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Measurement of Collins Asymmetries in e+e- Annihilation at Belle
D. Gabbert Illinois and RBRCM. G.Perdekamp Illinois and RBRCK. Hasuko RIKEN and RBRCS. Lange Frankfurt M. Leitgab IllinoisD. Mertens Illinois A. Ogawa BNL and RBRCR. Seidl Illinois and RBRC V. Siegle RBRC
for the Belle Collaboration
Workshop on Exclusive Reactions at
High Momentum TransferMay 24th , Thomas Jefferson Laboratory
Measurement of Collins Asymmetries at Belle 2May 24th
----------- Outline ------------
o MotivationCollins FF for global QCD analysis of singletransverse asymmetries in pp and SIDIS
o Collins Asymmetries from e+e-
LEP first work on Delphi data in the 90s: Efremov, Smirnova, Tkatchev andBonivento, Matteuzzi, Kotzinian
Experimental techniqueResults
Measurement of Collins Asymmetries at Belle 3May 24th
Motivation Transversity Quark Distributionsfrom Collins- and Interference-Fragmentation
Collins- and IFF- asymmetriesin SIDIS and pp are of the form
~ Transversity (x) x CFF(z)
~ Transversity (x) x IFF(z)
Collins- and IFF- asymmetriesin e+e- are of the form
~ CFF(z1) x CFF(z2)
~ IFF(z1) x IFF(z2)
global QCD analysis
New HERMES results for Collins AsymmetriesM. Diefenthaler, DIS 2007, Munich
Measurement of Collins Asymmetries at Belle 4May 24th
Global Analysis: Extract Transversity Distributions
EIC JLAB, COMPASS, HERMES,
IFF versity transCollins versity trans
SIDIS
××
⇒
ity transversersity transv :Yan Drell A
GSI IFF ersity transv
Collins sity transver:Jetsin A Jets hadrons, inclusivefor A
pp
TT
T
N
×
××
⇒TransversityTensor Charge
FF ceInterferen x FF ceInterferen
Collins x Collins ee - ⇒+
Lattice QCD: Tensor Charge
Factorization +
Universality ?!
Belle
RHIC / GSI
Theo
ry
RBRC Transversity Workshop 2000
Measurement of Collins Asymmetries at Belle 5May 24th
First QCD Analysisof SIDIS + e+e- !
Anselmino, Boglione, D’Alesio,Kotzinian, Murgia, Prokudin, TurkPhys. Rev. D75:05032,2007
HERMES SIDIS+ COMPASS SIDIS
+ Belle e+e-
transversity dist.
+ Collins FF
Fit includes:
Measurement of Collins AsymmetriesIn e+e- at KEK B with the Belle
Phys.Rev.Lett.96:232002,2006 and update at Spin 2006 in Kyoto
Measurement of Collins Asymmetries at Belle 7May 24th
Collins Effect in Quark FragmentationJ.C. Collins, Nucl. Phys. B396, 161(1993)
φ
q
momentum hadron relative : 2
zmomentum hadron e transvers:
momentum hadron : spinquark : momentumquark :
h
sE
EEppsk
h
qh
h
h
q
=
=⊥
v
v
v
r
qsv
kv
hph v,
Collins Effect:Fragmentation of a transversely polarizedquark q into spin-less hadron h carries anazimuthal dependence:
⊥hpv
( )φsin∝
⋅×∝ ⊥ qh spk vvv
Measurement of Collins Asymmetries at Belle 8May 24th
General Form of Fragmentation Functions
( )h
qhh
hqhqh
hq zM
spkpzHzDpzD
vvv ⋅×
+= ⊥⊥
⊥⊥↑
ˆ ),()(),( 2,
1,
1
Number density for finding hadron h from a transversely polarized quark, q:
unpolarized FF Collins FF
Measurement of Collins Asymmetries at Belle 9May 24th
o Quark spin direction unknown: measurement ofCollins function in one hemisphere is not possiblesin φ modulation will average out.
o Correlation between two hemispheres withsin φi Collins single spin asymmetries results incos(φ1+φ2) modulation of the observed di-hadronyield.
Measurement of azimuthal correlations for pion pairsaround the jet axis in two-jet events!
Collins FF in e+e- : NeedCorrelation between Hemispheres !
