measurements of fragmentation functions at belle: results and prospects d. gabbert (university of...
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Measurements of fragmentation functions at Belle: results and prospects
D. Gabbert (University of Illinois and RBRC)M. Grosse Perdekamp (University of Illinois and RBRC)
K. Hasuko (RIKEN/RBRC) S. Lange (Frankfurt University)
A. Ogawa (BNL/RBRC) R. Seidl (University of Illinois and RBRC)
for the Belle Collaboration
(see hep-ex/0607014 and Phys.Rev.Lett.96:232002,2006 for Collins results)
Outline:
• The KEKB rings and the Belle Experiment
• Transverse Spin Physics
•Motivations
•Collins analysis and parameterizations
•Interference Fragmentation functions
• Unpolarized fragmentation functions
R.Seidl:Measurements of fragmentation functions at Belle: results and prospects 2 BNL, July 21th
KEKB: L>1.6x1034cm-2s-1 !!
• Asymmetric collider• 8GeV e- + 3.5GeV e+
• s = 10.58GeV ((4S))• e+e-S• Off-resonance: 10.52 GeV• e+e-qq (u,d,s,c)• Integrated Luminosity: >600 fb-1
• >60fb-1 => off-resonance
Belle detectorKEKB
R.Seidl:Measurements of fragmentation functions at Belle: results and prospects 3 BNL, July 21th
Good tracking and particle identification!
R.Seidl:Measurements of fragmentation functions at Belle: results and prospects 4 BNL, July 21th
SIDIS experiments (HERMES and COMPASS, EIC) measure q(x) together with either Collins Fragmentation function or Interference Fragmentation function
Important input for global Transversity Analysis
zH1
RHIC measures the same combinations of quark Distribution (DF) and Fragmentation Functions (FF) plus unpolarized DF q(x)
There are always 2 unknown functions involved which cannot be measured independently
The Spin dependent Fragmentation function analysis yields information on the Collins and the Interference Fragmentation function !
Universality appears to be provenin LO by Collins and Metz:
[PRL93:(2004)252001 ]+most recent work from
Amsterdam Group
R.Seidl:Measurements of fragmentation functions at Belle: results and prospects 5 BNL, July 21th
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
Event Structure at Belle
22
21112
2
2
21
21
14
1
2
1
3
cos
,
cm
aaa
yyyA
zDzDeyAQdzdzd
Xhheed
e+e- CMS frame:
Spin averaged cross section:
R.Seidl:Measurements of fragmentation functions at Belle: results and prospects 6 BNL, July 21th
Collins fragmentation in e+e- : Angles and Cross section cos() method
1hP
2hP
2cm
21
111
121T
221
21
4
1y1yyB
zHzHyBddzdzd
Xhheedσ
sin
cos q
2-hadron inclusive transverse momentum dependent cross section:
Net (anti-)alignment oftransverse quark spins
e+e- CMS frame:
e-
e+
GeV 5210ss
E2z h .,
[D.Boer: PhD thesis(1998)]
R.Seidl:Measurements of fragmentation functions at Belle: results and prospects 7 BNL, July 21th
Applied cuts, binning
• Off-resonance data – 60 MeV below (4S)
resonance– 29.1 fb-1
• Track selection:– pT > 0.1GeV– vertex cut:
dr<2cm, |dz|<4cm• Acceptance cut
– -0.6 < cosi < 0.9 • Event selection:
– Ntrack 3– Thrust > 0.8 – Z1, Z2>0.2
• Hemisphere cut
• QT < 3.5 GeV
(Ph 2 ˆ n ) ˆ n (Ph1 ˆ n ) ˆ n 0
z1
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
= Diagonal bins z1
z2
R.Seidl:Measurements of fragmentation functions at Belle: results and prospects 8 BNL, July 21th
Testing the double ratios with MC• Asymmetries do cancel out for MC• Double ratios of
compatible with zero• Mixed events also show zero
result• Asymmetry reconstruction works
well for MC (weak decays)• Single hemisphere analysis yields
zeroDouble ratios are safe to use
•uds MC (UL/L double ratios)
•uds MC (UL/C double ratios) •Data ()
R.Seidl:Measurements of fragmentation functions at Belle: results and prospects 9 BNL, July 21th
Other Favored/Unfavored Combinations charged pions or
• Unlike-sign pion pairs (UL):(favored x favored + unfavored x unfavored)
• Like-sign pion pairs (L):(favored x unfavored + unfavored x favored)
• ± pairs(favored + unfavored) x (favored + unfavored)
• P.Schweitzer([hep-ph/0603054]): charged pairs are similar (and easier to handle) (C):
(favored + unfavored) x (favored + unfavored)
Challenge: current double ratios not very sensitive to favored to disfavored Collins function ratio Examine other combinations:
Favored = u,d,cc.
