measurement of sivers asymmetry for di-jets at star in polarized p+p collisions at 200gev

Jan Balewski, IUCFDIS 2007

April 16-20, 2007Munich, Germany

Measurement of Sivers asymmetry for di-jets at STAR

in polarized p+p collisions at 200GeVOutline• Motivation • Theory predictions• Principle of measurement, L2 trigger• Fast M-C model of Sivers spin asymmetry • Measured SSA• Conclusions

2 Di-jets at STAR Jan Balewski (IUCF)

Where does the proton’s spin come from?

u u

d

p

½ = ½ + G + Lq + Lg

p is made of 2 u and 1d quark

S = ½ = Sq

Explains magnetic moment of baryon octet

BUT partons have an x distribution and there are sea quarks and gluons

Check via electron scattering and find quarks carry only ~1/3 of the proton’s spin!

Jets,pions,ALLDi-jets, Sivers AN


HERMES transverse spin SIDIS asymmetries 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, 094025 (2004))

Both beams polarized, x +z x z 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 pp Motivation for pp Di-Jet Di-Jet MeasurementMeasurement

zx

y

Colliding beams

proton spin

parton kTx

TN SkpA


History of Predictions of Sivers AN for Di-jets @ STAR

pp dijet + X s = 200 GeV 8 pT(1,2) 12 GeV |(1,2)| 1.0

D. Boer & W. Vogelsang, PRD 69, 094025 (2004) – various models

Bacchetta, Bomhof, Mulders, Pijlman

AN, W=sin()

ISI

ISI+FSI

SPIN 2006

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

HERMES SIDIS

ISI only


Detector

=0

Endcap EM Calorimeter

Beam-Beam Counters

Time Projection Chamber

-2<η< 2

Barrel EM Calorimeter

-1<η< 1

1<η< 2

2<|η|< 5

Solenoidal MagneticField 5kG

=2= -1

Tracking

Lum. Monitor Local Polarim.

Triggering

Triggering

= - ln(tan(/2)

STARSTAR

+z beam -z beam

yz


L0 Jet Trigger

1

2

3

4

jet patch

size:1x1 x

7

8

9

10

11

12

+13,…,18

Luminosity 3*1031/cm2/sAllocated Jet Rate to tape only 15 Hz

1x1 Jet patch ET/GeV

4 8

Rate: 150 Hz, needs prescale by 20 or…smarter trigger

Rate: 3 Hz, sent to tape18 Hardwired 1x1 Jet patches


STA

RS

TAR

STAR Di-Jet Software Trigger (developed in 2006)

Rates @ STAR • 9.4 MHz : RHIC bunches collide • 300 kHz : min-bias collisions trigger BBCs • 150 Hz : L0 single jet exceeds 4 GeV ET

in a hardwired 1x1 ( x ) EMC patch• 10 Hz : L2 di-jet trigger with ET > 3.5 GeV

for a pair of back-to-back sliding 0.6x0.6 patches ~80 muSec decision time

=0o

L2 algoresult

Comparison with off-line similar

2005 data event

-1 0 +2

=180o

=360o

~18

0 o

back

-to-

back

di-j

et

Zaxis=ET/GeV

Barrel ||<1

Endcap EMC

=[1,2]


STAR Di-Jets Acquired in 2006 runSTAR Di-Jets Acquired in 2006 run 2006 p+p run, 1.1 pb1

2.6M di-jet triggered events

2 localized clusters, with ET

EMC > 3.5 GeV, | | > 60

Full, symmetric 1,2 coverage at STAR

X-axis

1

2

Is -distribution narrow enough

for precise determination of centroid ?

spin

Bisector RIGHT

Jet 1

Jet 2

Reco cos(bisector) measures

sign of net kTx for event

Signed azimuthal opening angle

LEFT

Sivers SSA


Fast M-C Captures Essence of Sivers SSA

•Back-to-back partonic collision yields a di-jet

•intrinsic kT1,2 smearing2D Gauss(0,kTx, kTy)

