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

1. STAR Exploring the Spin Structure of the Proton with Two-Body Partonic Scattering at RHIC For the J. Sowinski Collaboration Few Body 2006 8/24/06

2. Where does theproton’sspincome from? u u d pis made of 2uand1dquark S=½ =S Sq Explains magnetic moment of baryon octet p BUT partons have an x distribution and there are seaquarksandgluons Check viaelectron scatteringand findquarkscarryonly ~1/3of theproton’s spin! Sz=½= ½DS+DG +Lzq+Lzg

3. SMC Analysis, PRD 58, 112002 (1998) CTEQ5M 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) — + 0.03 + 0.03 + 0.03 - 0.03 - 0.02 - 0.05 + 1.17 + 0.42 + 1.43 -0.31 - 0.22 - 0.45 Parton Distribution Functions Gluons carry ~1/2 the momentum (mass)! 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. s++ - s+- ALL= s++ + s+- g STAR DG via partonic scattering from a gluon Know from DIS g-jet coinc.rare Measure • 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 ^ A ~ P3P3a g part LL LL pQCD Jets and p0s “DG” Prefer Heavy flavorrare ^ LL

5. Brahms pp2pp PHENIX STAR The RelativisticHeavyIonCollider ~4 km circ. Collider The first polarized p-p collider! 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 RHIC pC Polarimeters Absolute Polarimeter (H jet) BRAHMS PHOBOS Absolute Pbeam calibration to ~ 5% goal in Siberian Snakes Siberian Snakes progress PHENIX STAR Spin Rotators (longitudinal polarization) Spin flipper Spin Rotators (longitudinal polarization) Solenoid Partial Siberian Snake Pol. H- Source Helical Partial Siberian Snake LINAC BOOSTER AGS Internal Polarimeter AGS 200 MeV Polarimeter AGS pC Polarimeters Strong Helical AGS Snake Rf Dipole 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 STAR  s = 200 GeV pp Sampled Luminosities

7. STAR 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

8. Detector Lum. Monitor Local Polarim. Barrel EM Calorimeter -1<η< 1 STAR 2003 2005 2004 Triggering Beam-Beam Counters 2<|η|< 5 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. STAR GEANT detector particle pythia q,g parton What is a jet? Use Monte Carlo to correct data for comparison to theory (Resolution, trigger, efficiency, fragmentation …) • 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. STAR 2003 + 2004 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

11. STAR 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-ex0608030

12. STAR 2004 Prelim. jet cone=0.4 0.2<hjet<0.8 2003 Prelim. Inclusive Jets: LO (W. Vogelsang) STAR fraction pT/GeV • 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 First ALL Measurement for Inclusive Jet Production hep-ex0608030 Submitted for publication

13. Current Constraints on G Photon-gluon fusion results: Fit to STAR ALLjet vs. assumed G at input scale: W. Vogelsang Fit to PHENIX ALL vs. assumed G at input scale: W. Vogelsang COMPASS, HERMES, SMC photon-gluon fusion studies  ~ comparable G constraints to 2003+4 STAR jets and 2005 PHENIX  0 ALL

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

15. g jet Simulated data set Next Step is to Explore Dg(x) • 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. STAR 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)