Unfavored = d,u,cc.
Build new double ratios:
Unlike-sign/ charged pairs (UL/C)
UL/L
UL/C
R.Seidl:Measurements of fragmentation functions at Belle: results and prospects 10
BNL, July 21th
Systematic errors• Tau contributions• PID systematics
• MC double ratios• Charged ratios (
• Higher order terms• Double ratio-subtraction
methodtaken from UL/L analysisFurther studies:Reweighting asymmetries: underestimation of cos asymmetriesCorrelation studies:statistical errrors rescaled by 14%Beam polarization studiesconsistent with zero
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Final charm corrected results for e+ e- X (29fb-1 of continuum
data)• Significant non-zero
asymmetries • Rising behavior vs. z
• cos(+) double ratios only marginally larger
• UL/C asymmetries about 40-50% of UL/L asymmetries
• First direct measurements of the Collins function
Integrated results:
A0(UL/L) (3.06 ± 0.57 ± 0.55)%
A12(UL/L) (4.26 ± 0.68 ± 0.68)%
A0(UL/C) (1.270 ± 0.489 ± 0.350)%
A12(UL/C) (1.752 ± 0.592 ± 0.413)%
Final results
Preliminary results
R.Seidl:Measurements of fragmentation functions at Belle: results and prospects 12
BNL, July 21th
Fits to the HERMES data by the Torino Group
• A. Prokudin fitted the HERMES (Sivers and) Collins Data
• Either Soffer bound for transversity:2q(x) = | q + q |
• Or q(x) = q(x) • GRV1998 for
unpolarized PDFs• Kretzer FF• Gaussian kT
Assumption
HERMES 02-04 Data
COMPASS 02 Data
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BNL, July 21th
Fits to our UL/L data:
Efremov, Goeke, Schweitzer: [hep-ph/0603054]:
•Taking reasonable transversity from QSM•Gaussian intrinsic transverse momentum dependenceHERMES, COMPASS and Belle results are consistent with each other
However: Fit errors on Collins function still large
Probably improvement by UL/C data (not yet included)
Can we do better?
R.Seidl:Measurements of fragmentation functions at Belle: results and prospects 14
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What about the data under the resonance?
• More than 540 fb-1 of on_resonance data
• (4S) is just small resonance
• More than 75% of hadronic cross sectionfrom open quark-antiquark production
9.44 9.46
e+e- Center-of-Mass Energy (GeV)
0
5
10
15
20
25
(e+
e-
had
ron
s)(n
b)
(1S)10.0010.02
0
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(2S)10.34 10.37
0
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(3S)10.54 10.58 10.62
0
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(4S)9.44 9.46
e+e- Center-of-Mass Energy (GeV)
0
5
10
15
20
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(e+
e-
had
ron
s)(n
b)
(1S)10.0010.02
0
5
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(2S)10.34 10.37
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(2S)10.34 10.37
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(3S)10.54 10.58 10.62
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(3S)10.54 10.58 10.62
0
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(4S)
R.Seidl:Measurements of fragmentation functions at Belle: results and prospects 15
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Why is it possible to include on_resonance data?Different Thrust distributions
ii
ii
p
npthrust
ˆ
• e+ e- q q (u d s) MC
• (4S) B+ B- MC
• (4S) B0 B0 MC
• Largest systematic errors reduce with more statistics • Charm-tagged Data sample also increases with statistics
R.Seidl:Measurements of fragmentation functions at Belle: results and prospects 16
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• e+e- (+-)jet1(-+)jet2X• Stay in the mass region around -mass• Find pion pairs in opposite
hemispheres• Observe angles 12
between the event-plane (beam, jet-axis) and the two two-pion planes.
• Transverse momentum is integrated(universal function, evolution easy directly applicable to semi-inclusive DIS and pp)
• Theoretical guidance by papers of Boer,Jakob,Radici and Artru,Collins
• Early work by Collins, Heppelman, Ladinski
Interference Fragmentation – thrust method
21221111 m,zHm,zHA cos
2
1
1hP
2hP
2h1h PP
R.Seidl:Measurements of fragmentation functions at Belle: results and prospects 17
BNL, July 21th
Different model predictions for IFF
•Jaffe et al. [Phys. Rev. Lett. 80 (1998)] : inv. mass behavior out of -phaseshift analysissign change at mass
-originally no predictions on actual magnitudes
-Tang included some for RHIC-Spin
• Radici et al. [Phys. Rev. D65 (2002)] : Spectator model in the s-p channel no sign change observed (updated model has Breit-Wigner like asymmetry)
PRELIMINARY
f1, h1 from spectator model
f1, h1=g1 from GRV98 & GRSV96
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Unpolarized FFs
MC simulation, ~1.4 fb-1
2.
