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

Jan Balewski: Di-jet Sivers in p+p @ 200 GeV 1

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

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

STARJet 1

Jet 2

VerticalSpin

Sivers

+kTx

spin+

180o (deg)

Jan Balewski, IUCFfor STAR CollaborationSPIN2006, Kyoto, Japan

z

yx


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


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


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


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


(+) - (-) 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

beam 1 spin down

beam 1 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


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


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(


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


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?

Contributions from Collins fragmentation asymmetries in incompletely reconstructed jets conspire to cancel Sivers asymmetries for quarks, while gluon Sivers functions naturally small?

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


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.


BACKUP


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.

2 1

unpol4

3

1,2, 43

Yellow

SY0 N

xT

A

k1 2

unpol3

4

Blue

SY

z

y

2,1, 43

Cross-ratio forbidden 2-spin correlation cos(bisector)

STAR data both jets rotated by 90

rotation samples kT

y, parity-violating sp•kT correl’n

Null Tests


Systematic Corrections, Checks, Errors

1) L2 jet vs. parton resolution smearing:

Systematic uncertainty = 0.05 AN covers model-dependence of toy-model dilution factor already included.

2) Different di-jet yield extraction methods: Syst. Unc. = 0.2 (stat. unc.) covers observed

changes when integration range, baseline subtraction are varied.

ANfalse requires instrumental spin-dependent change in distribution

shape. Cross-ratio extraction, excellent L-R detector symmetry, bunch-to-bunch spin changes, etc., make these very small.

3) Beam polarization calibration: used online RHIC polarimeter results: PB = 0.57, PY = 0.59. Normalization unc. = 0.15 AN .

4) Other sources: syst. unc. < 0.001, confirmed by null tests & consistency of results for different polarization fill patterns.


STAR Excellent Coverage for Di-Jets

+60 deg

Jet 1ET> 3.6 GeV

Jet 2ET> 3.3 GeV

-60 deg

=+

=+2

= -1

TPC

EMC Barrel

EMC Endcap

BBC East BBC West

yellowblue

EMCal Geometry in L2 trigger

-1 0 1 2

EndcapEMC

x

y Barrel EMC

-units

L2jet


=0o

L2 algoresult

STAR Triggered on Di-Jet Found On-line

Comparison with off-line similar

L2 di-jet logic:• 5500 EMC towers• sliding jet patches• jet size : 0.6x0.6 (x)• jet1 Et>3.6, jet 2 Et>3.3 GeV• decision : 60 muSec/event• event rate ~8 Hz

2005 data event

L0 di-jet logic:• BBC East.West• Etot >14 GeV• hardwired jet patches• jet size : 1x1 (x)• one jet Et>4.0 GeV• event rate ~120 Hz

L0-L2 logic benefits:• large acceptance combined with trigger rejection power• full statistics on-line trigger monitoring of EMC• compact trig info saved for each event

-1 0 +2

=180o

=360o

~18

0 o

back

-to-

back

di-j

et

Z=Et/GeV

Jet is weighted with EM ET


Toy M-C Matched to Real Data

Toy M-C

pp200Real L2 di-jet events

Jet 1

Jet 2

()=220

()=21o

Bac

k-to

-bac

k di

-jet

Tails are different


3 Alternative Sivers Spin Observables Sivers kTx leads to displacement of (other beam unpolarized)

180o (deg)

eps

N>,+

spin+spin-

Width of influenced by DSA

Blue + Yell +

Z-axis

++

kTx destructive

Blue + Yell - +-

kTxconstructive

N<,-

N<,+ N>,-

),0(cos

),0(cos

),0(cos

),0(cos

,

,

,

,

bis

bis

bis

bis

NN

NN

NN

NN

epsR w/ more

-slices AN()

Based on Toy M-C

rel. sensitivity of 7

1

7

,,

,,,1

1

NN

NNR

R

RepsR

eps

eps

Sivers Sensitive Observables

independent

x

widens

narrows


•3M events•Fit: A-B*sin(2)

