Transcript Messung transversaler Spinstrukturen bei COMPASS

Measurement of Transverse
Spin Effects at COMPASS
Anselm Vossen
ALU Freiburg
On Behalf of the COMPASS Collaboration
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
Structure of the Nucleon
Collins effect
m’
m
Lz
+
*
Helicity base:
independent amplitudes:
~ q, q
+
Lz-1
 HD
 h
T 1 q
?
q
__
*
_
 q
Hadronic
T Tensor
N
+
_
Measuring Transversity
• Transversity base:
• Probabilistic interpretation

+
+
_
T q
_

=

↑
↑
↑
↑

_

↓
↓
↑
↑

Difference in densities for ↑, ↓ quarks
in ↑ nucleon
0
h

D
HERMES, BELLE:
T q , T q  0
Collins Fragmentation - Modell
http://cerncourier.com/main/article/44/8/19/1
K. Bruhwel, TJNAF
Collins Effekt in String Fragmentation
(X. Artru)
Collins Effect in SIDIS
Ph transverse momentum of the hadron
S’ transverse spin of
the struck quark
“Collins angle”
m’
m
S’
* 
C
Ph
C = h - s’
x
z
• Fragmentation Function for a transversely
polarised quark into unpolarised hadron
Dhq  Δ 0TDhqsin ΦC
spin dependent part : “Collins fragmentation function”
Number of
Events:

Nh C  = Nh0  1  AhC  sinC

A Coll 
2
0 h
e

Δ
q

Δ
q q T TDq
2
h
e

q

D
q q
q
Collins Asymmetries
Collins: Comparison with Theory
A.V. Efremov, K. Goeke and P. Schweitzer, PRD73 (2006) 094025
Tq from Chiral Quark Soliton Model
• Collins FF from Fit to HERMES Data
 unfav FF ~ - fav FF
 Tu, Td cancel in deuterium
COMPASS 2002-2004
Transversity signal in 2 Hadron
Correlations
• Azimuthal Asymmetry measurement
similar to Collins Asymmetry extraction
• (z2P1+z1P2)/(z1+z2) =Ph
m’
m
*
S’
2H
Ph
x
z
Results, z Binning
Results, xBj binning
Predictions 2H Asymmetries
Bacchetta, Radici: Phys.Rev. D74 (2006) 114007
p-p- production
Sivers Effect
+
}
x
x
M. Burkardt, PRD, 66, 114005(2002)
Sivers Effect
S = h - s
C = h - s' = h  s - p
“Sivers angle”
m’
m
*
S
S
Ph
x
Modulation in S of the hadron distribution
z
Correlation between the nucleon spin and quark motion
Sivers effect ≠0 ⇒ Quark angular momentum ≠ 0
modulation in S of the hadron distribution

Nh  S  = Nh0  1  AhS  sin S

A Siv 
2
T
h
e

Δ
q

D
q q 0 q
2
h
e

q

D
q q
q
Sivers Asymmetries
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
COMPASS 2002-2004
Δ u- Δ d
T
0
T
0
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 Td and 0Td
J.D. Bjorken
St. Croix, 1987
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.
• COMPASS measures this year with proton target
•
•
•
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
SM2
NEW TECHNOLOGIES E/HCAL
MuonWall
E/HCAL
SM1
MuonWall
RICH
m beam
Polarised Target
2002-2004: 6LiD (Deuteron)
dilution factor f = 0.38 polarization PT =50%
The RICH detector
5m
photon
detectors
C4F10
threshold momenta:
●
pp = 2 GeV/c
●
pK = 9 GeV/c
●
pP =17 GeV/c
fully efficient for transverse data
in 2003 and 2004
COMPASS Transversly Polarised
Target
two 60 cm long cells
with opposite polarization (systematics)
dN/dz
4000
2000
0
-1000
0
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
COMPASS Experiment @ 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
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
Predictions for 2H Signal
Radici @ QCD N’06, Bacchetta, Radici: hep-ph/0608037
ARS
ARS
Results, MInv Binning
Transverse Spin @ COMPASS
• Transverse Spin Measurements provide new insights into Spin
Structure and Quark dynamics inside the nucleon
• COMPASS provides key measurements
Sivers (kT dependent unpol. PDF)
COMPASS 2002-2004
Predictions
M. Anselmino et al., PRD72 (2005) 094007
Efremov, Goeke, Schweitzer, PRD73 (2006) 094025
Collins (Transversity)
COMPASS 2002-2004
Predictions
xBj
xBj