Measurement of Collins Asymmetries at Belle 10May 24th
e-
e+Jet axis: Thrust
<Nh+,-> = 6.4
GeV 5210ss
E2z h ., ==Near-side Hemisphere:
hi , i=1,Nn with zi
Far-side: hj , j=1,Nf with zj
Θ
Event Structure at Belle
( ) ( ) ( ) ( )
( )( ) ( )Θ+=⎟
⎠⎞⎜
⎝⎛ +−=
=Ω
→ ∑−+
22
21112
2
2
21
21
141
21
3
cos
,
cm
aaa
yyyA
zDzDeyAQdzdzd
Xhheed ασ
e+e- CMS frame:
Spin averaged cross section:
Measurement of Collins Asymmetries at Belle 11May 24th
Collins Fragmentation: Angles and Cross Section cos(φ1+φ2) Method
ϕ1
1hPr
2hPr
ϕ2−π
ϕ1
ϕ2
( ) ( ) ( ) [ ]( ) [ ]( )
( ) ( ) Θ=−=
+=Ω
→ ⊥⊥−+
2cm
21
111
121T
221
21
41y1yyB
zHzHyBddzdzd
Xhheedσ
sin
cos ϕϕLq
2-hadron inclusive transverse momentum dependent cross section:
Net anti-alignment oftransverse quark spins
Θ
e+e- CMS frame:
e-
e+
Observables: N12(φ1+φ2), A12(z1,z2)
Measurement of Collins Asymmetries at Belle 12May 24th
Collins Fragmentation: Angles and Cross Section cos(2φ0) Method
ϕ0
1hPr2hP
r
( ) ( ) ( ) ( )
( ) ( ) Θ=−=
⎢⎣
⎡⎥⎦
⎤⋅−⋅⋅=
Ω→ ⊥⊥−+
2cm
21
110
T2
21
21
41y1yyB
MMHH22yB
ddzdzdXhheedσ
sin
ˆˆcos TTTT pkphkhq
IϕL
2-hadron inclusive transverse momentum dependent cross section:
Net anti-alignment oftransverse quark spins
Θ •Independent of thrust-axis
•Convolution integral I over transverse momenta involved
e+e- CMS frame:
e-
e+
[Boer,Jakob,Mulders: NPB504(1997)345]
A0
Observables: N0(φ0), A12(z1,z2)
Measurement of Collins Asymmetries at Belle 13May 24th
Hadronic Events observed with Belle
∑∑
=
⋅=
ii
ii
p
npthrust v
v
particles
ˆ~ 1 ~ 0.5
1.5 x 109 hadronic events in analysis
On resonance: 547fb-1 , √s=10.52 GeV (published)Off resonance: 29fb-1 , √s=10.58 GeV (preliminary)
Measurement of Collins Asymmetries at Belle 14May 24th
z2
QTe- e+
( ) ( )∑ ⎥⎦
⎤⎢⎣
⎡+
∝Ω 21110
222
2
21
)2cos(sin zDzDQQ
qdqdzdzd
dN
T
T
T
φθ
z1 21 pp vv −=
Frame:
Gluon Emission
Measurement of Collins Asymmetries at Belle 15May 24th
Examples of Fits to Azimuthal Asymmetries
D1 : spin averaged fragmentation function,
H1: Collins fragmentation function
N(φ)/N0
2φ0 (φ1+φ2)
Measurement of Collins Asymmetries at Belle 16May 24th
Method to eliminate gluon contributions: Double ratios for unlike- and like sign pions
Double ratio method:
)cos()(
)(
)(cos1)(sin~ 02
2
2
2
φθ
θ⎥⎥
⎦
⎤
⎢⎢
⎣
⎡
⋅+⋅
⋅+⋅
+ ∑∑
UnfUnfFavFavq
UnfUnfFavFavq
DDDDe
HHHHeF
A0(z1,z2)
Measurement of Collins Asymmetries at Belle 17May 24th
Two Double Ratios
• Unlike-sign pion pairs (U):(favored x favored + unfavored x unfavored)
• Like-sign pion pairs (L):(favored x unfavored + unfavored x favored)
• π±π0 pairs(favored + unfavored) x (favored + unfavored)
• A. Efremov et al. ([hep-ph/0603054]): charged ππ pairs are similar (and easier to handle) (C):
(favored + unfavored) x (favored + unfavored)
UL
Double ratios unlike/like-sign not very sensitive in separating favored and disfavored Collins functions look at unlike-sign/charge-sum
UC
Observables:A12(UL), A12(UC)A0(UL) , A0(UC)
published and input tofit by Anselmino et al.
