The Helicity Structure of the Nucleon

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The Helicity Structure of the Nucleon from Lepton
Nucleon Scattering
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The contemporary experiments
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News on the spin structure of the nucleon
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Deep Inelastic Scattering
Important kinematic variables
q
cross section
Spin 1
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How to measure Quark Polarizations

• Virtual photon g 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

Asymmetry definition
inclusive DIS only e info used
semi-inclusive DIS eh info used
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World data on inclusive DIS
Combine p and d to get n
or 3He

• What can we learn on the PDFs

Compass hep-ex/0609038
Hermes hep-ex/0609039
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HERMES Integrals

• Saturation in deuteron integral is assumed
• use only deuterium

from hyperon beta decay (a80.5860.031)
From neutron beta decay a31.2690.003
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Compass QCD-fit
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JLAB High x behaviour
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Polarised quark distributions
Correlation between detected hadron and struck
qf Flavor Separation
Inclusive DIS
Semi-inclusive DIS
Extract Dq by solving
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Polarized Quark Densities

• First complete separation of
• pol. PDFs without assumption on
• sea polarization

• Du(x) gt 0

Dd(x) lt 0
good agreement with NLO-QCD
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Results for DQ and DS
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Results for DQ and DS

• Earlier HERMES conclusions of
• unpolarized strange sea confirmed
• factor 2 smaller error bars

• Errors very sensitive to FF input

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NLO FIT to DIS SIDIS Data
D. De Florian et al. hep-ph/0504155
NLO _at_ Q210 GeV2

• SIDIS data improves description of all Dq,
  especially light sea
• Kretzer FF favor SU(3) symmetric sea, not so for
  KKP
• DS 30 in all cases

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Unpolarized Gluon Distribution

• big Q2-x lever arm
• very accurate G(x)

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How to measure DG

• fixed target experiments
• small Q2-x lever arm
• even sign of DG unknown

• Indirect from scaling violation

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The golden channels
Idea Direct measurement of DG
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The golden channels
Idea Direct measurement of DG
Isolate the photon gluon fusion process

• detection of hadronic final states with high pT
• high pT pairs of hadrons
• single high pT hadrons

hh vs. h
h more inclusive ? pQCD NLO calculations
(easier) possible
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COMPASS Results

• Channel
• Cuts

hh with Q2gt1GeV2

• Considered sub-processes
• LO-MC LEPTO

• 10 of stat. Q2gt1GeV2

0.330.07
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COMPASS Results

• Channel
• Cuts

hh with Q2lt1GeV2

• Considered sub-processes
• LO-MC PYTHIA 6.2

used for Dg/g extraction
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HERMES Results

• Considered sub-processes
• LO-MC PYTHIA 6.2

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HERMES Results

• HERMES cross section
• kT, pT standard values

• M.Anselmino et al. Phys. Rev. D71,074006

• Dg/g results for different final states

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Summary
talk by H. Avakian
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BACKUP SLIDES
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Measured Asymmetries
Deuterium
Proton
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Purities
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Kaon Multiplicities
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AAC low x-extrpolation
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DG/G 0.410.180.03 at x0.170.11 and
m22.1GeV2

• Old analysis
• pairs of high pT hadrons pTh1gt1.5GeV and
  pTh2gt0.8GeV
• MC used Pythia-5
• only polarized proton data
• no determination of systematic uncertainty due
  to MC-model