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

Sivers +kT x. . z.  . y. x. spin+. Vertical Spin. Measurement of Sivers Asymmetry for Di-jets in 200 GeV pp Collisions at STAR. Jet 1. Outline Motivation Principle of measurement, trigger Toy model of Sivers asymmetry Measured SSA & DSA Comparison with theory

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

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  1. Sivers +kTx  z  y x spin+ Vertical Spin Measurement of Sivers Asymmetry for Di-jets in 200 GeV pp Collisions at STAR Jet 1 • Outline • Motivation • Principle of measurement, trigger • Toy model of Sivers asymmetry • Measured SSA & DSA • Comparison with theory • Conclusions  Jet 2 Jan Balewski, IUCF for STAR Collaboration SPIN2006, Kyoto, Japan  (deg) 180o 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 beam 1 spin up beam 1 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? • Contributions from Collins fragmentation asymmetries in incompletely reconstructed jets conspire to cancel Sivers asymmetries for quarks, while gluon Sivers functions naturally small? • 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

  12. BACKUP Jan Balewski: Di-jet Sivers in p+p @ 200 GeV

  13. STAR EMC  coverage -1 +2 proton proton STAR data both jets rotated by 90 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 y z 4 3 SY unpol unpol 1 2 2 1 Blue Yellow Cross-ratio  forbidden 2-spin correlation  cos(bisector) SY 4 3 Measured Sivers AN for Di-jets Null Tests AN – L/R asymmetry integrated over STAR detector pseudorapidity Jan Balewski: Di-jet Sivers in p+p @ 200 GeV

  14. Beam polarization calibration: used online RHIC polarimeter results: PB = 0.57, PY = 0.59.Normalization unc. = 0.15 AN . • Other sources: syst. unc.<  0.001, confirmed by null tests& consistency of results for different polarization fill patterns. Systematic Corrections, Checks, Errors ANfalse requires instrumental spin-dependent change in  distribution shape. Cross-ratio extraction, excellent L-R detector symmetry, bunch-to-bunch spin changes, etc., make these very small. • L2 jet vs. parton  resolution smearing: • Systematic uncertainty = 0.05 AN covers model-dependence of toy-model dilution factor already included. • Different di-jet yield extraction methods: • Syst. Unc. = 0.2  (stat. unc.)covers observed changes when  integration range, baseline subtraction are varied. Jan Balewski: Di-jet Sivers in p+p @ 200 GeV

  15. EMC Barrel  = -1 EMC Endcap y TPC   =+2 x BBC East BBC West Jet 1 ET> 3.6 GeV yellow blue -60 deg EMCal Geometry in L2 trigger +60 deg Barrel EMC -1 -units =+ 0 1 2 Endcap EMC Jet 2 ET> 3.3 GeV STAR Excellent Coverage for Di-Jets L2 jet Jan Balewski: Di-jet Sivers in p+p @ 200 GeV

  16. STAR Triggered on Di-Jet Found On-line • L0 di-jet logic: • BBC East.West • Etot >14 GeV • hardwired jet patches • jet size : 1x1 (x) • one jet Et>4.0 GeV • event rate ~120 Hz Comparison with off-line  similar Z=Et/GeV =360o 2005 data event Jet  is weighted with EM ET • L2 di-jet logic: • 5500 EMC towers • sliding jet patches • jet size : 0.6x0.6 (x) • jet1 Et>3.6, jet 2 Et>3.3 GeV • decision : 60 muSec/event • event rate ~8 Hz =180o ~180 o back-to-back di-jet L2 algo result • L0-L2 logic benefits: • large acceptance combined with trigger rejection power • full statistics on-line trigger monitoring of EMC • compact trig info saved for each event =0o  -1 0 +2 Jan Balewski: Di-jet Sivers in p+p @ 200 GeV

  17. Toy M-C Matched to Real Data ()=220 Toy M-C Back-to-back di-jet ()=21o Jet 1 Tails are different pp200 Real L2 di-jet events Jet 2 Jan Balewski: Di-jet Sivers in p+p @ 200 GeV

  18. Sivers Sensitive Observables independent 3 Alternative Sivers Spin Observables Width of  influenced by DSA Sivers kTx leads to displacement of  (other beam unpolarized) ++ Yell + Blue + narrows eps  spin- spin+ kTx destructive  x N<,- N>,+ Z-axis +- Yell - widens Blue + 180o  (deg) N>,- N<,+ kTx  constructive Based on Toy M-C rel. sensitivity of 7  1 7 • epsR w/ more -slices  AN() Jan Balewski: Di-jet Sivers in p+p @ 200 GeV

