Transcript Collins and Sivers asymmetries on the deutron from COMPASS

The International Workshop on Transverse Polarisation Phenomena in
Hard Processes
Collins and Sivers asymmetries
on the deuteron from
COMPASS data
Igor Horn, University of Bonn
on behalf of the
COMPASS Collaboration
Villa Olmo (Como), 7−10th. September 2005
Outline
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Motivation
Experimental configuration
Data selection
Results from 2002 beamtime
Phenomenological models
Statistical accuracy with 2003/4 data
Summary and outlook
Physics at COMPASS
With the muon beam
Gluon Polarization G/G
transverse spin distribution
functions Tq (x)
With hadron beams
Primakoff-Reactions
- polarizability of  and K
glueballs and hybrids
Flavor dependent polarized
quark helicity densities q(x)
 physics
Diffractive VM-Production
charmed mesons and baryons
- semi-leptonic decays
- double-charmed baryons
Spin structure functions
3 distribution functions are necessary to describe the spin
structure of the nucleon at LO (twist-2), all of equal
importance!
q(x)
q(x)
Tq(x)
decouples from leading twist DIS because helicity of quark must flip;
not observable in inclusive DIS;
How to measure ΔTq(x)?
epe’X
Impossible in DIS
ppl+l-X
epe’hX
Direct
Measurement
Convolution with
fragmentation function
1) Semi-inclusive DIS (COMPASS and HERMES)
2) Scattering of nucleons: Drell-Yan (RHIC)
3) Scattering of nucleon and antinucleon: Drell-Yan (GSI)
Developments in transversity
In the last ten years:
• Measurements at DESY, CERN, Jlab and RHIC
• Great development in the theory of transversity: transversity is not
suppressed at high energies;
• Remarkable role of ΔTq(x), complementary to Δq(x).
In the last couple of years:
• Role of the kT dependent distribution functions clarified (Cahn and Sivers
effects, …).
Key features of transversity:
- Probes relativistic nature of quarks
- No gluon analog for spin-1/2 nucleon
- Different Q2 evolution and sum rule than Δq(x)
Transverse spin physics at
COMPASS
Possible quark polarimeters:
• Azimuthal distribution of single hadrons
• Azimuthal dependence of the plane containing
hadron pairs (see A.Mielech ’s talk)
• Measurement of transverse polarization of spin ½
baryons, e.g.  hyperon (see A. Ferrero’s talk)
Azimuthal asymmetries of single
hadrons
1) Collins effect predicts an azimuthal asymmetry in
fragmentation of transversely polarized quarks with finite
transverse momentum to un-polarized hadrons
2) Sivers effect predicts an azimuthal asymmetry from the unpolarized quarks, -- an azimuthal modulation of quark
transverse momentum for a transversely polarized nucleon.
Collins and Sivers angles
Collins:
AColl ~ sin C
C = h - S’ = h + S –
Sivers:
S
ASiv ~ sin S
= h -  S
S -- azimuthal angle of spin vector
of initial-state quark/nucleon
S’ -- azimuthal angle of spin vector
of fragmenting quark
with S’’ =  -S (spin flip)
h -- azimuthal angle of hadron momentum
Collins and Sivers effects
Collins and Sivers effect distinguishable
AUTsin( - s‘ )
AColl
AUTsin( - s )
sin  Coll
UT
A


DNN  f  P
sin  Siv
UT
A
ASiv 

f P
2
h
e


q


D
a a T a a
2
h
e

q

D
a a a a
2
h
e

f

D
 a a 1T a a
2
h
e

q

D
a a a a
Experimental configuration
trigger-hodoscopes
DW45
SM2 dipole
straws
Polarized Target
Muon-filter2,MW2
RICH_1 HCAL1
Gem_11
SM1 dipole
ECAL2,HCAL2
MWPC Gems Scifi
Muon-filter1,MW1
straws,MWPC,Gems,SciFi
Veto
Silicon
SciFi
Beam:
Luminosity:
Gems,SciFi,DCs,straws
Micromegas,DC,SciFi
2 . 108 µ+/ spill (4.8s / 16.2s)
~5 . 1032 cm-2 s-1
Beam momentum: 160 GeV/c
Beam polarisation: ~76%
The frozen spin Target
– 4He Dilution
refrigerator (T~50mK)
3He
superconductive
Solenoid (2.5 T)
Dipole (0.5 T)
For transversity measurements
the polarization was reversed once
a week 1 to 2
1
2
two 60 cm long Target Cells
with opposite polarization
Relaxation time > 2000 hrs
6LiD
2002,2003,2004:
Achieved polarization: ~50%
Dilution factor:
~0.38
Kinematical variables and cuts
Primary vertex: m, m’ + hadron
To ensure that we have DIS events and avoid large radiative corrections
Q2 1 (GeV/c)2
Q2=2.4 (GeV/c)2
0.1 Y 0.9
Y=0.33
Q2 > 1(GeVc)2
W 5 GeV/c2
W=9.4 GeV/c2
xBj0.035
2002 data
Event selection: further cuts
Hadron selection:
energy deposit in hadron calorimeters (if present) > 5 GeV(HCAL1) or 8 GeV
(HCAL2)
● penetration length < 10 X
0
● no π / K / p separation by RICH for the 2002 data
●
Kinematic cuts on hadrons:
• pt > 0.1 GeV/c; z > 0.25; z > 1- ∑ zi (leading hadron analysis)
(zi = fraction of available energy carried by hadron)
• pt > 0.1 GeV/c; z > 0.20
(analysis with all hadrons)
Event selection
2002 data
z 0.25
pT0.1 GeV/c
pT0.51 GeV/c
z 0.45
2002 statistics:
Positive hadrons ~ 8.7 ·105
Negative hadrons ~7.0 ·105
Total
~ 1.6 ·106
Monte Carlo studies
MC events generated with Lepto 6.5.1
Data
MC
rms: 0.068 rad
Taken into account:
Trigger geometry
Tracking efficiencies
Good agreement between MC and real data!
all reconstructed hadrons
correctly reconstructed leading hadrons
correctly reconstructed leading hadron,
but leading hadron is not charged π
(~20% of the final sample, mainly K and p
(RICH not used in analysis) )
z=0.25
Extraction of Collins and Sivers
asymmetries
Rate asymmetries: N

