Transcript Slide 1
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? p is made of 2 u and 1d quark S = ½ = S Sq u u Explains magnetic moment of baryon octet d 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 2 Parton Distribution Functions SMC Analysis, PRD 58, 112002 (1998) CTEQ5M 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 First Moments at Q02=1 GeV2: — = 0.19 ± 0.05 ± 0.04 DS(MS) DS(AB) = 0.38 + 0.03 + 0.03 + 0.03 - 0.03 - 0.02 - 0.05 DG(AB) = 0.99 + 1.17 + 0.42 + 1.43 - 0.31 - 0.22 - 0.45 3 (just one example of many) DG via partonic scattering from a gluon STAR g g-jet coinc. rare Jets and p0s ALL = s++ - s+s++ + s+- Measure Know from DIS ALL~ Pg 3P part3a “DG” Prefer Heavy flavor rare ^ LL pQCD • Dominant reaction mechanism • Experimentally clean reaction mechanism ^ • Large a LL • But jet and p0 rates are sufficient to give significant DG const. in 4 first RHIC pol. p data The Relativistic Heavy Ion Collider ~4 km circ. Collider Brahms pp2pp PHOBOS PHENIX STAR 24GeV s 500GeV The first polarized p-p collider! Heavy ions • Au-Au • Lighter ions • Asymmetric d-Au 4+ detectors • STAR Retired • PHENIX • PHOBOS • Brahms • pp2pp (p-p only) 5 Dramatic Improvements in Polarized Beam 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 RHIC pC Polarimeters Absolute Polarimeter (H jet) BRAHMS PHOBOS Siberian Snakes PHENIX Absolute Pbeam Siberian Snakes calibration to ~ 5% goal in progress STAR Spin Rotators (longitudinal polarization) Pol. H Source Solenoid Partial Siberian Snake LINAC Spin Rotators (longitudinal polarization) Helical Partial Siberian Snake BOOSTER AGS 200 MeV Polarimeter AGS Internal Polarimeter AGS pC Polarimeters Rf Dipole Strong Helical AGS Snake Long. L dt Trans. L dt 2002 2003 2004 2005 [pb–1] 0.3 0.3 0.4 3.1 2006 8.5 [pb–1] 0.15 0.25 0 0.1 3.3 (slow det.) 6.8 (fast det.) Year STAR s = 200 GeV pp Sampled Luminosities Spin flipper Pbeam 15% 30% 40-45% 45-50% 60% 6 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 7 Triggering Barrel EM Calorimeter Detector STAR -1<η< 1 Lum. Monitor Local Polarim. Beam-Beam Counters 2<|η|< 5 h = - ln(tan(q/2) h= -1 h=0 h=2 Triggering Endcap EM Calorimeter Forward Pion Detector 1<η< 2 -4.1<η< -3.3 Time Projection Chamber Tracking -2<η< 2 Solenoidal Magnetic Field 5kG 2003 2005 2004 8 What is a jet? Use Monte Carlo to correct data for comparison to theory 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 parton pythia STAR particle detector GEANT (Resolution, trigger, efficiency, fragmentation …) e, g p , p, etc q,g 9 2003 + 2004 Results Jet Shape • (Dr) = Fraction of jet pT in sub-cone Dr • 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 10 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 hep-ex0608030 underlying event ~25% corr. not shown STAR 11 First ALL Measurement for Inclusive Jet Production 2004 Prelim. 2003 Prelim. • 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 Inclusive Jets: LO (W. Vogelsang) fraction STAR hep-ex0608030 Submitted for publication STAR pT/GeV 12 Current Constraints on DG Photon-gluon fusion results: COMPASS, HERMES, SMC photon-gluon fusion studies ~ comparable DG constraints to 2003+4 STAR jets and 2005 PHENIX p 0 ALL Fit to STAR ALLjet vs. assumed DG at input scale: W. Vogelsang Fit to PHENIX ALLp vs. assumed DG at input scale: W. Vogelsang 13 STAR Projections from Collected Data 2005 Data • Jet patch triggers • Enhanced EM calorimeter coverage DG=G GRSV-std DG=-G DG=0 L = 6 pb-1 P=0.6 2006 Data • Software triggers • Full EM calorimeter coverage -1<h<2 including trigger • DiJets • Direct g-jet sample 14 Next Step is to Explore Dg(x) g jet 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 15 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 16