Measurement of Collins Asymmetries at Belle 18May 24th
Final Charm Corrected Results for e+ e- π π X (29fb-1, off-resonance Data)
• Significant non-zero asymmetries
• Rising behavior vs. z• UL/C asymmetries about 40-
50% of UL/L asymmetries• First direct measurements of
the Collins function• UL/L data published
A0(UL/L) (3.06 ± 0.57 ± 0.55)%
A12(UL/L) (4.26 ± 0.68 ± 0.68)%
A0(UL/C) (1.27 ± 0.49 ± 0.35)%
A12(UL/C) (1.75 ± 0.59 ± 0.41)%
Phys.Rev.Lett.96:232002,2006
Measurement of Collins Asymmetries at Belle 19May 24th
Preliminary Charm Corrected Results for e+ e- π π X (547 fb-1 ,on-resonance)
• Significance largely increased
• Behavior unchanged• Reduced systematics• Precise measurements of the
Collins function
Integrated results:A0(UL/L) (2.67 ± 0.10 ± 0.26)%
A12(UL/L) (3.55 ± 0.08 ± 0.15)%
A0(UL/C) (1.11 ± 0.11 ± 0.22)%
A12(UL/C) (1.46 ± 0.09 ± 0.13)%
Preliminary
Measurement of Collins Asymmetries at Belle 20May 24th
Summary and Outlook
• Observation of large azimuthal asymmetries in light quark frag-mentation. Updated statistics:
∫Ldt = 29 fb-1 547 fb-1
• Double ratios reliably cancel contributions from detector acceptance and gluon radiation.
important input for the transverse spin physics programs at RHIC, HERMES, COMPASS and JLab
• Future spin dependent FFs: update Collins FFmeasure pT dependenceinterference fragmentationCollins FF for VMsLambdas
• Precision measurement of spinaveraged fragmentation func-tions as input to RHIC program with inclusive hadrons and futureprecision measurements in SIDIS.
Summary: Outlook:
Measurement of Collins Asymmetries at Belle 21May 24th
Belle MC
<1% of data sample work in progress
Belle Impact on the Knowledge of FFs
2/
sEz
h
=2/
sEz
h
=
FF
FF
Compilation of dataavailable for the char-ged hadron FF
Belle: Charged h+/-, pions, kaons, protons
precision at high z!
h+,-pionskaonsprotons
Input also for precision measurements of quarkhelicity distributions in SIDIS, in particular ata possible future electron-polarized proton collider.
Measurement of Collins Asymmetries at Belle 22May 24th
What is Known Experimentally ?
Hirai, Kumano,Nagai, Sudohep-ph/0612009(including errors!)
Earlier work:
KretzerBinnewies, Potter, AlbinoKniehl, Kramer, PotterAlbino, Kniehl, Kramer
Measurement of Collins Asymmetries at Belle 23May 24th
Motivation I ∆G from QCD Analysis of ALLfor inclusive hadrons: π0,+,-, η, K+,-
PHENIX π0 cross section a |η|<0.35Phys.Rev.Lett.91:241803,2003
Deviation connected to uncertaintiesin FFs gluon FF!
Idea: QCD analysis ofLEP and Belle data willfix uncertainty in know-ledge of FFs (gluon FF, flavor separation)
Measurement of Collins Asymmetries at Belle 24May 24th
o Very basic QCD process: Fundamental test case for any approach to solve QCD at soft scales.
o Tests of universality and factorization betweene+e-, DIS and p-p collisions
o Symmetry properties
o Evolution is fundamental QCD prediction
o Connection between microscopic and macroscopic observables:
Probe/analyzer for transverse quark spincritical input for transverse proton spin programat DESY, CERN, JLab and RHIC
Why is the Collins Fragmentation Function Interesting?