+/- 1(A)

epsN=0

UnpolarizedYield()

eps N

()

eps N

()

eps N

()

Blue SSA

Yellow SSA ‘mixed’ DSA

Di-jet Results Sivers epsN()=AN ()*P

May 2006


2006 pp200 Transverse Run @ RHIC

Yellow bXing (uses revTick)

STAR event count

Yel

low

A

bort

gap

Blu

e

Abo

rt g

ap

spin8 stateSTAR spin sorting (uses V124)

Imperfections : few ppm

Available STAR events:• both TPC & EMC active• few M 1-jet trig, ET>8.0 GeV• 3M di-jet trig, 2xET>3.7 GeV• few M di-jet w/o TPC (crude run QA)

Example of di-jet events for one of 4

RHIC fill polarization patterns { \\ , \\+1, // , //+1 }

STAR magnetincident

( ~10 days )

May 9April 6

WCM intensity

No spinInfo

@ STAR good STAR datagood data

days in 2006

other valueB –Y – 51B –Y + 53B +Y – 83B +Y + 85


Run QA

Pass 1 (results presented in May of 2006)• use run Browser: TPC & BTOW & ETOW• triggers: BJP0, EJP0•classify runs based on fill pattern 1-4• drop run if mean -180o more than: - 1 deg AND it is more than 2 sigma effect, or - 3 deg (no matter what sigma) - in any of the subsamples: bjp0 or ejp0

Polpatt

avaliable: #runs, #events

accepted: #runs, #events

Px0 73, 725K 61 , 667K

Px1 102, 1226K 89 ,1150K

Px2 113, 1255K 90 , 1060K

Px3 114, 1338K 92 , 1083K

Pass 2 (new TrgJetTree in hand)• use run Browser: BTOW & ETOW (include no-TPC runs)• triggers: BJP0, EJP), minB, zB• use official off-line QA (from Jim & Co.)• suggestions?


Offline Evaluation of Di-jets

Input: 84k BJP1 triggered events from 2005( L0 BJP1: ET>4 GeV , only BTOW-W)

L2 algo: uses BTOW E+W

Offline jet finder : no cut

all BJP1 trigET2>3 GeVET2>4 GeVET2>5 GeV

• Bias introduced by L2 di-jet 2006 trigger

•Compare 2006 L2 vs. off-line di-jets - Issam Qattan, IUCF

•Avoid any -shape model, fit ’= a*+b to match + & - spin state distributions

(perhaps it will work?)

all BJP1 trigET2>3 GeVET2>4 GeVET2>5 GeV

ET EMC / ET TOT

2005 data

2005 data


Expectation: Sivers Asy Depends on Partonic Process

Endcap+Barrel

Barrel+Barrel

E+E

B+E

Favors quark jets

Available info (trigger level)•Eta1,2•ET1,2•Et tot•Tower multiplicity•Additional info

Example:Asymmetric Endcap-Endcap jetsQuark –blue beamGluon – yellow beam

Favorsquark-gluon

Z=# ofevents

Selecting regions in & pT changesprobabilities of partonic processes.

Justin Stevens


Is the Di-jet Spin Sorting Correct? ( Find MinB BBC SSA)

+Z

2

6

WestBBC

X6

2

EastBBC

Y

Bluebeam

Yellowbeam

Top view

Blue-Left

Yellow-Left

West-Blue5.6

West-Yellow0.3

East-Blue0.1

East-Yellow5.8

Pol patt 1,2,3,4 YES !

12M MinB evens


An Integrated vs. Averaged

Single-Transverse Spin Asymmetries

Werner VogelsangBNL Nuclear Theory

Spin 2006, Kyoto, 10/02/2006


(2) Non-universality in ppjet jet XBacchetta, Bomhof, Mulders, Pijlman

• careful analysis of gauge links in 22 QCD scattering

• color structure of amplitudes plays role

• complicated, but simplifies if moment is taken:

• then everything has structure as in unpol. case, but with different color weights:


• phenomenology is beginning :

AN

Unpol.partoniccross sections

Taking into accountProcess-dependence

See FengYuan’s talk

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

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