1. New Results for Transverse Spin Effects at COMPASS
Anselm Vossen
ALU Freiburg
EINN 2007, Milos

2. Agenda Short Introduction to New results from 2002-2004 data
The Compass Experiment
Transverse Spin Effects at Compass
New results from data
Collins effect for K0
Transversity from Leading 2 Hadron Asymmetries
Sivers effect for K0

3. Compass Experiment (2004) high energy beam large angular acceptance
broad kinematical range
beam: 160 GeV/c
intensity 2·108 µ+/spill (4.8s/16.2s)
two stages spectrometer
Large Angle Spectrometer (SM1),
Small Angle Spectrometer (SM2)
tracking, calorimetry, PID
MuonWall
SM2
E/HCAL
E/HCAL
SM1
MuonWall
Polarised Target or
RICH
m beam

4. Transverse Data taken 2002 - 2004
10 Mill.
DIS events
transversely
polarized
deuteron target
~ 20% of the running time
RICH PID starting 2003

5. Measuring Transversity in SIDIS_ +
Collins effect m’ _
Lz-1 Lz _ + * * ? m _ + q
Helicity base:
independent amplitudes: _ +

6. Measuring Transversity
Transversity base:
Probabilistic interpretation  ↑  ↓ ↑   _ _ + = _ +
Difference in densities for ↑, ↓ quarks in ↑ nucleon
Azimuthal SSA on Transv. Pol. nucleon

7. 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:

8. Selection of SIDIS Events
COMPASS
transverse data
DIS cuts:
Q2 > 1 (GeV/c)2
0.1< y < 0.9
W > 5 GeV/c2
HERMES
K0 selection
2 charged particles
-0.3 (GeV/c)2 < M-Mk0 < 0.3 (GeV/c)2
pt rel. to p1+p2>25 MeV/c (Armenteros plot)
z > 0.2
pt/GeV/c
(pl1-pl2)/(pl1+pl2)

9. Collins, All Kaons

10. Transversity signal in 2 Hadron Correlations
Azimuthal Asymmetry measurement similar to Collins Asymmetry extraction
(z2P1+z1P2)/(z1+z2) =Ph S’ Ph
F2H m m’ z x g*

11. Event Selection Leading 2 Hadron
Q2>1
0.1 < y < 0.9
W > 5 GeV/c2
Leading 2h
h1 leading, h2 subleading in event
z1,2>0.1 (current fragmentation region)
xF 1,2> 0.1
0.25 < z1+z2< (no exclusive events)

12. Predictions for
Expecting Signal around -Mass

13. Compass Results
+-
- +
++
- - 

14. Kaon-Pion Combinations

15. Like Sign Kaon Combinations

16. Kaon-Kaon Combinations
K+/K-: /
K-/K+: /
K+/K+: /
K-/K-: / f

17. Sivers Effect + } x x
M. Burkardt, PRD, 66, (2002)

18. Sivers Effect m’
“Sivers angle” m S g* FS Ph x
Modulation in FS of the hadron distribution
Correlation between the nucleon spin and quark motion
Sivers effect ≠0 ⇒ Quark angular momentum ≠ 0 z
modulation in FS of the hadron distribution

19. All K0s

20. Summary/Outlook for transverse spin physics at COMPASS
So far
Precise COMPASS data, identified hadrons with deuteron target available
Collins and Sivers from All and Leading π+, π-, K+, K-, K0
2 Hadron Correlations from Pions and Kaons
Each Combination gives new constraints on Distribution and Fragmentation functions
6 more transverse Spin dependent Asymmetries in TMD SIDIS XSection
COMPASS currently measures with proton target!
Flavor separation

21. Backup

22. Single Hadron Collins, Sivers

23. Leading K0, Collins

24. Leading K0

25. XBj Binning

26. XBj Binning

27. Hadron Pairs Invariant Mass Spectra
all hadron pairs: M

28. The polarised 6LiD-Target
Two 60 cm long target cells
with opposite polarization
Superconducting
Dipole (0.5 T)
3He – 4He dilution
refrigerator
Vertex distribution:
T=50mK
Polarization: %
Dilution factor: 0.38
Transverse target polarization:
Reversed one a week

29. Ring Imaging Cherenkov Detector
Identification of π, K and protons
Cherenkov thresholds: π ≈ 3 GeV/c
K ≈ 9 GeV/c
p ≈ 17 GeV/c
2σ π/K separation at 43 GeV/c
6 m
5 m
3 m
Radiator:
C4F10
Mirror
wall
CsI
MWPC

30. Results for all hadrons

31. Results beyond Collins and Sivers

32. Results beyond Collins and Sivers

33. Results beyond Collins and Sivers

34. Particle ID in Data
Hadron identification is based on RICH response: several studies performed on the stability in time of the detector.
Cherenkov thresholds: π ~ 3 GeV/c
K ~ 9 GeV/c
p ~ 17 GeV/c
2 σ π/K separation at 43 GeV/c
In the hadron sample:
~76% pions
~12% kaons

35. Frame independent definition of φR
A. Baccetta,
hep-ph/
where P1T and P2T are the transverse
components of the hadron momenta
with:
we define:

36. Comparison with Theory
Model calculations for COMPASS kinematics (M. Radici, QCDN 06,
hep-ph/ ):
Model for transversity:
Soffer, Stratmann, Vogelsang,
P.R. D65 (02)
Korotkov, Nowak, Oganessian,
E.P.J. C18 (01) 639
Schweitzer et al.,
P.R. D64 (01)
Wakamatsu
P.L. B509 (01) 59
ΔTq(x)
xBj
Prediction for COMPASS:
small asymmetries
on the deuteron
ARS Mh

37. Predictions for Asymmetries on the Proton
Predictions for two-hadrons asymmetries on the proton at COMPASS:
Soffer, Stratmann, Vogelsang,
P.R. D65 (02)
Korotkov, Nowak, Oganessian,
E.P.J. C18 (01) 639
Wakamatsu
P.L. B509 (01) 59
- ARS
(M. Radici, hep-ex/ )