• Performed Systematic studies:– Smearing– Tracking efficiency– Acceptance– Energy scale
• Particle ID study not yet finished
• AKK: Try flavor tagged/enhanced analysis?
R.Seidl:Measurements of fragmentation functions at Belle: results and prospects 19
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Summary and outlook
• Collins analysis:– Analysis procedure passes all null
tests – Systematic uncertainties
understood Significant nonzero
asymmetry with double ratios are observed
• UL/C about half as large UL/L double ratios
• Data can be used for more sophisticated analysis
• Paper is published• Important input for
Transversity measurements now and in the future
• Leading order fits ongoing• On resonance >10 x
statisticslong paper on Collins results
• Interference fragmentation function analysis started
• Unpolarized FF analysis almost complete but lacks manpower
• Many more QCD/Spin related studies possible (VM Collins, timelike DVCS, Polarimetry, kT-dependent FFs, “Sivers”FF, Event Shapes )
Results: Prospects:
R.Seidl:Measurements of fragmentation functions at Belle: results and prospects 20
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R.Seidl:Measurements of fragmentation functions at Belle: results and prospects 21
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• Spin composition of the nucleon:– Contribution of quark spin
small– Contribution of gluon spin
also small?– Orbital angular
momentum? Sivers effect?
• Single spin asymmetries small in pQCD– …but large and nonzero!– Understandable by gauge
link contributions
• Sivers effect:– Unpolarized quarks in transversely
polarized nucleon– Non-centered quark distribution in
impact parameter spaceRescattering creates left-right
asymmetry
• Boer-Mulders effect:• Transversely polarized quarks in
unpolarized nucleon• Vacuum polarization effect?
• Collins effect:• Fragmentation of transversly
polarized quark into unpolarized hadron
• Connects microscopic quantitiy (transverse spin) to macroscopic quantity (azimuthal asymmetries)
QCD: the right theory, but many surprisesespecially in spin …
R.Seidl:Measurements of fragmentation functions at Belle: results and prospects 22
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J.C. Collins, Nucl. Phys. B396, 161(1993)
q
momentum hadron relative : 2
z
momentum hadron e transvers:
momentum hadron :
spinquark :
momentumquark :
h
sE
EE
p
p
s
k
h
qh
h
h
q
qs
k
hph
,
Collins Effect:Fragmentation with of a quark q with spin sq into a spinless hadron h carries anazimuthal dependence:
hp
sin
qh spk
Collins effect in quark fragmentation
R.Seidl:Measurements of fragmentation functions at Belle: results and prospects 23
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The Collins Effect in the Artru Fragmentation Model
π+ picks up L=1 tocompensate for thepair S=1 and is emittedto the right.
String breaks anda dd-pair with spin-1 is inserted.
A simple model to illustrate that spin-orbital angular momentum coupling can lead to left right asymmetries in spin-dependent fragmentation:
R.Seidl:Measurements of fragmentation functions at Belle: results and prospects 24
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Typical hadronic events at Belle
ii
ii
p
npthrust
ˆ
R.Seidl:Measurements of fragmentation functions at Belle: results and prospects 25
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Examples of fits to azimuthal asymmetries
D1 : spin averaged fragmentation function,
H1: Collins fragmentation function
N()/N0
No change in cosine moments when including sine and higher harmonics
Cosine modulationsclearly visible
2 )
R.Seidl:Measurements of fragmentation functions at Belle: results and prospects 26
BNL, July 21th
Methods to eliminate gluon contributions: Double ratios and subtractions
Double ratio method:
Subtraction method:
Pros: Acceptance cancels out
Cons: Works only if effects are small (both gluon radiation and signal)
Pros: Gluon radiation cancels out exactly
Cons: Acceptance effects remain
)(
)(
)(
)(
)(cos1
)(sin:
2
2
2
2
2
2
FavUnfUnfFav
q
FavUnfUnfFav
q
UnfUnfFavFav
q
UnfUnfFavFav
q
DDDDe
HHHHe
DDDDe
HHHHeFA
2 methods give very small difference in the result
R.Seidl:Measurements of fragmentation functions at Belle: results and prospects 27
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Small double ratios in low thrust data sample
• Low thrust contains radiative effects
• Collins effect at least smeared for low thrust
Strong experimental indication that double ratio method works
A0
A12
R.Seidl:Measurements of fragmentation functions at Belle: results and prospects 28
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Results for e+ e- X for 29fb-1
• Significant non-zero asymmetries
• Rising behavior vs. z• cosdouble ratios
only marginally larger• First direct
