Jan Balewski: Di-jet Sivers in p+p @ 200 GeV 1 Outline Motivation Principle of measurement, trigger Model of Sivers asymmetry Measured SSA & DSA Comparison

Jan Balewski: Di-jet Sivers in p+p @ 200 GeV

1

Outline• Motivation • Principle of measurement, trigger• Model of Sivers asymmetry • Measured SSA & DSA • Comparison with theory• Conclusions

Measurement of Sivers Asymmetry for Di-jets in 200 GeV pp Collisions at

STAR

Jan Balewski, IUCFfor STAR Collaboration

RHIC Spin Physics, RIKEN, Japan

September 29, 2006


2

Sivers effect:

Sensitive to parton orbital angular momentum

Needs ISI and/or FSI to evade time-reversal violation.

HERMES transverse spin SIDIS asymmetries non-zero u and d quark

Sivers functions of opposite sign, different magnitude.

Sivers effect in pp spin-dependent sideways boost to di-jets,

suggested by Boer & Vogelsang (PRD 69)

Both beams polarized, xa xb can distinguish q vs. g Sivers effects.

Do we observe q Sivers consistent with HERMES? Tests universality.

First direct measurement of gluon Sivers effects.

Motivation for Di-Jet Sivers Effect Motivation for Di-Jet Sivers Effect MeasurementMeasurement

0

processobserved

partonTprotonproton kpS


3

Sivers Mechanism of SSA

yx

Spin dependent kTX offset

• deflects both jets in the same direction• reduces average di-jet opening angle• can be measured from correlation between di-jet bisector vs. spin direction• from accumulated -spectra (spin)

intrinsickT

unpol proton 2

proton 1

intrinsic kT

p1

p2

Tp

Tp

180open

jet 1

jet 2

2 2 partonicscattering

p3

p4

180open

1

2

spin

di-jetbisector

kTx

>

180

for

k T

x > 0

spin

+ spin-dep. kT

x

Polarized


4

Is -distribution narrow enough

for precise determination of centroid ?

Have we measured di-jets?

EMC-based Di-jet Trigger in STAR

=+2

= -1

TPC

EMC Barrel

EMC Endcap

BBC East BBC West

Yellow beamBlue

spin

bisector

Jet 1

Jet 2

Reco cos(bisector) measures

sign of net kTx for event

2/43

Endcapessential for q vs.g Siversdistinction

L0 jet patch trigger + L2 EMC-only dijet-finder 2.6M event sample from 1.1 pb–1 of transverse p+p collisions during 2006 run

3 (deg)

4

(deg

)

|3-

4| <

60

L2 trigger thresholds: ETEMC > 3.6, 3.3 GeV


5

How close EMC-only jets resemble full reco jets ?

How close full recon jets resemble original

scattered partons ?

(rad)

Jet finder •TPC+EMC

• jet cone radius 0.6

-resolution, pT of Reconstructed Di-jets

Typical xT ~ 0.05 - 0.10; 1+2

range 0.01 < xBj < 0.4

Data: offline di-jet reco for~2% of all runs

PYTHIA+GEANT: full reco jet vs. parton

EMC-only resol’n <<

kT-induced width

8.5

06.0

9.3

(full reco) – (L2) [deg]

Full width ~21


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(+) - (-) vs. AN ?

Toy Model of Sivers Spin Effect Generates AN

)()(

)()(

)()(

)()(

1

11)( :asymmetry Sivers Integrated

,

,

,

,

,

NN

NN

NN

NNr

r

r

PA

YB

YB

YB

YB

YBN

•Throw 1e6 2-parton events in transverse plane•Match to pT distribution from full jet reco.•Gaussian + exp. tail kT distrib’n reproduces shape •Sivers spin-dep. kT

x offset spin-dep. distrib’n shift•Vary kT

x offset, see how pol’n observables change•Can introduce resolution smearing, see its effects

AN=measure of L-R

asymmetry of kT distribution in L-R symmetric detector

Blue beam spin down

Blue beam spin up

(+) - (-)

)( N)( N

method ratio

-crossby extracted

)( NA

Intrinsic kTx,y width =

0.75 GeV/c () = 6.3 resol’n smear reproduces observed width

smearing dilutes AN by only 10% of its value EMC-only analysis OK


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Measured Sivers AN for Di-jets

• AN – L/R asymmetry

integrated over STAR detector pseudorapidity

STAR EMC coverage

Blue(+z) Yellow(-z)

proton proton

-1 +2

• Event weights by |cos(bisector)| account for beam pol’n compo-nent kT plane.

STAR data both jets rotated by 90

rotation samples kT

y, parity-violating sp•kT correl’n

Null Tests


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V & Y use these Sivers fcns. (assuming zero gluon Sivers) to predict SSA for pp dijet + X, integrated over kT distributions within both protons. Note strong y- and weak pT - dependence.

Quark Sivers Functions Constrained by SIDIS Quark Sivers Functions Constrained by SIDIS ResultsResults Sivers sensitivity in depend-

ence of SIDIS yield on (hadron – proton spin ).

HERMES positive Sivers SSA for +, but 0 for –, fitted by Vogelsang & Yuan with u and d quark Sivers fcns. of different sign and magnitude:

xxxud

xxxuuVY

T

T

186.1)(/

181.0)(/:1)2/1(

)2/1(

W. Vogelsang and F. Yuan, PRD 72, 054028 (2005).

Jet 1 rapidity Jet pT (GeV/c)

xxxdd

xxxuuVY

T

T

176.2)(/

175.0)(/:2)2/1(

)2/1(


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Measured Sivers AN for Di-jets vs. Theory

• Model w/o hadronization, integrated over STAR , 5<pT<10 GeV/c, includes only quark Sivers -- predicts AN ~ AN

HERMES where q Sivers dominates• Sign of predictions reversed to adhere to Madison AN sign conventio

STAR measured AN all consistent with zero both quark and gluon Sivers effects much smaller in pp di-jets than in HERMES SIDIS !!

Emphasizes (80%+) gluon Sivers

Emphasizes (50%+ ) quark Sivers


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Theory - exp’t discrepancy raises questions!Are observed di-jet Sivers SSA much smaller than predictions because:

ISI & FSI both important in pp jets and tend to cancel?

Need q Sivers or different q Sivers x, kT - shapes in HERMES fits?

If ISI / FSI cancel at mid-rapidity, does their balance change at high to yield sizable Sivers contribution to observed pp 0X SSA?

S

Initial state interaction

p

p

q

q

l

l +

*SIDIS

Drell-Yan

ISI for Drell-Yan vs. FSI for SIDIS opposite sign predicted for Sivers SSA


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Summary and Conclusions Large STAR EMC acceptance large 2006 triggered di-jet

data set.

Di-jet ANSivers all consistent with zero quark Sivers effects ~

5-10 smaller than expected from SIDIS. Gluon Sivers comparably small.

Sensitivity of analysis to ANSivers 0 confirmed by toy model

simulations. Beam spin-sorting verified via known non-zero SSA.

Results constrain treatments of Sivers effect and of transverse spin asymmetries in pp collisions.

Extend di-jets in future to higher , where ANmeas

(pp 0X)

sizable.

Documents

Jan Balewski: Di-jet Sivers in p+p @ 200 GeV 1 Outline Motivation Principle of measurement, trigger Model of Sivers asymmetry Measured SSA & DSA Comparison