New Results for Transverse Spin Effects at COMPASS

1
New Results for Transverse Spin Effects at
COMPASS

• Anselm Vossen
• ALU Freiburg

EINN 2007, Milos
2
Agenda

• Short Introduction to
• The Compass Experiment
• Transverse Spin Effects at Compass
• New results from 2002-2004 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 2108 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 taken2002 - 2004
10 Mill. DIS events
5
Measuring Transversity in SIDIS
Collins effect
m

?
?
?
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
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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
8
Selection of SIDIS Events
COMPASS 2002-2004 transverse data

• DIS cuts
• Q2 gt 1 (GeV/c)2
• 0.1lt y lt 0.9
• W gt 5 GeV/c2

HERMES

• K0 selection
• 2 charged particles
• -0.3 (GeV/c)2 lt M??-Mk0 lt 0.3 (GeV/c)2
• pt rel. to p1p2gt25 MeV/c (Armenteros plot)
• z gt 0.2

pt/GeV/c
(pl1-pl2)/(pl1pl2)
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Collins, All Kaons
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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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Event Selection Leading 2 Hadron

• Q2gt1
• 0.1 lt y lt 0.9
• W gt 5 GeV/c2
• Leading 2h
• h1 leading, h2 subleading in event
• z1,2gt0.1 (current fragmentation region)
• xF 1,2gt 0.1
• 0.25 lt z1z2lt0.9 (no exclusive events)

12
Predictions for
Expecting Signal around ?-Mass
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Compass Results
??- ?- ?
?? ?- ?-
?
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Kaon-Pion Combinations
K-? ?K-
K?- ?-K
K
K
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Like Sign Kaon Combinations
K? ?K
K-?- ?-K-
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Kaon-Kaon Combinations

• K/K- 0.0273 /- 0.0552
• K-/K -0.0890/- 0.0583
• K/K0.2482 /- 0.1390
• K-/K- 0.4257 /- 0.1798

f
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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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All K0s
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Summary/Outlookfor transverse spin physics at
COMPASS

• So far
• Precise COMPASS data, identified hadrons with
  deuteron target available
• Collins and Sivers from All and Leading p, p-,
  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

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Backup
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Single Hadron Collins, Sivers
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Leading K0, Collins
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Leading K0
25
XBj Binning
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XBj Binning
27
Hadron Pairs Invariant Mass Spectra
all hadron pairs 5.3 M
28
The polarised 6LiD-Target
Vertex distribution
T50mK
Polarization 50 Dilution factor 0.38
Transverse target polarization Reversed one a
week
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Ring Imaging Cherenkov Detector
Identification of p, K and protons Cherenkov
thresholds p 3 GeV/c K 9 GeV/c p
17 GeV/c
2s p/K separation at 43 GeV/c
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Results for all hadrons
31
Results beyond Collins and Sivers
32
Results beyond Collins and Sivers
33
Results beyond Collins and Sivers
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Particle ID in 2003-2004 Data

• Hadron identification is based on RICH response
  several studies performed on the stability in
  time of the detector.
• Cherenkov thresholds p 3 GeV/c
• K 9
  GeV/c
• p 17
  GeV/c
• 2 s p/K separation at 43 GeV/c
• In the hadron sample
• 76 pions
• 12 kaons

35
Frame independent definition of fR

• A. Baccetta,
• hep-ph/0608037

where P1T and P2T are the transverse components
of the hadron momenta
with
we define
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Comparison with Theory
Model calculations for COMPASS kinematics (M.
Radici, QCDN 06, hep-ph/0608037)
Model for transversity

• Soffer, Stratmann, Vogelsang,
• P.R. D65 (02) 114024
• Korotkov, Nowak, Oganessian,
• E.P.J. C18 (01) 639
• Schweitzer et al.,
• P.R. D64 (01) 034013
• Wakamatsu
• P.L. B509 (01) 59

?Tq(x)
xBj
Prediction for COMPASS

• small asymmetries
• on the deuteron

ARS
Mh
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Predictions for Asymmetries on the Proton
Predictions for two-hadrons asymmetries on the
proton at COMPASS

• Soffer, Stratmann, Vogelsang,
• P.R. D65 (02) 114024
• Korotkov, Nowak, Oganessian,
• E.P.J. C18 (01) 639
• Wakamatsu
• P.L. B509 (01) 59

- ARS
(M. Radici, hep-ex/0608037)