Measurement of Sivers Asymmetry for Di-jets in 200 GeV pp Collisions at STAR

1. Measurement of Sivers Asymmetry for Di-jets in 200 GeV pp Collisions at STAR • Outline • Motivation • Principle of measurement, trigger • Model of Sivers asymmetry • Measured SSA & DSA • Comparison with theory • Conclusions Jan Balewski, IUCF for STAR Collaboration RHIC Spin Physics, RIKEN, Japan September 29, 2006 Jan Balewski: Di-jet Sivers in p+p @ 200 GeV

2. Motivation for Di-Jet Sivers Effect Measurement • Sivers effect: • Sensitive to parton orbital angular momentum • Needs ISI and/or FSI to evade time-reversal violation. • HERMES transverse spin SIDIS asymmetries  non-zero u and d quark Sivers functions of opposite sign, different magnitude. • Sivers effect in pp  spin-dependent sideways boost to di-jets, suggested by Boer & Vogelsang (PRD 69) • Both beams polarized, xa  xb  can distinguish q vs. g Sivers effects. • Do we observe q Sivers consistent with HERMES? Tests universality. • First direct measurement of gluon Sivers effects. Jan Balewski: Di-jet Sivers in p+p @ 200 GeV

3. unpol proton 2 p3 2 2 partonic scattering Polarized jet 1 spin jet 2 + spin- dep. kTx Sivers ON p4 spin 1  > 180 for kTx > 0 y x di-jet bisector kTx 2 Sivers Mechanism of SSA proton 1 intrinsic  kT p2 intrinsic  kT  p1 • Spin dependent kTX offset • deflects both jets in the same direction • reduces average di-jet opening angle • can be measured from correlation between di-jet bisector vs. spin direction • from accumulated-spectra (spin) Jan Balewski: Di-jet Sivers in p+p @ 200 GeV

4. L0 jet patch trigger + L2 EMC-only dijet-finder  2.6M event sample from 1.1 pb–1 of transverse p+p collisions during 2006 run EMC Barrel  = -1 EMC Endcap TPC  =+2 Reco cos(bisector) measures sign of net kTxfor event BBC East BBC West spin Yellow beam Blue Jet 1   bisector  4 (deg) Endcap essential for q vs. g Sivers distinction Jet 2 |3-4| < 60   3 (deg) EMC-based Di-jet Trigger in STAR L2 trigger thresholds: ETEMC > 3.6, 3.3 GeV Is -distribution narrow enough for precise determination of centroid ? Have we measured di-jets? Jan Balewski: Di-jet Sivers in p+p @ 200 GeV

5.  (rad) EMC-only  resol’n << kT-induced  width Full  width ~21    (full reco) –  (L2) [deg] -resolution, pT of Reconstructed Di-jets Data: offline di-jet reco for ~2% of all runs PYTHIA+GEANT: full reco jet vs. parton • Jet finder • TPC+EMC • jet cone radius 0.6 Typical xT ~ 0.05 - 0.10; 1+2 range  0.01 < xBj < 0.4 How close full recon jets resemble original scattered partons ? How close EMC-only jets resemble full reco jets ? Jan Balewski: Di-jet Sivers in p+p @ 200 GeV

6. Intrinsic kTx,y width = 0.75 GeV/c  () = 6.3 resol’n smear reproduces observed  width Blue beam spin up Blue beam spin down  smearing dilutes AN by only 10% of its value  EMC-only analysis OK Toy Model of Sivers Spin Effect Generates AN (+) - (-) • Throw 1e6 2-parton events in transverse plane • Match to pT distribution from full jet reco. • Gaussian + exp. tail kT distrib’n reproduces  shape • Sivers spin-dep. kTx offset spin-dep.  distrib’n shift • Vary kTx offset, see how pol’n observables change • Can introduce  resolution smearing, see its effects AN=measure of L-R asymmetry of kT distribution in L-R symmetric detector (+) - (-) vs. AN ? Jan Balewski: Di-jet Sivers in p+p @ 200 GeV

7. STAR EMC  coverage -1 +2 proton proton Yellow(-z) Blue(+z) • Event weights by |cos(bisector)| account for beam pol’n compo-nent  kT plane.  rotation samples kTy, parity-violating sp•kT correl’n Measured Sivers AN for Di-jets Null Tests STAR data both jets rotated by 90 • AN – L/R asymmetry integrated over STAR detector pseudorapidity Jan Balewski: Di-jet Sivers in p+p @ 200 GeV

8. Quark Sivers Functions Constrained by SIDIS Results • V & Y use these Sivers fcns. (assuming zero gluon Sivers) to predict SSA for pp  dijet + X, integrated over kT distributions within both protons. Note strong y- and weak pT - dependence. • Sivers sensitivity in depend-ence of SIDIS yield on (hadron– proton spin ). • HERMES positive Sivers SSA for  +, but  0 for  –, fitted by Vogelsang & Yuan with u and d quark Sivers fcns. of different sign and magnitude: W. Vogelsang and F. Yuan, PRD 72, 054028 (2005). Jet 1 rapidity Jet pT (GeV/c) Jan Balewski: Di-jet Sivers in p+p @ 200 GeV

9. Emphasizes (80%+) gluon Sivers • Model w/o hadronization, integrated over STAR , 5<pT<10 GeV/c, includes only quark Sivers -- predicts AN ~ ANHERMES where q Sivers dominates • Sign of predictions reversed to adhere to Madison AN sign conventio • STAR measured AN all consistent with zero  both quark and gluon Sivers effects much smaller in pp  di-jets than in HERMES SIDIS !! Measured Sivers AN for Di-jets vs. Theory Emphasizes (50%+ ) quark Sivers Jan Balewski: Di-jet Sivers in p+p @ 200 GeV

10. Drell-Yan S SIDIS l- p  * q q Initial state interaction p l+ ISI for Drell-Yan vs. FSI for SIDIS  opposite sign predicted for Sivers SSA Theory - exp’t discrepancy raises questions! • Are observed di-jet Sivers SSA much smaller than predictions because: • ISI & FSI both important in pp  jets and tend to cancel? • Need q Sivers or different q Sivers x, kT - shapes in HERMES fits? • If ISI / FSI cancel at mid-rapidity, does their balance change at high  to yield sizable Sivers contribution to observed pp   0X SSA? Jan Balewski: Di-jet Sivers in p+p @ 200 GeV

11. Large STAR EMC acceptance  large 2006 triggered di-jet data set. • Di-jet ANSivers all consistent with zero  quark Sivers effects ~ 5-10  smaller than expected from SIDIS. Gluon Sivers comparably small. • Sensitivity of analysis to ANSivers 0 confirmed by toy model simulations. Beam spin-sorting verified via known non-zero SSA. • Results constrain treatments of Sivers effect and of transverse spin asymmetries in pp collisions. • Extend di-jets in future to higher , where ANmeas(pp   0X) sizable. Summary and Conclusions Jan Balewski: Di-jet Sivers in p+p @ 200 GeV