Spin Physics at COMPASS

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Spin Physics at COMPASS
Christian Schill Universität Freiburg
on behalf of the COMPASS collaboration
Selection of Results Polarized DIS and polarized
distribution functions Direct measurements of
gluon polarization Dg/g Transversity
measurements
Rutherford Centennial Conference on Nuclear
Physics 2011 Manchester, August 8, 2011.
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COMPASS polarized target system
solenoid 2.5T dipole magnet
0.5T acceptance 180 mrad
3He 4He dilution refrigerator (T50mK)
6LiD or NH3 50/90 polarization 40/16
dilution factor Longitudinally or
transversely polarized
160 GeV long. polarized muon beam
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Longitudinal spin asymmetries in DIS

• Inclusive scattering

• Semi-inclusive scattering

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Proton A1 asymmetries

• incl. semi-incl. asymmetries for
  identified ps and Ks

K
p
incl.
K -
p
PLB 693 (2010) 227
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Deuteron A1 asymmetries
Inclusive semi-inclusive asymmetries
for identified ps and Ks
K
p
incl.
K -
p
PLB 680 (2009) 217224
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Proton deuteron g1(x,Q2)
From first moment of g1d

• unique kinematical domain
• important for global QCD analyses

• ?Sa00.330.030.05 (evol. Q28)

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Flavour distributions
PLB693 (2010) 227 PRD80 (2009) 034030
LO analysis of 5p5d asymmetries, DSS FF Line
NLO DSSV not including these data
D. de Florian, R. Sassot, M. Stratmann, and W.
Vogelsang Phys.Rev.Lett.101072001,2008
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Flavour asymmetry?
PLB693 (2010) 227

• Rather small effect, ?u gt ?d

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Flavour distributions Ds Ds ?
PLB693 (2010) 227
Varenna, July 2011
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Gluon polarization ?g
quarks gluons orbital
unknown
small 0.15
poorly known
How to measure ?g?
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Photongluon fusion (PGF)

• Gluon polarization is measurable in PGF

N
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Hadron production

• Analysis of hadron-pair asymmetries
• open charm single D meson AROMA
• cleanest process wrt physics background
• high-pT hadron pairs with Q2 gt 1 GeV2 LEPTO
• high-pT hadron pairs with Q2 lt 1 GeV2 PYTHIA

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Results for ?g

• All results compatible with zero!
• Confirmed by RHIC results in pp

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Transverse Spin Effects
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Transversity PDFs

• Transversity DTq(x)
• Transverse quark spin distribution in
  transversely polarized nucleon
• Measurement
• Semi-inclusive DIS
• transversely polarized target
• Collins azimuthal asymmetry

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Collins asymmetry
p
positive negative
x
PT
z
K
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Collins Asymmetry Global Fit

• Fit to COMPASS d, HERMES, BELLE (FF, ee-)
• Compared to COMPASS proton data

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Proton Sivers Asymmetry

• non-zero asymmetry for pos. hadrons on the proton

h
h-
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Summary

• COMPASS is a major player in the nucleon spin
  physics
• PRESENT
• Many important results with µ beam
• Lots of data are being analyzed, additional
  results to come
• FUTURE ( gt2012) COMPASS phase II
• GPD measurements (µ and µ- beams)
• DRELL-YAN measurements

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Proposal for 2012 and beyond

• Study of the Generalized Parton Distributions
  (GPDs)
• Natural extension of
• Form factors PDFs (spatial
  distributions) (momentum
  distributions)
• - Deeply Virtual Compton Scattering (DVCS)
• - Hard Exclusive Meson Production (HEMP) r, w,
  f, p, h

Access through
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Measurements of TMDs using Drell-Yan
Single Target Spin Asymmetry AUT
Drell-Yan (pp ) gives direct access to
chiral odd PDFs like the Sivers function
COMPASS Projected
Models for Sivers function
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Spares
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F2(x,Q2)
g1(x,Q2)
p
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Proton Deuteron g1(x) world data
Proton data - world
Deuteron data - world
Subm. Phys. Lett. B (hep-ex/1001.4654)
Phys. Lett. B 647 (2007) 8
COMPASS data 2007
COMPASS data 2002 2006
From first moment of g1d

• ?Sa0 0.330.030.05 (evol. to Q28)

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Test of the Bjorken sum rule
3 Par. NLO Fit
gA/gV 1.2694 0.0028 from neutron beta decay
(PDG)
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Results for ?g next to leading order
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Collins Asymmetries
new
proton
h
h-
h
deuteron
h-
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Transversity ?Tq(x) from hadron pairs

• Alternative couple ?Tq(x) to chiral odd
  2-hadron interference FF

cross-section asymmetry
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Two-hadron asymmetry

• large asymmetries
• interference FF and transversity sizable

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Comparison to a recent Fit

• Recent fit (dominated by HERMES, COMPASS p not
  yet in)

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GPDs at COMPASS kinematical coverage

• Unique features
• Both µ and µ- polarized beams

ZEUS H1
Luminosity limit
COMPASS
HERMES
JLab
L1032 with a 2.5m LH2 target
Complementary to HERA and HERMES/JLAB
Prediction for 1 year of data taking
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Generalized Parton Distributions and DVCS

• Cross section
• Experimental apparatus must detect
• Incident and scattered lepton
• Outgoing real photon
• Recoiling soft proton

known
ds ??Deep VCS Bethe-Heitler?2