  19. Di-jet Results Sivers epsN()=AN ()*P May 2006 epsN() Unpolarized Yield() Blue SSA epsN() epsN=0 epsN() +/- 1(A) Yellow SSA ‘mixed’ DSA • 3M events • Fit: A-B*sin(2) Jan Balewski: Di-jet Sivers in p+p @ 200 GeV

  20. May 9 April 6 WCM intensity No spin Info @ STAR STAR magnet incident ( ~10 days ) good STAR data good data days in 2006 2006 pp200 Transverse Run @ RHIC STAR event count • Available STAR events: • both TPC & EMC active • few M 1-jet trig, ET>8.0 GeV • 3M di-jet trig, 2xET>3.7 GeV • few M di-jet w/o TPC • (crude run QA) Yellow Abort gap Blue Abort gap Yellow bXing (uses revTick) spin8 state STAR spin sorting (uses V124) • other value • B –Y – 51 • B –Y + 53 • B +Y – 83 • B +Y + 85 Example of di-jet events for one of 4 RHIC fill polarization patterns { \\ , \\+1, // , //+1 } Jan Balewski: Di-jet Sivers in p+p @ 200 GeV Imperfections : few ppm

  21. Run QA • Pass 1 (results presented in May of 2006) • use run Browser: TPC & BTOW & ETOW • triggers: BJP0, EJP0 • classify runs based on fill pattern 1-4 • drop run if mean -180omore than: - 1 deg AND it is more than 2 sigma effect, or - 3 deg (no matter what sigma) - in any of the subsamples: bjp0 or ejp0 • Pass 2 (new TrgJetTree in hand) • use run Browser: BTOW & ETOW (include no-TPC runs) • triggers: BJP0, EJP), minB, zB • use official off-line QA (from Jim & Co.) • suggestions? Jan Balewski: Di-jet Sivers in p+p @ 200 GeV

  22. Offline Evaluation of Di-jets all BJP1 trig ET2>3 GeV ET2>4 GeV ET2>5 GeV 2005 data • Input: 84k BJP1 triggered events from 2005 • ( L0 BJP1: ET>4 GeV , only BTOW-W) L2 algo: uses BTOW E+W Offline jet finder : no  cut • Bias introduced by L2 di-jet 2006 trigger • Compare 2006 L2 vs. off-line di-jets - Issam Qattan, IUCF • Avoid any -shape model, fit ’= a*+b to match + & - spin state distributions • (perhaps it will work?) all BJP1 trig ET2>3 GeV ET2>4 GeV ET2>5 GeV 2005 data ETEMC / ETTOT Jan Balewski: Di-jet Sivers in p+p @ 200 GeV

  23. Justin Stevens Endcap+Barrel E+E B+E Barrel+Barrel Expectation: Sivers Asy Depends on Partonic Process Favors quark-gluon Z=# of events Favors quark jets Selecting regions in  & pT changes probabilities of partonic processes. • Available info (trigger level) • Eta1,2 • ET1,2 • Et tot • Tower multiplicity • Additional info • Example: • Asymmetric Endcap-Endcap jets • Quark –blue beam • Gluon – yellow beam Jan Balewski: Di-jet Sivers in p+p @ 200 GeV

  24. East BBC West BBC 2 6 2 6 Top view X Blue-Left Blue beam  +Z Y Yellow beam Yellow-Left Is the Di-jet Spin Sorting Correct?( Find MinB BBC SSA) Off-Line 12M MinB evens On-Line West-Blue 5.6  West-Yellow 0.3  YES ! Pol patt 1,2,3,4 East-Blue 0.1  East-Yellow 5.8  Jan Balewski: Di-jet Sivers in p+p @ 200 GeV

  25. An Integrated vs. Averaged Jan Balewski: Di-jet Sivers in p+p @ 200 GeV

  26. Single-Transverse Spin Asymmetries Werner Vogelsang BNL Nuclear Theory Spin 2006, Kyoto, 10/02/2006

  27. • careful analysis of gauge links in 22 QCD scattering • color structure of amplitudes plays role • complicated, but simplifies if moment is taken: • then everything has structure as in unpol. case, but with different color weights: (2) Non-universality in ppjet jet X Bacchetta, Bomhof, Mulders, Pijlman Jan Balewski: Di-jet Sivers in p+p @ 200 GeV

  28. Taking into account Process-dependence Unpol.partonic cross sections • phenomenology is beginning : AN See Feng Yuan’s talk Jan Balewski: Di-jet Sivers in p+p @ 200 GeV

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