W. W. Jacobs for STAR Collaboration IUCF and Dept of Physics, Indiana Univ.

1. STAR Probing the Proton’s Spin Structure with Hard Scattering, Jets and the STAR Detector at RHIC: Recent Results W. W. Jacobs for STAR Collaboration IUCF and Dept of Physics, Indiana Univ.

2. The STAR detector • The polarized proton collider RHIC Topics: “Slice” of STAR Spin Physics Status & Overview • Introduction • STAR longitudinal spin - Recent results • STAR Transverse spin di-jets - Recent results • Look at near future and outlook • STAR EMC Calorimetry! • Summary

3. Spin Problem & DIS Gluon Helicity Preference Constraints DS º D + D + D u u d + D + D + D d s s Only ~30% of proton spin arises from q and q helicity preferences ! All fixed-target data World data (2005) on g1p = ½ ei2 [qi (x,Q2) + qi (x,Q2)] Only valence quarks are strongly polarized limited info on scaling violations,  on shape or integral of gluon helicity preferenceg(x,Q2).

4.   RHIC p+p: Pert QCD Probe of Spin-Dep Partonic Structure LO pQCD pp  hX “soft” parton distribution functions “soft” frag. function ∧ “hard” dQCD parton-parton Theory ingredients: pQCD factorization + large parton-level 2-spin sensitivity • Prefer dominant/”clean” reaction mechanism w/ large “aLL”(-jet) • But jet and p0 rates are sufficient to give significant DG constrain in initial RHIC polarized p data

5. 2005 STAR Preliminary  PRL 97, 252001 (2006) 2003+2004 PLB 637, 161 (2006) STAR Inclusive cross-section (jets, , and ±) 1st RHIC inclusive Jet x-sec jets Hadron inclusive production also direct photon incl. @ PHENIX; forward incl. @ STAR & incl chg. hadrons @ BRAHMS ± Good agreement with NLO pQCD over many orders of magnitude pQCD works!

6. 100 GeV beam proton beams • Each bunch filled with a distinct polarization state • Spin Rotators at STAR IR allow for transverse and longitudinal spin orientation • Bunch Xings every 100-200ns • CNI polarimeters + Hydrogen Jet target provide run by run & absolute polarization • HJT calibr to ~ 5% goal in progress RHIC pC Polarimeters Absolute Polarimeter (H jet) BRAHMS PHOBOS Siberian Snakes Siberian Snakes PHENIX STAR Spin Rotators (longitudinal polarization) Spin flipper Spin Rotators (longitudinal polarization) Solenoid Partial Siberian Snake Pol. H- Source Helical Partial Siberian Snake LINAC BOOSTER AGS Internal Polarimeter AGS 200 MeV Polarimeter AGS pC Polarimeters Strong Helical AGS Snake Rf Dipole Relativistic Heavy Ion Collider …world’s 1st Collider

7. Level 0 High Tower & Jet Triggering in STAR +13,…,18 7 1 8 12 2 jet patch 3 size:1x1  x  11 9 4 10 4 8 1x1 Jet patch ET/GeV ENDCAP EMC BARREL EMC Trigger either on HT:  1 (of 4800 BEMC or 720 EEMC) tower ET > thresh - Or JP:  1(of 12 BEMC or 6 EEMC) hard-wired jet patch  ET > thresh. [note: also a minbias cond] 2006 rate ~ 150 Hz, combine with L2 trigger to fit in limited bandwidth 2006 rate ~ 2.5 Hz, sent to tape without prescaling Allocated Jet Rate to tape: ~15 Hz

8. Theoretically allowed range in Δg: • GRSV-STD: Higher order QCD analysis of polarized DIS experiments! Inclusive ALL measurements (, ±, and jets) • Predicted ALL sensitivity for different G scenarios  ±  jets

9. Neutral pions (in Jets); Inclusive 0 ALL • use  invariant mass spect. (data from HT trig) • MC 0 line shape • low inv. mass bkgrd • comb. bkgrd & residual fit •  assoc w/ full Jets if within a Jet cone (0.4 in , ) but typ. ~ 5 deg. 2005 data • /ndf compared to NLO calculations • (ignoring systematic errors): • GRSV Std: 0.8 • GRSV Max: 2.5 • GRSV Min: 0.8 • GRSV Zero: 0.4  GRSV max scenario disfavored

10. Jet Reconstruction in STAR Full jet reconstruction uses “midpoint-cone” algorithm (hep-ex/0005012): Data Simulation detector “geant” jets GEANT “pythia” jets particle • Use cone radius: • = 0.4 for half-BEMC 2003-5 • = 0.6-0.7 for full B+EEMC 2006 pythia parton theory • Search over all possible seeds (pTseed > 0.5 GeV) for stable groupings • Check midpoints between jet-jet pairs for stable groupings • Split/merge jets based on Eoverlap • Add all track/tower 4-momenta • Data well described by MC

11. Inclusive Jet ALL and x-sec Analysis Issues • Use Simulation (MC) to provide correction to RAW jet yield - trigger and jet inefficiencies - jet resolution & bin migration - undetected particles (n + ) - PYTHIA 6.205 CDF Tune A - GEANT (Geisha) • Verification of DATA/MC agreement essential • Minbias • JP2 • HT2 STAR Preliminary The shape of the Fraction of Neutral Energy in the Jet (EMF) is sensitive to the trigger bias as well as contributions from beam background.