j
( j )  F  n    a j ( j )  (1   j  sin  j )
 C  f  PT  D NN  A Coll
 S  f  PT  A Siv
j = C, S;
F muon flux
n
target density
σ
cross-section
aj acceptance  efficiency
f
dilution factor
PT target polarization
DNN depolarization factor
Asymmetries for two target cells are ( separately ) extracted,
using periods of opposite polarization, then combined:
N j ( j )  r  N j ( j )

j

j
N ( j )  r  N ( j )
  j  sin j
a j (  j )

j
a ( j )
 con st
Systematics
►
Ratio of the acceptances and efficiencies for both
target cells vs. Coll, Siv does NOT change between two
spin orientations
►
Results STABLE under the following tests:
Splitting the target cells in two parts
Splitting the data in high and low hadron momenta
Using a different method to extract the raw asymmetry
Different binning
●
●
●
●
►
Systematic errors are smaller than statistical errors
Results:
Phys. Rev. Lett. 94, 202002 (2005)
A Sivers
A Collins
Statistical Errors
x
z
pT
[ Gev/c ]
Results:
Phys. Rev. Lett. 94, 202002 (2005)
A Sivers
A Collins
Statistical Errors
x
z
pT
[ Gev/c ]
Interpretations
• Small asymmetries, some explanations:
– Cancellation between proton and neutron;
– Too small Collins mechanism.
• If 0TDhq  0 and large as from preliminary
measurement by BELLE (hep-ex/0507063), this is
evidence for cancellation in isoscalar target;
• Phenomenological fits of the data from HERMES
and prediction for COMPASS by Vogelsang and
Yuan (hep-ph/0507266), Efremov et al. (hepph/0412353, Phys.Part.Nucl.35:S139-S142,2004)
and Anselmino et al. (hep-ph/057181)
Sivers single-spin asymmetries
M. Anselmino et al. hep-ph/057181
Prediction of Sivers asymmetries on
a proton target
M. Anselmino et al. hep-ph/057181
Asymmetry predicted to be larger with z>0.4, 0.2<pT<1 GeV/c and x>0.02
Sivers single-spin asymmetries
on a deuteron target
W. Vogelsang and F. Yuan hep-ph/0507266
Collins single-spin asymmetries
on a deuteron target
W. Vogelsang and F. Yuan hep-ph/0507266
Fit Parameters: Set I for CFF
Collins single-spin asymmetries
on a deuteron target
W. Vogelsang and F. Yuan hep-ph/0507266
Fit Parameters: Set II for CFF
Collected data
Data sample with transverse spin:
Year
2002
2003
2004
Number of SPS spills
~45300
~42900
~94000
Data sample increased in years 2003/4:
– trigger system enlarged;
– 2004 longer run.
E-calorimeter information is
available for 2004 data
Number of good DIS events
~1.6 ∙ 106
~4 ∙ (2002)
~2 ∙ (2003)
π
RICH particle
identification is available
for 2003-2004 data!
K
p
Expected statistical accuracy
for Collins asymmetries
Summary and Outlook
• Collins and Sivers single-spin asymmetries shown from 2002 data
published by PRL;
• The 2002 asymmetries are small and compatible with the non zero
proton asymmetries measured by HERMES;
• Existing phenomenological models are in a good agreement with
COMPASS and HERMES data;
• Making use of existing deuteron data (2002-2004), accuracy will
improve by a factor ~ 3, results are coming soon;
• Data (of comparable statistics) will be collected on a transversely
polarized proton target (NH3) in 2006 and will allow a flavor
separation.
Thank you for your attention!