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

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

Parton Distribution Functions SMC Analysis, PRD 58, 112002 (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) + 0.03 + 0.03 + 0.03 - 0.03 - 0.02 - 0.05 + 1.17 + 0.42 + 1.43 - 0.31 - 0.22 - 0.45 — 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

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

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

Dramatic Improvements in Polarized Beam Performance 2003  2006  > 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

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

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

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

Cross Section Correction Factors 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 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 STAR

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

Current Constraints on G Photon-gluon fusion results: COMPASS, HERMES, SMC photon-gluon fusion studies  ~ comparable G constraints to 2003+4 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

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

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

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