12. 10 20 30 pT(GeV) Effects of Jet Resolution On average PARTICLE Jets are reconstructed in the DETECTOR with 20% increase in pT due to ~25% jet resolution + steep jet pT distribution • Systematic offsets in pT cause dilution of the jet asymmetry which depend on the size of the asymmetry! Trigger Bias • The trigger biases jets toward higher neutral energy. This may change the ratios of qq+qg+gg and therefore change the asymmetries Trigger Bias: JP << HT ALL/ALL larger at low pT Total Systematic:

13. 2005 STAR preliminary Systematic error band Measured Jet PT (GeV/c) 2005 Inclusive jet ALL at mid-rapidity g = g (max) g = -g (min) g = 0 GRSV-STD • Error bars are statistical • Systematic band includes 25% scale error from current polarization uncertainties due to online values • Results in good agreement with ’03/’04 ALL data* in region of overlap but ~ 4 times more precise and pT range nearly doubled [* PRL 97,252001 (2006)]

14. Constraint on G Vogelsang and Stratmann GRSV DIS GRSV DIS best fit=0.24 1 = -0.45 to 0.7 PRD 63, 094005 (2001) • Significant new constraints on G when compared to predictions derived from one global fit to DIS data

15. Inclusive Jet ()Data from 2006 -> Greater Discr’m Power for g DG=G jet GRSV-std DG=-G DG=0 Projected statistical uncertainties for STAR 2006 inclusive jet ALL • High-statistics (esp. at high pT) inclusive jet and 0ALL data from 2006 will select among g models, assuming a shape of g(x,Q2). • Need global analysis including these ALL results!  • Significant increase in sampled luminosity • Polarization typically ~60% • acceptance in BEMC increased by a factor of 2 significant increase in figure of merit! • also analysis w/ Endcap EMC • + vs. - analysis in Barrel

16. Di-Jet Asymmetry Measurements with STAR; Sivers Transverse Spin Asymmetries f sproton pproton kTparton ? kTparton ? Do quarks and/or gluons have transverse motion preferences in a proton polarized transverse to its momentum? • Characterize by Sivers effect: • Sensitive to parton orbital angular momentum. • Needs ISI and/or FSI to evade time-reversal violation • Assuming QCD factorization, subsume ISI/FSI contributions in gauge-invariant kT -dependent parton distribution fcns.

17. Motivation for pp  Di-Jet Measurement y proton spin x z Colliding beams parton kTx • HERMES transverse spin SIDIS asymmetries  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, 094025 (2004)) • Both beams polarized, x +z  x z  can distinguish high-x vs. low-x (primarily gluon) Sivers effects. • Do we observe q Sivers consistent w/ HERMES, after inclusion of proper pQCD-calculable ISI/FSI gauge link factors for pp  jets? Tests limited TMD “universality”. • First direct measurement of gluon Sivers effects. • Expect Collins and Boer-Mulders asy’m contributions small: no charge-sign bias from EMC-only L2 trigger jet reconstruction & small role of q-q scattering at probed pT • 2006 p + p run at RHIC; 3 wks transverse spin @ STAR

18. STAR EMC-Based (Level 0 + 2) Di-Jet Trigger in 2006 • 2006 p+p run, 1.1 pb1 • 2.6M di-jet triggered events • 2 localized clusters    = 0.6  0.6, with ETEMC > 3.5 GeV, | | > 60; ET wt’ed centroid Reco cos(bisector) measures sign of net kTxfor event spin Jet 1   bisector Jet 2  EMC Barrel  = -1 EMC Endcap TPC  =+2 BBC East BBC West Blue (+z) Yellow (-z) beam Full, symmetric 1,2 coverage Broad 1,2 coverage Endcap essential for hi-x vs. lo-x Sivers distinction Signed azimuthal opening angle 

19. Fast MC Simulations Illustrate Di-Jet Sivers Effects f = 0.85 dilution corrected in data • 2-parton events, transverse plane • match full jet reco. pT distribution • Gaussian + exp’l tail kT distribution fits  distribution • random kTx,y (rms = 1.27 GeV/c) for each parton • Sivers spin-dep. kTx offset   shift, L-R di-jet bisector asymetry • 1-spin effects vary linearly with kTx offset

