1. Exploring the Spin Structure of the Proton with Two-Body Partonic Scattering at RHIC
J. Sowinski
For the
STAR
Collaboration
Few Body /24/06

2. Where does the proton’s spin come from?u d
p is made of 2 u and 1d quark
S = ½ = S Sq
Explains magnetic moment of baryon octet p
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!
Sz = ½ = ½ DS + DG + Lzq + Lzg

3. Parton Distribution Functions
SMC Analysis, PRD 58, (1998)
CTEQ5M
Gluons carry ~1/2 the momentum (mass)!
First Moments at Q02=1 GeV2:
DS(MS) = 0.19 ± 0.05 ± 0.04
DS(AB) = 0.38
DG(AB) = 0.99
(just one example of many) —
Maybe we shouldn’t be surprised that
quarks carry only ~1/3 of proton’s spin
DG is poorly constrained, even solutions with zero crossing allowed

4. DG via partonic scattering from a gluon
STAR g
ALL =
s++ - s+-
s++ + s+-
Know from DIS
g-jet coinc. rare
Measure
A ~ P 3P 3a ^ g
part LL LL
Jets and p0s
pQCD
“DG”
Prefer
Dominant reaction mechanism
Experimentally clean reaction mechanism
Large a
But jet and p0 rates are sufficient to give significant DG const. in first RHIC pol. p data
dG in partonic scattering
Qg scatter to jets know quark pol from DIS all from pQCD
Other rare channels have advantages
But L vs time start with pi0s and jets
Heavy flavor rare ^ LL

5. The Relativistic Heavy Ion Collider
~4 km circ. Collider
The first polarized p-p collider!
PHENIX
STAR
Brahms
pp2pp
PHOBOS
Heavy ions
Au-Au
Lighter ions
Asymmetric d-Au
4+ detectors
STAR
PHENIX
PHOBOS
Brahms
pp2pp (p-p only)
Retired

6. Dramatic Improvements in Polarized Beam Performance
2003   > 2 orders of magnitude improvement in FOM = P 4L relevant to 2-spin asymmetries!
BRAHMS
PHENIX
AGS
BOOSTER
Spin Rotators
(longitudinal polarization)
Solenoid Partial Siberian Snake
Siberian Snakes
200 MeV Polarimeter
AGS Internal Polarimeter
Rf Dipole
RHIC pC Polarimeters
Absolute Polarimeter (H jet)
AGS pC Polarimeters
Strong Helical AGS Snake
Helical Partial Siberian Snake
Spin flipper
STAR
PHOBOS
Pol. H- Source
LINAC
Absolute Pbeam calibration to ~ 5% goal in
progress
Factor ~ 5--6 remains to reach “enhanced design” goals
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
STAR Detector
Large solid angle
Not hermetic
Tracking in 5kG field
EM Calorimetry
“Slow” DAQ (100Hz)
Sophisiticated triggers
STAR

8. Time Projection Chamber
Lum. Monitor
Local Polarim.
Triggering
Detector
STAR
Barrel EM Calorimeter
-1<η< 1
Beam-Beam Counters
2<|η|< 5
2004
2003
2005
h = - ln(tan(q/2)
h=0
h= -1
h=2
Triggering
Endcap EM Calorimeter
Forward Pion Detector
1<η< 2
-4.1<η< -3.3
Time Projection Chamber
-2<η< 2
Solenoidal Magnetic
Field 5kG
Tracking

9. What is a jet?
Use Monte Carlo to correct data for comparison to theory
STAR
(Resolution, trigger, efficiency, fragmentation …)
parton
particle
detector
GEANT
pythia
q,g
STAR large solid angle to get jets
Multiple processes contribuite
But signif results from 05 data in sense can discriminate between std (large) and 0
Midpoint Cone Algorithm
Add 4 momenta of tracks and towers in cone around seed
R = 0.4 (h , f) year < 2006
Split and merge for stable groups

10. Cross Section Correction Factors
Results
Jet 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
STAR

11. First inclusive jet cross section result at RHIC
2004 p+p run
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
hep-ex
STAR

12. First ALL Measurement for Inclusive Jet Production
2003 (pol.~0.3) (pol. ~ 0.4) total 0.4 pb-1
Total systematic uncertainty ~0.01
Backgrounds
Relative Luminosity
Residual transverse asymmetries
Beam Polarization
Trigger Bias
jet cone= <hjet<0.8
2004 Prelim.
2003 Prelim.
STAR
Inclusive Jets: LO
(W. Vogelsang)
fraction
pT/GeV
hep-ex
STAR
Submitted for publication

13. Current Constraints on G
Photon-gluon fusion results:
COMPASS, HERMES, SMC photon-gluon fusion studies  ~ comparable G constraints to STAR jets and 2005 PHENIX  0 ALL
Fit to STAR ALLjet vs. assumed G at input scale: W. Vogelsang
Fit to PHENIX ALL vs. assumed G at input scale: W. Vogelsang

14. Projections from Collected Data
STAR
2005 Data
Jet patch triggers
Enhanced EM calorimeter coverage
L = 6 pb-1 P=0.6
DG=G
GRSV-std
DG=-G
DG=0
2006 Data
Software triggers
Full EM calorimeter coverage -1<h<2 including trigger
DiJets
Direct g-jet sample

15. Next Step is to Explore Dg(x)
jet g
Simulated data set
Exploit 2 body kinematics
Detect g and jet in coinc.
Measure ujet, Eg and ug
Extract x1, x2 and u*
Assume larger of x1 and x2 = xquark
Assume lesser = xgluon
Make cut that one x > 0.2
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 g-jets
Large coincident solid angle is crucial

16. Conclusions
RHIC has made tremendous progress in delivering polarized protons over past few years
Initial inclusive jet ALL results are providing significant constraints on DG
Much better jet statistics are already in hand from 2005 and 2006 data
Future studies with di-Jets and g-jet coinc. are expected to probe the shape, Dg(x)
STAR