Using Dijets to Measure the Gluon Sivers Functions at STAR

1. Using Dijets to Measure the Gluon Sivers Functions at STAR Renee Fatemi Massachusetts Institute of Technology February 28, 2005

2. OUTLINE • STAR has made strong contributions to transverse physics in forward rapidities (see L. Bland’s talk) • STAR has been investigating ways to extend transverse spin studies at mid-rapidity. Leading Charged Particle asymmetries were consistent with zero and interpretation was complicated by possible contributions from Sivers/Collins/higher twist effects. • Vogelsang/Boer Proposal (early 2004) to Measure Single Spin Asymmetry of Dijet opening angle deviation from = provides a clean way to access Sivers at mid-rapidity. • STAR is preparing to make this measurement in 2006 run. Why STAR and Why Now? • Estimated Statistical Errors for 2006

3. SP   Jet #2 AN  Jet #1 Sivers Effect in Dijets Deviations from  =  due toPartonic kT Gluon = (U + D) / 2 Gluon = 0 Gluon = D Gluon = D + √kT2 = 2.5 Maximal Effects at  = 0.4-0.5. This region experimentally available! W.Vogelsang and D.Boer Phys Rev D 69 (2004) 094025

4. Why STAR? Why 2006? BEMC x2 + Endcap! Large - coverage of STAR Time Projection Chamber + Barrel + Endcap Electromagnetic Calorimeter makes STAR a natural choice for a di-jet measurement. BEMC fully installed - but not fully in trigger - ONLY in 2005! Investigated Sivers dijet analysis in 2003 data but it was clear that sample was statistically limited due to triggering only on half barrel. Several Level 0 jet triggers were implemented, tested and understood in 2005 -- these form the basis for Level 2 triggers. Needed time to develop and test L2 trigger algorithms in order to maximize di-jet sample

5. Circleoffline jets Level 2 Dijet Trigger Algorithms • Goal: Reconstructed di-jet rate of 10Hz which is factor of 5 above 2005 rate. Do this by taking 1/2 dijet sample from L0 Jet Patch Triggers and 1/2 sample from L2. • L2: Pass events from L0 (Jet Patch * ETOT) Trigger. If reconstruct two localized clusters of transverse energy surpassing thresholds (ET1,ET2) with thrust axis separation || > 1 radian keep the event. • L2 treats BEMC+EEMC as uniform detector - in effect taking jets covering  = [-1,2] and  = [0,2] • L2 algo biases towards jets with higher neutral energy. L0 triggered sample biased towards lower than average neutral jet energy. We are equipped to estimate this trigger bias in simulation. ET1> 3 GeV L2 algo tested on 2005 data - shows excellent agreement with offline analysis. Thresholds are variable but currently set at ET1=ET2 ==3 GeV. % of dijet events in sample increases from 10-80% for ET2>5!

6. Background Shielding 2003/2004 Inclusive Jet Analysis led to identification of background via enhanced neutral energy in jet spectrum. The background is asymmetric in phi --resulting in asymmetric trigger rates in the Endcap Calorimeter. Background location could produce large -possibly spin dependent- systematic for di-jet measurement Tracking developments allowed identification of “straight tracks” which do not originate from the collision vertex to be identified. These tracks reproduce the asymmetric background pattern in the EEMC and point back to an origin 40 m upstream in blue beam. Steel Shielding added near quadrupole triplet in beam tunnels on both side of STAR in order to absorb high-energy hadrons.

7. STAR JetFinding Algorithms are on Firm Ground Inclusive Jet X-sec agrees within systematic error bars with NLO pQCD predictions Good agreement between data & PYTHIA+GEANT simulations. We are capable of separating trigger and detector effects from physics effects 3) Spin Sorting mechanism tested and incorporated into jet analysis framework 4) Necessary tools are ready for relatively quick di-jet analysis STAR preliminary

8. STAR 2006 Beam Use Request • Place a world class constraint on gluon polarization in the proton • Delineate the roles of the parton orbital motion/transversity in creating the transverse single spin asymmetry (A_N) observed for inclusive forward pi0 production • First significant measurement of Sivers Effect asymmetry in di-jet production • 18 cryogenic weeks • 13 weeks physics running • 1.5 wk calibration/trigger commissioning • 11.5 wks running = 45/15 pb-1 delivered/sampled • 5/10pb-1transverse/long

9. Summary and Discussion • STAR Collaboration is invested in studying all aspects of spin puzzle. • Transverse running has high priority for STAR in 2006. • Recent developments in hardware, shielding and triggering should enable us to produce first statistically significant measurement of dijet  asymmetry. • Do we need to worry about Collins effects from first gluon scattered from polarized quark? How does this effect vary with cone size? • If size of partonic kT is independent of hard scattering scale (jet pT) then shouldn’t this asymmetry become smaller at higher jet pT? • How will Sivers Functions be extracted from this asymmetry? What is the status of the necessary factorization theorems? • Can we extract Lg from Sivers?

10. Backup slides

11. J1 h1 kT  h1+h2 h2 J2 Di-hadron Analysis Higher statistics and simpler analysis make Di-hadrons cheaper. But is the correlation with kT strong enough? Uses Real LCP, nLCP kT = kT1+kT2 Use PYTHIA 6.2 to simulate pp collisions. Find LCP and next to LCP (nLCP). Require 0.4 < pT < 5 GeV. If they are separated by 180 +/- 600 then find opening angle, , between their bisector and 1 of the initial parton kT directions. PYTHIA • Correlation 1.3/1 - weak for ideal case • kT seems to point in direction of LCP  (degrees)

12. Sivers at Mid-rapidity? Need an observable which is correlated with Partonic kT. The Leading Charged Particle (LCP) is a high statistics candidate! kT = kT1+kT2 Uses True LCP • Use PYTHIA 6.2 to simulate pp collisions for s = 200 GeV • Identify true LCP in event with 0.4 < pT < 5 GeV • Calculate vector sum of Initial Partonic kT • Calculate opening angle, , between LCP and kT directions PYTHIA 2.5/1  (degrees)