Post on 16-Jan-2016
transcript
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Updates on Transversity Experiments and Interpretations
Jen-Chieh Peng
Transversity Collaboration Meeting, JLab, March 4, 2005
University of Illinois
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Outline• Recent results of SSA with transversely
polarized targets from HERMES and COMPASS– What are the implications on the transversity, Collins
fragmentation function, and Sivers quark distributions?
– Can existing models explain the SSA data from HERMES for both the longitudinally polarized and the transversely polarized targets?
– What are the implications for the Hall-A experiments?
– What are the implications for polarized and unpolarized Drell-Yan experiments.
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Transversity distribution, Sivers distribution, and Collins fragmentation function in Semi-Inclusive DIS
with transversely polarized target
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26 4
Q
sxd
),()(])1(1{[ 211
,
22 h
qqq PzDxfey
),()()sin()1(||
),()()2sin(4
)1(||
),()()2cos(4
)1(
2
,11
2
2
,1
)1(1
22
2
2
,1
)1(1
22
2
hqq
qqq
lS
lh
h
hT
hqq
qqLq
lh
hN
hL
hqq
qqq
lh
hN
h
PzHxhezM
PyS
PzHxheMMz
PyS
PzHxheMMz
Py
)},()()cos()2
11(||
),()()2
11(||
),()()3sin(6
)1(||
),()()sin()2
11(||
21
)1(1
,
2
21
,1
2
,
21
)2(1
223
3
21
)1(1
,
22
hqq
Tqq
qlS
lh
N
hTe
hq
qqLe
qqh
qqTq
lS
lh
hN
hT
hqq
Tqq
qlS
lh
N
hT
PzDxgezM
PyyS
PzDxgeyyS
PzHxheMMz
PyS
PzDxfezM
PyyS
Unpolarized
Polarized target
Polarzied beam and
target
SL and ST: Target Polarizations; λe: Beam Polarization
Sivers
Transversity
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Observation of Single-Spin Azimuthal Asymmetry
Longitudinally polarized target
ep → e’πx HERMES
<ST> ~ 0.15
Collins effect: Correlation between the quark’s transverse spin with pion’s pT in the fragmentation process.
Sivers effect: Correlation between the transverse spin of the proton with the quark’s transverse momentum.
Other higher twist effects could also contribute.
Origins of the azimuthal asymmetry (correlation between the target nucleon transverse spin and the pion transverse momentum)?
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Model prediction of transversityChiral-quark soliton model
Similar to helicity distributions
Chiral-quark soliton model predicts Sivers distribution is zero!
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Comparison with HERMES longitudinal SSA dataProton data Deuteron data
Chiral-quark soliton model can describe the SSA data very well (by including only the transversity term)
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Makins DNP04 talk
π
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Disfavored Collins function = 0
dis1 ( ) 0H z
Comparison between the HERMES transversely polarized
target SSA data with Chiral-quark soliton model hep-ex/0412420
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Comparison between the HERMES transversely polarized target data with the Chiral-quark soliton model
disf fav1 1( ) 1.2 ( )H z H z
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Comparison between the HERMES transversely polarized data with the Chiral-quark soliton model
disf fav1 1( ) 5 ( )H z H z
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Implications of the HERMES SSA data withtransversely polarized target
• Anselmino et al. showed that the Hermes SSA data for longitudinally polarized data can be explained by Sivers effect alone (without the transversity/Collins effect).
• However, the extracted Sivers function is much larger (and of opposite sign) compare to the HERMES SSA data with transversely polarized data.
hep-ph/0412316
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First results from COMPASS transversely polarized 6LiD
•Effects are expected to be small at small x
•Some cancellations between proton and neutron are expected
hep-ex/0503002
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Extraction of the Sivers distribution
Fits to the HERMES data on Sivers moments
(1) 0.3 51
(1) 51
Dashed curv
Soli
e: (
d curve: ( ) 0.4 (1
0.1 (1 )
)
)
u
T
T
ux
xf x x x
f x x x
(hep-ph/0412353)
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SSA with Transversely Polarized Drell-Yan
• Prediction by Anselmino, D’Alesio, Murgia (hep-ph/0210371) for a negative AN.
• |AN| increases with rapidity, y, and with dilepton mass, M.
Analysing power (AN) is sensitive to Sivers function
q qqqqq
q qqqTqDYN xfxfe
xfxfeA
)()(
)()(2
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Sivers function in Drell-Yan is expected to have a sign opposite to that in DIS!(Brodsky, Hwang, Schmidt, hep-ph/0206259; Collins, hep-ph/0204004)
p↑ + p → l+ l- + X√s = 200 GeV
AN
y
Is this measurement feasible at RHIC?
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Expected statistical sensitivity for Drell-Yan AN
• Might be feasible to determine the sign of the Sivers function at RHIC
• Should consider fixed-target polarized Drell-Yan too
Assuming 400 pb-1 50% polarization
6 < M < 10 GeV
p↑ + p → l+ l- + x √s = 200 GeV
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Sivers functions from SSA in polarized Drell-Yan
(1) 0.3 51
(1) 51
Dashed curv
Soli
e: (
d curve: ( ) 0.4 (1
0.1 (1 )
)
)
u
T
T
ux
xf x x x
f x x x
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Cos2Ф Dependence in Unpolarized Drell-Yan
• RHIC would provide unpolarized p-p Drell-Yan data too
• Fixed-target unpolarized p-p Drell-Yan data also exist
Large cos2Ф dependences have been observed in π – induced Drell-Yan
This azimuthal dependence could arise from a product of KT-dependent distribution function h1
┴
( Boer, hep-ph/9902255; Boer, Brodsky, Hwang, hep-ph/0211110)
In quark-diquark model, h1┴ is identical to Sivers function
No Cos2Ф depenence for unpolarized p-p Drell-Yan has been reported yet (The effect from h1
┴ is expected to be smaller)
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(Conway et al.) 252 GeV π- + W
2 211 cos sin 2 cos sin cos 2
2
d
d
*h N X X
Lam-Tung sum rule : 1 2 0
Lam-Tung rule is violated and can not be explained by pion bound state effect
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Brandenburg, Nachtmann and Mirkes proposed correlation between quark kT and its transverse spin
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0 4 41 2 4 T
T T
Q
Q m
is a measure of the correlation between the transverse spins of the qq
D. Boer pointed out that h1┴ provides such correlation
221 12 2
( , ) ( )T TkC HTT H
T C
M Mh x k c e f x
k M
-2
2.3GeV,
1, 1GeV
C
H T
M
C
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Unpolarized p-p and p-d dimuon production
Fermilab E866, √s = 38.8 GeV
J/Ψ
Ψ’
Υ
~ 2.5 x 105 Drell-Yan events
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Ф – coverage of the E866 dimuon data
J/Ψ events Drell-Yan events
Not corrected for acceptance yet
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Summary• Transversity distribution remains an interesting
frontier in understanding spin structure in nucleon.• Study of the T-odd Sivers structure function and the
Collins fragmentation function are important for their own sake, and for extracting information on transversity.
• First results of SSA using transversely polarized p and d targets are intriguing. The proposed Hall-A measurement on 3He should provide very useful new information.
• New Semi-Inclusive DIS experiments at JLab will continue to probe the flavor structure of unpolarized and polarized parton distributions.