Measurement of Collins Asymmetries at Belle 25May 24th
Measurement of Collins Asymmetries at Belle 26May 24th
• KEKB– Asymmetric collider– 8GeV e- + 3.5GeV e+
– √s = 10.58GeV (Υ(4S))e+e- Υ(4S) Β⎯Β
– Off-resonance: 10.52 GeVe+e- q⎯q (u,d,s,c)
– Integrated Luminosity: > 600 fb-1
> 60fb-1 => off-resonance
Belle detectorKEKB
KEKB: L>1.6 x 1034cm-2s-1 !!
•Average Trigger rates:Υ(4S) BB 11.5Hzq⎯q 28 Hzμμ + ττ 16 HzBhabha 4.4 Hz2γ 35 Hz
1.5 x 109 hadronic events in analysis
Measurement of Collins Asymmetries at Belle 27May 24th
Good tracking and particle identification!
Measurement of Collins Asymmetries at Belle 28May 24th
• Good detector performance (acceptance, momentum resolution, pid)
• Jet production from light quarksoff-resonance (60 MeV below resonance) (~10% of all data)
• Intermediate EnergySufficiently high scale (Q2 ~ 110 GeV2)
- can apply pQCDNot too high energy (Q2 << MZ
2)-avoids complication from Z interference
• Sensitivity = A2sqrt(N) ~ x 25 (100) compared to LEPABelle / ALEP ~ x 2 (A scales as ln Q2)∫LBelle dt / ∫LLEP dt ~ x 46 (600)
Total number of hadronic events ~ 1.5 x 109
9.44 9.46
e+e- Center-of-Mass Energy (GeV)
0
5
10
15
20
25
σ(e+ e
- →ha
dron
s)(n
b)
ϒ(1S)10.0010.02
0
5
10
15
20
25
ϒ(2S)10.34 10.37
0
5
10
15
20
25
ϒ(3S)10.54 10.58 10.62
0
5
10
15
20
25
ϒ(4S)9.44 9.46
e+e- Center-of-Mass Energy (GeV)
0
5
10
15
20
25
σ(e+ e
- →ha
dron
s)(n
b)
ϒ(1S)10.0010.02
0
5
10
15
20
25
ϒ(2S)10.34 10.37
0
5
10
15
20
25
5
10
15
20
25
ϒ(2S)10.34 10.37
0
5
10
15
20
25
ϒ(3S)10.54 10.58 10.62
0
5
10
15
20
ϒ(3S)10.54 10.58 10.62
0
5
10
15
20
25
ϒ(4S)
Belle is Well Suited for FF Measurements:
Measurement of Collins Asymmetries at Belle 29May 24th
Applied Cuts, Binning
• Originally off_resonance data, now also on_resonance data (29.1 547 fb-1)
• Track selection:– pT > 0.1GeV– vertex cut:
dr<2cm, |dz|<4cm• Acceptance cut
– -0.6 < cosθi < 0.9• Event selection:
– Ntrack ≥ 3– Thrust > 0.8 – Z1, Z2>0.2
• Hemisphere cut
• QT < 3.5 GeV• Pion PID selection
(Ph 2 ⋅ ˆ n ) ˆ n ⋅ (Ph1 ⋅ ˆ n ) ˆ n < 0
z2
0 1 2 3
4 5 6
7 8
9
0.2
0.3
0.5
0.7
1.0
0.2 0.3 0.5 0.7 1.0
5
86
1
2
3
z1
Measurement of Collins Asymmetries at Belle 30May 24th
Experimental Issues
• Cos2φ moments have two contributions:– Collins Can be isolated either by subtraction or double ratio method– Radiative effects Cancels exactly in subtraction method, and in LO of double
ratios• Beam Polarization
zero? cos(2φ) asymmetries for jets or γγ
• False asymmetriesfrom weak decays Study effect in τ decays, constrain through D tagging
• False asymmetries from misidentified hemispheres QT or polar angle cut
• False asymmetries from acceptance Cancels in double ratios, can be estimated in charge ratios,
fiducial cuts• Decaying particles lower z cut
Measurement of Collins Asymmetries at Belle 31May 24th
Why is it Possible to Include Data on Resonance?Different Thrust Distributions
∑∑ ⋅
=
ii
ii
p
npthrust
ˆ
• small B contribution (<1%) in high thrust sample •>75% of X-section continuum under
Υ (4S) resonance•29 fb-1 547 fb-1
