuned to HERMES unpolarized data

18. Method of Δq extraction Extracted System of asymmetries and purities solved through 2 minimization of polarizations

19. The published HERMES 5-flavor Δq extraction • First complete separation • of pol.PDFs without assumption • on sea polarization • Δu(x) > 0 • polarized parallel to the proton • Δd(x) < 0 • polarized anti-parallel to the proton • Δu(x) and Δd(x) good agreement • with LO-QCD fit to inclusive data • All the sea quark polarizations are • consistent with zero in a measured • region

20. Isoscalar extraction of ΔS ⇒the largest sensitivity to the strange quark polarization • Need a longitudinally polarized deuterium target • - strange quark in proton and neutron identical • - fragmentation simplifies • All needed information can be extracted from HERMES data alone: • - inclusive A1d(x,Q2) and kaon A1dK(x,Q2) double spin asymmetries • - kaon multiplicities • Only assumptions used: • - isospin symmetry between proton and neutron • - charge-conjugation invariance in fragmentation

21. Isoscalar extraction of ΔS: • Extract isoscalar combinations of ΔQ(x) and ΔS(x)

22. Isoscalar extraction of ΔS: • Extract isoscalar combinations of ΔQ(x) and ΔS(x) • Inclusive asymmetry A1,dand kaon asymmetriesA1K+andA1K-

23. Isoscalar extraction of ΔS: • Extract isoscalar combinations of ΔQ(x) and ΔS(x) • Inclusive asymmetry A1,d and kaon asymmetries A1K+and A1K- • Inclusive purities are simple combinations of unpolarized PDFs. • Kaon purities can be computed from the unpolarized kaons multiplicity • assuming only charge symmetry in fragmentation

24. Isoscalar extraction of ΔQ and ΔS: Preliminary • No dependence on MC simulation • Result is consistent with full • 5 flavor extraction • No hint of negative strange sea in measured range Preliminary Inclusive DIS

25. Conclusions Inclusive DIS • Hermes has measured g1 for proton and deuteron for 0.0041<x<0.9 and 0.18<Q2<20GeV2 • Proton data precision is comparable with CERN and SLAC • Deuteron data is the most precise so far • The deutron integral is observed to saturate Phys.Rev.D75(2007)012007 • HERMES did complete separation of pol.PDFs without assumption on sea polarization • Δu(x) and Δd(x) good agreement with LO-QCD fit to inclusive data • All the sea quark polarizations are consistent with zero • In measured range 0.023<x<0.6 : • Phys.Rev.D71(2005)012003 Semi-inclusive DIS • Isoscalar method on d target provides an extraction of strange sea • No dependence on MC simulation • Result is consistent with full 5 flavor extraction • No hint of negative strange sea in measured range Semi-inclusive DIS No MC