• sum kT from both ISkT=kT1+kT2

• sum: partonic pT + intrinsic kT

Model Generator:Tp

Tp

yx

180open

jet 1

jet 2

Add Spin dependent kTX offsets

180open

1

2

spin

bisector

avr kTx

Bac

k-to

-bac

k je

ts


Fast M-C Tuned to Real Di-jets

Data

Fast M-C Data

Fast M-C

y

x


D. Boer & W. Vogelsang, PRD 69, 094025 (2004)

-

Fast M-C: shift in equivalent to Sivers AN

Displaced peak for pol=+ and -

SSA AN results

naturally asthe difference

Assume non-zero Sivers <kTX>=100 MeV/c

‘Calibration’ of A N vs.

amplitude Sivers <k TX >


Error-weighted avg. of 16 independent AN(>) values for |cos(bisector)| slices, with effective beam polarization for each = Pbeam |cos(bisector)|

rotation samples kT

y, parity-violating sp•kT correl’n

STAR data both jets rotated by 90

Null Tests

STAR Di-Jet Results Integrated Over STAR Di-Jet Results Integrated Over PseudorapidityPseudorapidity

Sivers asymmetries consistent with zero with stat. unc. = 0.002

Fast M-C model sensitivity to Sivers kTx offset few MeV/c 0.004 (kT

x)

Systematic uncertainties smaller than statistics

All null tests, including forbidden 2-spin asym. cos(bisector), consistent with zero

Validity of spin-sorting confirmed by reproducing known non-zero AN for inclusive forward charged-particle production (STAR BBC’s)

AN+z AN

-z 2-spin AN+z AN

-z 2-spin

Emphasizes (50%+ ) quark Sivers

STAR Results vs. Di-Jet Pseudorapidity SumSTAR Results vs. Di-Jet Pseudorapidity Sum

V&Y calcs. include:

no hadronization

5 < pTparton < 10 GeV/c

STAR acceptance

HERMES-fitted q Sivers

no gluon Sivers fcns.

They predict:

AN ~ ANHERMES where q

dominates, if only ISI included (à la Drell-Yan)

AN sign reversal, factor ~2 reduction when gauge links include FSI as well

Reverse V&Y signs here to obey Madison convention!

STAR AN all consistent with zero both quark and gluon Sivers effects ~order of magnitude smaller in pp di-jets than SIDIS quark Sivers asym.!

Emphasizes (80%+) gluon Sivers

Emphasizes (50%+ ) quark Sivers

Spin

Polquark

Unpol

qluon+z

+z

y

x


Bacchetta, Bomhof, Mulders, Pijlman

AN , yields weighted w/ |sin()|

ISI

SPIN 2006

Bomhof, Mulders, Vogelsang, Yuan, hep-ph/070127731 Jan 2007

ISI+FSIu-quark

d-quark

u+d

Theory Few Months Later

Delicate cancellations of Sivers fcn for:• u and d quarks and

• ISI and FSIreconciles theory & experiment

ISI+FSI


STAR Results : weighted ASTAR Results : weighted ANN

AN

Di-jet yield

=180 deg

Di-jet yieldW=|sin()|

Theory calculation for AN more robust,better characterization of initial state kT.Describes forward inclusive pi0's and

analogous k_T-weighted SIDIS asymmetries moderately well

AN

(weighted)


Summary & Outlook

•2x larger statisticsInclude HT & JP1 triggers

• energy dependence• quark/gluon enhancement• u-,d-quark flavored jets

Full jet reco : charge tracks & EM

• STAR acquired 2.6M di-jet events w/ transversely polarized proton beams at sqrt(s)=200 GeV

•Experimental values of Sivers SSA for quarks & gluons is consistent w/ zero +/-0.002

•Early large predictions tuned to non-zero HERMES quark Sivers SSA has been reduced due to accounting for cancellation between ISI & FSI and u- and d-quarks Sivers fcns.

•More detailed analysis is ongoing ….


Backup Slides


CNI Measured On-line 60%+ Polarization at RHIC in 2006

CNIpol

Year 2006

Year 2002


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

spinInfo

problem good STAR datagood data

days in 2006

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


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


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

measurement of sivers asymmetry for di-jets at star in polarized p+p collisions at 200gev

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