measurement of the Collins function
• Integrated results:– cos(2) method
(3.06±0.57±0.55)%– cos(2) method
(4.26±0.68±0.68)%
z1
z2
Systematic error
A0
A12
Final results
R.Seidl:Measurements of fragmentation functions at Belle: results and prospects 29
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• Good detector performance (acceptance, momentum resolution, pid)
• Jet production from light quarks off-resonance (60 MeV below resonance)
(~10% of all data)• Intermediate Energy
Sufficiently high scale (Q2 ~ 110 GeV2)- can apply pQCD
Not too high energy (Q2 << MZ2)
-avoids additional complication from Z interference• Sensitivity = A2sqrt(N) ~ x19 (60) compared to LEP
ABelle / ALEP ~ x2 (A scales as ln Q2)
LBelle / LLEP ~ x23 (230)
9.44 9.46
e+e- Center-of-Mass Energy (GeV)
0
5
10
15
20
25
(e+
e-
had
ron
s)(n
b)
(1S)10.0010.02
0
5
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(2S)10.34 10.37
0
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(3S)10.54 10.58 10.62
0
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(4S)9.44 9.46
e+e- Center-of-Mass Energy (GeV)
0
5
10
15
20
25
(e+
e-
had
ron
s)(n
b)
(1S)10.0010.02
0
5
10
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(2S)10.34 10.37
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(2S)10.34 10.37
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(3S)10.54 10.58 10.62
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(3S)10.54 10.58 10.62
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(4S)
Belle is well suited for FF measurements:
R.Seidl:Measurements of fragmentation functions at Belle: results and prospects 30
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What is the transverse momentum QT of the virtual photon?
• In the lepton CMS frame e-=-e+ and the virtual photon is only time-like:q=(e-+p+)=(Q,0,0,0)
• Radiative (=significant BG) effects are theoretically best described in the hadron CMS frame wherePh1+Ph2=0
q’=(q’0,q’)• Inclusive Cross section for
radiative events (acc. to D.Boer):
Ph1
e+e-
Ph2
e-’
e+’
q’
P’h1 P’h2
qT
Lepton-CMS
Hadron-CMS
R.Seidl:Measurements of fragmentation functions at Belle: results and prospects 31
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Raw asymmetries vs QT
1hP
2hP
1hP
2hP
• QT describes transverse momentum of virtual photon in CMS system
•Significant nonzero Asymmetries visible in MC (w/o Collins)
•Acceptance, radiative and momentum correlation effects similar for like and unlike sign pairs
•uds MC () Unlike sign pairs
•uds MC () Like sign pairs
R.Seidl:Measurements of fragmentation functions at Belle: results and prospects 32
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Experimental issues
• Cos2 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(2Lab) asymmetries for jets or • False asymmetries
from weak decays Study effect in 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
R.Seidl:Measurements of fragmentation functions at Belle: results and prospects 33
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Take simple parameterization to test sensitivity on favored to disfavored Ratio
Favored/Disfavored contribution Sensitivity
2
cb
R.Seidl:Measurements of fragmentation functions at Belle: results and prospects 34
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• Similar to previous method• Observe angles 1R2R
between the event-plane (beam, two-pion-axis) and the two two-pion planes.
• Theoretical guidance by Boer,Jakob,Radici
Interference Fragmentation – “ “ method
R2R1221111 m,zHm,zHA cos
R2
R1
1hP
2hP
3hP
4hP
2h1h PP
4h3h PP
R.Seidl:Measurements of fragmentation functions at Belle: results and prospects 35
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What would we see in e+e-?
Simply modeled the shapes of these predictions in an equidistant Mass1 x Mass2 binning
m1
m1
m2
m2
“Jaffe” “Radici”
R.Seidl:Measurements of fragmentation functions at Belle: results and prospects 36
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• Spin composition of the nucleon:– Contribution of quark spin
small– Contribution of gluon spin
also small?– Orbital angular
momentum? Sivers effect?
• Single spin asymmetries small in pQCD– …but large and nonzero!– Understandable by gauge
link contributions
• Sivers effect:– Unpolarized quarks in transversely
polarized nucleon– Non-centered quark distribution in
impact parameter spaceRescattering creates left-right
asymmetry
• Boer-Mulders effect:• Transversely polarized quarks in
unpolarized nucleon• Vacuum polarization effect?
• Collins effect:• Fragmentation of transversly
polarized quark into unpolarized hadron
• Connects microscopic quantitiy (transverse spin) to macroscopic quantity (azimuthal asymmetries)
QCD: the right theory, but many surprisesespecially in spin …