Measurement of Transverse Spin Effects at COMPASS

1
Measurement of Transverse Spin Effects at COMPASS

• Anselm Vossen
• ALU Freiburg

On Behalf of the COMPASS Collaboration
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Outline

• Transversity
• Possible Measurements
• Transversity at COMPASS
• Theory predictions
• kT dependent Quark Distribution q(x,kT)
• Sivers Effect
• Sivers Asymmetries at COMPASS
• Theory predictions
• Outlook

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Structure of the Nucleon
Collins effect
m

?
?
?
m
_

q
Helicity base independent amplitudes
Hadronic Tensor
N
_

4
Measuring Transversity

• Transversity base
• Probabilistic interpretation

?
?
_
_


_

Difference in densities for ?, ? quarks in ?
nucleon HERMES, BELLE
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Collins Fragmentation - Modell
K. Bruhwel, TJNAF
Collins Effekt in String Fragmentation (X. Artru)
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Collins Effect in SIDIS

• Ph transverse momentum of the hadron
• S transverse spin of the struck
  quark

m
m
S
g
FC
Ph
x

• Collins angle ?C ?h - ?s

z

• Fragmentation Function for a transversely
  polarised quark into unpolarised hadron

spin dependent part Collins fragmentation
function
Number of Events
7
Collins Asymmetries
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Collins Comparison with Theory

• A.V. Efremov, K. Goeke and P. Schweitzer, PRD73
  (2006) 094025
• DTq from Chiral Quark Soliton Model
• Collins FF from Fit to HERMES Data
• ? unfav FF - fav FF
• ? DTu, DTd cancel in deuterium

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Transversity signal in 2 Hadron Correlations

• Azimuthal Asymmetry measurement similar to
  Collins Asymmetry extraction
• (z2P1z1P2)/(z1z2) Ph

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Results, z Binning
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Results, xBj binning
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Predictions 2H Asymmetries
Bacchetta, Radici Phys.Rev. D74 (2006) 114007
p-p- production
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Sivers Effect


x
M. Burkardt, PRD, 66, 114005(2002)
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Sivers Effect
m
Sivers angle
S
m
g
FS
Ph
x
z
Modulation in FS of the hadron distribution Corre
lation between the nucleon spin and quark motion

Sivers effect ?0 ? Quark angular momentum ? 0
modulation in FS of the hadron distribution
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Sivers Asymmetries
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Sivers Comparison with Theory

• M. Anselmino et al., PRD72 (2005) 094007
• including transverse momenta, FF from literature
• 13 parameter fit to HERMES and COMPASS 2002 data
  to extract the Sivers functions
• comparison with COMPASS 2002-2004 data ok

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Conclusion/Outlook

• Measurement of Transversity and Sivers effect
  provides new insights in soft QCD effects
• Experiments have shown unexpected effects
• COMPASS data on deuteron is important to extract
  DTd and D0Td
• COMPASS measures this year with proton target

Polarization data has often been the
graveyard of fashionable theories. If
theorists had their way, they might
just ban such measurements altogether
out of self-protection.
J.D. Bjorken St. Croix, 1987
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COMPASS 2004 Setup

• high energy beam
• Large angular acceptance
• Broad kinematical range

two stages spectrometer Large Angle Spectrometer
(SM1), Small Angle Spectrometer (SM2) tracking,
calorimetry, PID NEW TECHNOLOGIES
MuonWall
SM2
E/HCAL
E/HCAL
SM1
MuonWall
RICH
Polarised Target
m beam
2002-2004 6LiD (Deuteron) dilution factor f
0.38 polarization PT 50
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The RICH detector
photon detectors

• threshold momenta
• pp 2 GeV/c
• pK 9 GeV/c
• pP 17 GeV/c

fully efficient for transverse data in 2003 and
2004
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COMPASS Transversly Polarised Target
two 60 cm long cells with opposite polarization
(systematics)
dN/dz
4000
2000
0
0
-1000
1000
zvtx (mm)
2002-2004 6LiD (Deuteron) dilution factor f
0.38 polarization PT 50 during data taking
with transverse polarization polarization
reversal in the 2 cells after 5 days
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COMPASS Experiment _at_ CERN
COmmon Muon and Proton
Apparatus for
Structure and
Spectroscopy
SPS

• fixed target experiment at the CERN SPS
• 240 physicists from 28 Institutes, 11 Countries

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• ?h azimuthal angle of the hadron
• ?s azimuthal angle of the transverse
  spin of the struck quark
• ?s p - ?s
• ?s azimuthal angle of the transverse
  spin of the initial quark

• Collins angle ?C ?h - ?s ?h
  ?s - p

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Predictions for 2H Signal
Radici _at_ QCD N06, Bacchetta, Radici
hep-ph/0608037
ARS
ARS
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Results, MInv Binning
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Transverse Spin _at_ COMPASS

• Transverse Spin Measurements provide new
  insights into Spin Structure and Quark dynamics
  inside the nucleon
• COMPASS provides key measurements

Collins (Transversity)
Sivers (kT dependent unpol. PDF)
M. Anselmino et al., PRD72 (2005) 094007
Efremov, Goeke, Schweitzer, PRD73 (2006) 094025
xBj
xBj