•many systematic errors reduce with more statistics • Charm-tagged Data sample also increases
9.44 9.46
e+e- Center-of-Mass Energy (GeV)
0
5
10
15
20
25
σ(e+ e
- →ha
dron
s)(n
b)
ϒ(1S)10.0010.02
0
5
10
15
20
25
ϒ(2S)10.34 10.37
0
5
10
15
20
25
ϒ(3S)10.54 10.58 10.62
0
5
10
15
20
25
ϒ(4S)9.44 9.46
e+e- Center-of-Mass Energy (GeV)
0
5
10
15
20
25
σ(e+ e
- →ha
dron
s)(n
b)
ϒ(1S)10.0010.02
0
5
10
15
20
25
ϒ(2S)10.34 10.37
0
5
10
15
20
25
5
10
15
20
25
ϒ(2S)10.34 10.37
0
5
10
15
20
25
ϒ(3S)10.54 10.58 10.62
0
5
10
15
20
ϒ(3S)10.54 10.58 10.62
0
5
10
15
20
25
ϒ(4S)−+ BB00 BB
e+e- qq̅, q∈uds
e+e- cc̅
Measurement of Collins Asymmetries at Belle 32May 24th
Collins Asymmetries I: 4x4 z1, z2 binning
• 547 fb-1 charm corrected data sample, • UL and UC double ratios
PRELIMINARY
PRELIMINARY
A0 (cos(2φ0)) moments A12 (cos(φ1 + φ2)) moments
Measurement of Collins Asymmetries at Belle 33May 24th
Collins Asymmetries II: sin2 θ/(1+cos2 θ) Binning (UL)
• Nonzero quark polarization ~ sin2 θ
• Unpolarized de-nominator ~ 1+cos2 θ
• Clear linear behavior seen when using either thrustz or 2nd hadron as polar angle
• Better agreement for thrust axis (~approximate quark axis)
• UC plots similar
thrustz
θ2
PRELIMINARY
Measurement of Collins Asymmetries at Belle 34May 24th
Collins Asymmetries III:QT Binning (UL)
• Reduced asymmetries in low thrust sample
• At low thrust significant B contribution(for t<0.8 ~20 % B for t>0.8 < 1 % B)
• A12 thrust axis dependent
• High QT (>3.5 GeV) asymmetries from beam related BG
• UC plots similar
Thrust>0.8
Thrust<0.8 (not corrected for heavy quark contributions)
PRELIMINARY
Measurement of Collins Asymmetries at Belle 35May 24th
Improved Systematic Errors (UC)• Tau contributions• PID systematics∗
• MC double ratios• Charged ratios (π+π+ /π−π− )• Higher order terms• Double ratio-subtraction method
Reweighting asymmetries: underestimation of cos(φ1+φ2)
asymmetries rescaled by 1.21Correlation studies:
statistical errrors rescaled by 1.02 (UL) and 0.55 (UC)Beam polarization studies
consistent with zero
A0 (cos(2φ0)) moments
A12 (cos(φ1 + φ2)) moments
Measurement of Collins Asymmetries at Belle 36May 24th
Collins Asymmetry Collins FF
QCD analysis: (I) Efremov, Goeke, Schweitzer (Phys.Rev.D73:094025,2006):
consistency between Belle, HERMES and COMPASSasymmetries.
(II) Anselmino, Boglione, D’Alesio, Kotzinian, Murgia,Prokudin and Tuerk (hep-ph/0701006 ) extract transversity distributions and the Collins FF (see tomorrow).
(III) UIUC fit to Belle data (work in progress):
Measurement of Collins Asymmetries at Belle 37May 24th
Spin Averaged FF + Collins Asymmetry
Kretzer FFs (also KKP)
Experimental double ratio:
Measurement of Collins Asymmetries at Belle 38May 24th
Fits of the Belle Collins Asymmetries A0 with∫Ldt = 29 fb-1
Measurement of Collins Asymmetries at Belle 39May 24th
Fits of the Belle Collins Asymmetries A12 with∫Ldt = 29 fb-1
Measurement of Collins Asymmetries at Belle 40May 24th
Results for the Collins FF
fav
fav
DH
fav
fav
DH
fav
fav
DH
A0 A12
0 < z < 1
fav
fav
DH
fav
fav
DH
dis
dis
DH
dis
dis
DH