B. S. Page for the Collaboration

1. STAR Status of Neutral Dijet Analysis on Data from 200GeV Proton Proton Collisions Using the STAR Detector at RHIC B. S. Page for the Collaboration

2. Investigating the Proton’s Spin at STAR Sz = ½ = ½ DS + DG + Lzq + Lzg Contributions to proton spin from quark spin DS is smaller than expected At RHIC we have been measuring the contribution from the gluon spin DGprimarily via qq or qg scattering Individual particles or the jets can be detected The large solid angle of STAR favors jets

3. Constraints on ΔG Sz = ½ = ½ DS + DG+ Lzg+ Lzq • Work horse of STAR ΔG measurement has been inclusive jets • Good constraints on ΔG for 0.05<x<0.2 • Low x behavior and shape still poorly constrained

4. Forward Dijet Advantages • Dijet kinematics allow access to partonic x at leading order • Partonic x sensitivity will provide constraints on the shape of Δg as a function of x • Selection of favorable kinematics will allow access to lower x values

5. STAR Detector Barrel EM Calorimeter -1<h< 1 Beam-Beam Counters Triggering Special interest for Dijet program 2<|h|< 5 h = - ln(tan(q/2) h=0 h= -1 h=2 Triggering Endcap EM Calorimeter VPD 1<h< 2 Time Projection Chamber -2<h< 2 Solenoidal Magnetic Field 5kG Tracking

6. Dijet Kinematics By Region Endcap-West Barrel Endcap-East Barrel Asymmetric Barrel Symmetric Barrel Endcap-Endcap FMS- endcap x1/x2 ^ cosq* aLL Select valence q uark

7. How to Proceed • Favorable kinematics at high pseudorapidity • TPC tracking falls off at high pseudorapidity • Forced to use only neutral component of jet • Thrust axis still well determined • Good h1+h2 gives x1/x2 • Explore these issues using Trigger Data Files • Contains jets found in calorimeters by Level 2 jet algorithm • No tracking information: good jet angles but poor PT • Large data set • Need vertex position to correct particle ET and pseudorapidity • Use VPD and BBC to get vertex

8. VPD and BBC Time Differences • VPD and BBC detectors located around beam line on both sides of STAR magnet • Time difference between first hit on each side will give Z vertex of event • Use fast offline data files because they have tracking information • Plot vertex obtained from tracking Vs. VPD/BBC time difference • Can use linear fit to find Z vertex from time difference information

9. Corrected Vertex Distributions Armed with fits found above, can rerun and compare PPV vertex with VPD / BBC vertex • VPD Vertex Distribution • PPV Vertex Distribution • BBC Vertex Distribution

10. Tracking and BBC Vertex Difference • The previous slide shows that we reproduce the vertex distribution well using BBC and VPD • How well do we reproduce kinematic variables using BBC and VPD? • Plots show event-by-event difference in kinematic variables when using BBC Vs. tracking vertex See good agreement between kinematic variables using BBC Vs. tracking vertex Jet 1 η Err Jet 1 ET Err 0.5*(η1 + η2) Err 0.5*|η1 – η2| Err

11. Yields after BBC Vertex Correction • Spectra of kinematic variables after BBC vertex correction applied • 0.5*(η1 + η2) corresponds to 0.5*Log(x1/x2) • 0.5*|η1 – η2| corresponds to |Cos(θ*)| High ET Jet Low ET Jet Jet ET Jet η 0.5*(η1 + η2) 0.5*|η1 – η2| X1/X2 7.39 20.1 1.00 0.14 0.20 0.54 |Cos(θ*)| 0.76

12. Summary • Dijet measurements allow selection of favorable kinematics and sensitivity to x dependence with forward jets giving access to low x values • Vertex finding using VPD and BBC time difference seems to be viable • A first look at dijets in the endcap has been made using Trigger Data • Production of 2009 data is ongoing • Simulation studies needed to fully understand jet properties • Ultimate goal is the extraction of asymmetries

13. Backup Slides

14. Dijet Projections:200GeV 50pb-1 P=60% • Detector regions select x1/x2 • Also âLL Left two panels same η1+η2 (x1/x2) but different η1-η2 (cosθ*) so smaller ALL in upper Same M X=0.06 X=0.16

15. Diff between PPV and VPD/BBC Vertex

16. VPD / BBC Vertex Vs PPV Vertex BBC slope is slightly off

17. PPV / BBC Difference Eta 1 Diff ET 1 Diff ET 2 Diff Eta Sum Eta 2 Diff Eta Diff

18. PPV / VPD Difference

19. Yields after VPD Correction

20. Detector Yields