20. STAR Results Integrated Over Pseudorapidity Null Tests STAR data - both jets rotated by 90 AN-z AN-z AN+z AN+z 2-spin 2-spin Error-weighted average of 16 independent AN(>) values for |cos(bisector)| slices effective  beam pol’n for each slice = Pbeam |cos(bisector)| STAR data  rotation samples kTy, parity-violating sp•kT correlation • Sivers asymmetries consistent with zero with stat. unc. = 0.002 • Fast MC  sensitivity to Sivers kTx offset  few MeV/c  0.002 (kTx)21/2 • Systematic uncertainties smaller than statistics • All null tests, including forbidden 2-spin asym.  cos(bisector), consistent with zero, as are physics asymmetries for all polarization fill patterns • Note: P_beam from online CNI analysis, with 20% calibr. uncertainty

21. What Did We Expect? Constraints from SIDIS Results W. Vogelsang and F. Yuan, PRD 72, 054028 (’05) Jet 1 rapidity Jet pT (GeV/c) • no hadronization • no gluon Sivers functions Theory of Transverse SSA Developing Very Rapidly! VY 2 SIDIS Sivers fit FSI only ISI+FSI ISI only Fits to HERMES SIDIS Sivers asym constrain u and d quark Sivers functions, for use in pp  dijet + X predictions. models (2) of Sivers fcn. x-dep • 5 < pTparton < 10 GeV/c • Initial State Inter’ns only (à la Drell-Yan) • Trento sign conv. (opposite Madison) • Bacchetta et al. [PRD 72, 034030 (2005)] deduce gauge link struct for pp  jets, hadrons: •  AN (ISI+FSI)   0.5 AN (ISI) • Gauge links more robust for SSA wt’ed by  pT or |sin  |, due to kT - factorization breakdown (Collins & Qiu, arXiv:0705.2141) Bomhof, Mulders, Vogelsang & Yuan, hep-ph/0701277

22. STAR Di-Jet Sivers Results vs. Jet Pseudorapidity Sum I Emphasizes (50%+ ) quark Sivers y Spin +z x +z Unpol qluon Pol quark Blue beam Yellow beam Extract analyzing powers averaged over  and b w/ fit to asym’s in |cos b| using cross ratio: Trans Spin Asym Typical xT ~ 0.05 - 0.10; 1+2 range  0.01 < xBj < 0.4

23. STAR Di-Jet Sivers Results vs. Jet Pseudorapidity Sum STAR AN all consistent with zero  both net high-x parton and low-x gluon Sivers effects ~10x smaller in pp  di-jets than SIDIS quark Sivers asym.! • All calcs. for STAR  acceptance • Reverse calc. AN signs for Madison convention • Scale Bomhof calcs by 1/|sin |  3.0 to get AN of unit max. magnitude • u vs d and FSI vs ISI cancellations  sizable SSA in inclusive fwd. h prod’n and SIDIS (weighted SSA) compatible with small weighted di-jet SSA -- test via LCP flavor select Blue beam Yellow beam

24. Near Term future, and RHIC run 8 & 9 xgluon Inclusive 0 200 500 101 GeV N.B. x-range sampled depends on g(x,Q2) ! -- M. Stratmann 102 0 10 30 20 pT (GeV) • Sivers -> Ongoing analysis incorporates TPC tracks for full jet reconstruction  allowance for cuts on jet pT , u vs. d filtering via leading hadron charge sign, etc. (w/o cuts consistent w/ EMC only) • Presently in detailed planning stages for RHIC run 8 (accelerator cool-down ~1 Nov. 2007) … as per the Beam Use Request (BUR) there will be a significant amount of polarized pp running (divided among longitudinal and transverse spin orientations) … as we also work on run 6 analysis! • Similarly we expect a signifcant amount of polarized pp beam in run 9 • Inclusive channels for longitudinal spin prog suffer from broad integration over x  model-dep. Gextraction as well as other systematic issues • With improved beam & detector performance, focus will now shift to jet-jet and  -jet coincidences for event-by-event constraints on colliding parton x1,2 .

25. 2008-12: Coincidence Measurements to Map g(x) Fully For example, simulations (L. Bland) of STAR capabilities for  - jet coincidences give rough indication of g discriminating power for various models of input gluon polarization. • Simplified LO analysis used for simulations here to illustrate sensitivity •  - jet and di-jet measurements @ s = 200 & 500 GeV, will map g(x 0.01-0.3,high Q2), when included in NLO treatments of entire spin structure database.

