Exploring the Spin Structure of the Proton with Two-Body Partonic Scattering at

RHIC

J. Sowinski

STARSTAR

For the

Collaboration

Few Body 2006 8/24/06

2

Where does the proton’s spin come from?

u u

d

p

Sz = ½ = ½ + G + Lzq + Lz

g

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!

3

CTEQ5M

SMC Analysis, PRD 58, 112002 (1998)

First Moments at Q02=1 GeV2:

(MS) = 0.19 ± 0.05 ± 0.04

(AB) = 0.38

G(AB) = 0.99

(just one example of many)

+ 0.03 + 0.03 + 0.03- 0.03 - 0.02 - 0.05

+ 1.17 + 0.42 + 1.43- 0.31 - 0.22 - 0.45

—

Gluons carry ~1/2 the momentum (mass)!

Maybe we shouldn’t be surprised thatquarks carry only ~1/3 of proton’s spin

G is poorly constrained, even solutions with zero crossing allowed

Parton Distribution Functions

4

A ~ P P a LL g part LL^

pQCD

MeasureKnow from DIS

“G”

G via partonic scattering from a gluon

• Dominant reaction mechanism

• Experimentally clean reaction mechanism

• Large a• But jet and 0 rates are

sufficient to give significant G const. in first RHIC pol. p data

Prefer

LL^

-jet coinc. rare

STARSTAR

Heavy flavor rare

Jets and 0s

ALL = ++ - +-

++ + +-

5

The Relativistic Heavy Ion Collider

PHENIXSTAR

Brahmspp2pp

PHOBOS

~4 km circ. Collider

Heavy ions• Au-Au• Lighter ions• Asymmetric d-Au

4+ detectors• STAR• PHENIX• PHOBOS• Brahms• pp2pp (p-p only)

The first polarized p-p collider!

500GeV24GeV s

Retired

6

Dramatic Improvements in Polarized Beam Dramatic Improvements in Polarized Beam Performance Performance 2003 2006 >

2 orders of magnitude improvement in FOM = P 4L relevant to 2-spin asymmetries!

Factor ~ 5--6 remains to reach “enhanced design” goals

BRAHMS

PHENIX

AGS

BOOSTER

Spin Rotators(longitudinal polarization)

Solenoid Partial Siberian Snake

Siberian Snakes

200 MeV PolarimeterAGS Internal Polarimeter

Rf Dipole

RHIC pC PolarimetersAbsolute Polarimeter (H jet)

AGS pC Polarimeters

Strong Helical AGS Snake

Helical Partial Siberian Snake

Spin Rotators(longitudinal polarization)

Spin flipper

Siberian Snakes

STAR

PHOBOS

Pol. H- SourceLINAC

Absolute Pbeam calibration to ~ 5% goal in progress

STAR s = 200 GeV pp Sampled

Luminosities

7

The STAR Detector at RHIC

At the heart of STAR is the world’s largest Time Projection Chamber

STARSTAR

STAR Detector• Large solid angle• Not hermetic• Tracking in 5kG field• EM Calorimetry• “Slow” DAQ (100Hz)• Sophisiticated triggers

8

200320042005

Detector

=0

Forward Pion Detector

Endcap EM Calorimeter

Beam-Beam Counters

Time Projection Chamber

-2<η< 2

Barrel EM Calorimeter

-1<η< 1

1<η< 2-4.1<η< -3.3

2<|η|< 5

Solenoidal MagneticField 5kG

=2= -1

Tracking

Lum. Monitor Local Polarim.

Triggering

Triggering

= - ln(tan(/2)

STARSTAR

9

STARSTAR

What is a jet?

Midpoint Cone Algorithm• Add 4 momenta of tracks and

towers in cone around seed• R = 0.4 (, ) year < 2006• Split and merge for stable groups

part

onpa

rtic

lede

tect

or

etcp

e

,,

,

GE

AN

Tpy

thia

q,g

Use Monte Carlo to correct data for comparison to theory

(Resolution, trigger, efficiency, fragmentation …)

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2003 + 2004 ResultsJet Shape

• (r) = Fraction of jet pT in sub-cone r

• Study of trigger bias• Study of data/MC

agreement• High Tower trigger • Bias decreases with pT

Cross Section Correction Factors

• MinBias correction ~ 1• Corrections (1/c(pT) can be

large for High Tower data

STARSTAR

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• Sampled luminosity: ~0.16 pb-1

• Good agreement between minbias and high tower data

• Good agreement with NLO over 7 orders of magnitude – slope

• Good agreement with NLO magnitude within systematic uncertainty

• Error bars: Statistical uncertainty from data

• Systematic error band

Leading systematic uncertainty

10% E-scale uncertainty 50% uncertainty on yield

• Out of cone hadronizaton and underlying event ~25% corr. not shown

First inclusive jet cross section result at RHIC

2004 p+p run

STARSTARhep-ex0608030

12

jet cone=0.4 0.2<jet<0.8

2004 Prelim.2003 Prelim.

STARSTAR

First ALL Measurement for Inclusive Jet Production

Submitted for publication

• 2003 (pol.~0.3) + 2004 (pol. ~ 0.4) total 0.4 pb-1

• Total systematic uncertainty ~0.01▪ Backgrounds▪ Relative Luminosity▪ Residual transverse

asymmetries▪ Beam Polarization▪ Trigger Bias

STARSTAR

Inclusive Jets: LO (W. Vogelsang)

fra

cti

on

pT/GeV

hep-ex0608030

13

Current Constraints on G

Fit to STAR ALLjet

vs. assumed G at input scale: W. Vogelsang

Fit to PHENIX ALL

vs. assumed G at input scale: W. Vogelsang

Photon-gluon fusion results:

COMPASS, HERMES, SMC photon-gluon fusion studies ~ comparable G constraints to 2003+4 STAR jets and 2005 PHENIX 0 ALL

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Projections from Collected Data

L = 6 pb-1 P=0.6

G=G

GRSV-std

G=-G G=0

2005 Data• Jet patch triggers• Enhanced EM

calorimeter coverage

2006 Data• Software triggers• Full EM

calorimeter coverage -1<<2 including trigger

• DiJets• Direct -jet

sample

STARSTAR

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Next Step is to Explore g(x)

• Exploit 2 body kinematics • Detect and jet in coinc.

• Measure jet, E and • Extract x1, x2 and *

• Assume larger of x1 and x2 = xquark

• Assume lesser = xgluon

• Make cut that one x > 0.2

jet

Simulated data set

• Large data sets at 200 and 500 GeV• 500 GeV => low x• Overlap gives same x with different

pT to check scaling• Di-Jets

• Similar kinematics• Less selective for gluons• Lower sensitivity but larger cross

section than -jets

Large coincident solid angle is crucial

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Conclusions• RHIC has made tremendous progress in

delivering polarized protons over past few years

• Initial inclusive jet ALL results are providing significant constraints on G

• Much better jet statistics are already in hand from 2005 and 2006 data

• Future studies with di-Jets and -jet coinc. are expected to probe the shape, g(x)

STARSTAR