1. Measurement of the Double Longitudinal Spin Asymmetry in Inclusive Jet Production in Polarized p+p Collisions at 200 GeV Outline Introduction STAR @ RHIC Data Analysis Spin Asymmetries Conclusion & Outlook Frank Simon (MIT) for the STAR Collaboration Lake Louise Winter Institute, February 2006

2. Frank Simon: Jet A LL in p+p @ RHIC 2 Introduction: Gluon Polarization in the Proton Spin Crisis: EMC measured that quarks only account for a small fraction of the spin of the proton => Where is the rest? Orbital Momentum? Gluons? Lepton does not couple directly to the gluons, gluon polarization so far poorly constrained by current fixed-target DIS data Alternative approach: polarized proton-proton collisions Direct coupling of gluons with gluons and quarks with gluons

3. Frank Simon: Jet A LL in p+p @ RHIC 3 Inclusive Jet Production in pp Collisions Jets: Spray of energetic particles Asymmetries: most systematics cancel in asymmetry measurement, good for looking for small effects Jager et.al, PRD70(2004)034010 Contribution of subprocesses: Connection of the asymmetry to  G not straight forward. Current strategy: Compare measured A LL to NLO calculations to determine  G based on the model “probability”

4. Frank Simon: Jet A LL in p+p @ RHIC 4 What is a Jet? parton particle detector Jet Finding Look for “splashes of energy” in the detector Add track/tower 4-momenta cleaned di-jet event

5. Frank Simon: Jet A LL in p+p @ RHIC 5 STAR @ RHIC  STAR IR RHIC polarimeters Siberian Snakes Siberian Snakes Spin Rotators (transverse/ longitudinal) PHENIX IR STAR detectors used in the analysis: Tracking: TPC -1 <  < 1 Calorimetry: EMC -1 0 only up to 2004 Trigger: Minimum bias trigger based on BBCs High Tower trigger : BBC coincidence and one EMC tower (0.05 x 0.05 in ,  ) above threshold Jet reconstruction: cone radius 0.4 p t of track/tower > 0.2 GeV/c 0.2 <  Jet < 0.8 |vertex-z| < 60 cm  = -ln[tan(  /2)]

6. Frank Simon: Jet A LL in p+p @ RHIC 6 Preliminary Cross Section preliminary jet cross section obtained from 2004 dataset comparison to NLO calculation large uncertainties due to jet energy scale, studies ongoing agreement (within systematics) over 7 orders of magnitude

7. Frank Simon: Jet A LL in p+p @ RHIC 7 A LL Measurements statistical significance: Required Ingredients: Beam polarization measurement (P 1, P 2 )RHIC polarimeters Direction of polarization vector at interaction Relative luminosities of different spin states: STAR Spin dependent yields N ++, N +-

8. Frank Simon: Jet A LL in p+p @ RHIC 8 Preliminary Inclusive Jet A LL Consistent results for 2003 and 2004 data Limited by statistical precision total systematic error ~0.01 (STAR) + beam polarization (RHIC) Sources of systematic uncertainties: background contribution, trigger bias, relative luminosity, residual (non-longitudinal) asymmetries, bunch to bunch systematic variations (random pattern analysis) + beam polarization Data set: ~0.3 pb -1 (2003 and 2004) recorded luminosity =0.3 (2003) and = 0.4 (2004) Final statistics: (after cuts) for 5 < jet p T < 17 GeV/c: 125k (2003) and 162k (2004) jets

9. Frank Simon: Jet A LL in p+p @ RHIC 9 Comparison to Theory Results limited by statistics (systematic uncertainties ~ 0.01 without polarization) STAR preliminary

10. Frank Simon: Jet A LL in p+p @ RHIC 10 Data Analysis in Progress 2005 data analysis well under way. Projections based on actual data sample (about 0.7 M HT2 and 2.2 M JP2 triggered Jets) Jet patch trigger optimized for jet detection Increased acceptance of Calorimeter In addition to jet analysis: Asymmetry measurements in inclusive  0 production in both central and forward calorimeter in progress Asymmetry in inclusive charged  production

11. Frank Simon: Jet A LL in p+p @ RHIC 11 Summary & Outlook Preliminary results for the measurement of double spin asymmetry A LL in inclusive jet production in polarized proton-proton collisions at sqrt(s)= 200 GeV over the measured jet p T range 5-17 GeV/c Measurement is consistent with evaluations based on DIS (GRSV-standard) The results for A LL are limited by statistical uncertainties Large positive gluon polarization is disfavored Upcoming results from the 2005 pp run will lead to significantly reduced uncertainties (increase in statistics and polarization) Additional results from neutral and charged  We are hoping for a productive pp-only run starting soon!