26. Summary • NLO pQCD describes hadron x-sections at RHIC for inclusive jets, p, (g and ±) allowing spin program to access G directly • Longitudinal spin: 2005 inclusive jet data provide significant new constraint on G when compared to predictions derived from one global fit to DIS data (GRSV-max scenario ruled out w/ ‘03/’04 data); 2006 data should provide sizable add’l constraint (as will global fits!) • STAR longitudinal spin program entering phase of correlation and direct g measurements, while continuing to expand the pT reach of the incl. channels; 2008-09, focus on ALL for di-jet and +jet production • Transverse spin: spin asym’s for ppdi-jet production  Sivers asym’s consistent w/ zero, whether dominated by valence or sea partons … data will constrain unified theoretical accounts of SSA in hard trans spin pQCD, and connection to parton orbital momentum. • Present pQCD calcs. reconcile small observed asym’s with larger effects seen in SIDIS (& pp  forward hadron), via cancelling ISI vs. FSI and u vs. d contr. • p+p in pQCD regime viable complement to DIS  more data coming!

27. BACKUPS

28. 2005 Inclusive Jet ALL GRSV curves* G = G G = -G G = 0 G =STD 2005 ALL is consistent with previous 2003/2004 results. Phys.Rev.Lett 97 252001 (2006)

29. EMC-Only Information OK For 1st Dijet Sivers Asymmetry • Jet finder • TPC+EMC • jet cone radius 0.6  (full reco) –  (L2) [deg] Net L2-to-parton (jet) = 6.3, (di-jet) = 9.0 Full offline di-jet reconstruction for ~2% of all runs shows triggered jet pTspectrum: Typical xT ~ 0.05 - 0.10; 1+2 range  0.01 < xBj < 0.4 and  angle resolution loss @ L2 OK: [()=3.9, ()=5.8] L2 vs. full jet << observed()  20, mostly from kT PYTHIA+GEANT  full jet reconstruction vs. parton-level resolution: [()=5.0, ()=0.10] full reco. jet vs. parton angles

30. Distinguishing Sivers from Collins Asymmetries Sivers Collins In SIDIS, can distinguish transverse motion preferences in PDF’s (Sivers) vs. in fragmentation fcns. (Collins) via asym. dependence on 2 azimuthal angles: HERMES results  both non-zero, but  + vs. – difference suggests Sivers functions opposite for u and d quarks.

31. Theory of Transverse SSA Developing Very Rapidly! VY 2 SIDIS Sivers fit FSI only ISI+FSI ISI only Bomhof, Mulders, Vogelsang & Yuan, hep-ph/0701277 Sivers fcns. from twist-3 qg correl’n fits to pp  forward hadron • Ji, Qiu, Vogelsang & Yuan [PRL 97, 082002 (2006)] show strong overlap between Sivers effects & twist-3 quark-gluon (Qiu-Sterman) correlations: • twist-3 fits to AN(p+p  fwd. h) can constrain Sivers fcn. moment relevant to weighted di-jet SSA • Kouvaris et al. [PRD 74, 114013 (2006)] fits give nearly complete u vs. d cancellation in weighted di-jet SSA d quark u+d u quark Bomhof, Mulders, Vogelsang & Yuan, hep-ph/0701277 Bacchetta, Bomhof, Mulders & Pijlman [PRD 72, 034030 (2005)] deduce gauge link structure for pp  jets, hadrons:  AN (ISI+FSI)   0.5 AN (ISI)  Gauge links more robust for SSA weighted by  pT or |sin  |, due to kT - factorization breakdown (Collins & Qiu, arXiv:0705.2141)

32. ¯ ¯ Future: W Production @ 500 GeV  u/ u and d/d to Illuminate Origin of the Nucleon’s qq Sea ¯ ¯ ¯ Projected uncertainties for quark and antiquark polarizations • Many non-perturbative models of nucleon structure predict sign & magni-tude difference between u and d polar-izations in nucleon sea, not yet seen. • Probe via single-spin parity-violating asym. AL for p + p  W  + X with respect to helicity flip of each beam. • Detect W± via isolated high-pT daughter e± or ± , no away-side jet • 2 asyms.  2 charges  pol’n of valence q, sea q separately for u,d. ¯

33. Detector at RHIC EndCap EMC Barrel EMC STAR TPC BBC East BBC West Blue beam Yellow beam  = -ln[tan(/2)]

34. STAR STAR Calorimeter Coverage FMS, EEMC and BEMC provides nearly complete EM coverage from -1 η +4

35. Yellow beam asymmetry Blue beam asymmetry proton spin proton spin + proton momentum proton momentum 2006 p+p run  STAR measurement of Sivers transverse single-spin asymmetry for di-jets -- shows smaller effects than predicted for observable sensitive to orbital components of parton motion in proton.