Spin Recent Results, Future Directions

1. STAR STAR SpinRecent Results, Future Directions Carl A. Gagliardi Texas A&M University for the Collaboration

2. What contributes to the proton spin? Consider a proton moving toward the right Proton spin    Δq(x) Δg(x) Polarized DIS: ~ 0.3 Both are poorly constrained Spin sum rule: Proton spin  STAR spin program: Exploring poorly determined components of the proton δq(x)   Transversity – very little data

3. 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 RHIC: the world’s first polarized hadron collider • Spin varies from rf bucket to rf bucket (9.4 MHz) • Spin pattern changes from fill to fill • Spin rotators provide choice of spin orientation • “Billions” of spin reversals during a fill with little if any depolarization

4. STAR detector in cross section BEMC FMS EEMC TPC FPD BBC

5. STAR STAR Essential benchmark – unpolarized cross sections PRL 97, 252001 PRL 97, 152302 • Mid-rapidity jet cross section is consistent with NLO pQCD over 7 orders of magnitude • Forward rapidity π0 cross section also consistent with NLO pQCD • Many other examples • pQCD works over a very broad kinematic range at RHIC energies

6. STAR longitudinal spin program • What is the polarization of the gluons in the proton? Three recent fits of equal quality: • ΔG = 0.13 ± 0.16 • ΔG ~ 0.006 • ΔG = -0.20 ± 0.41 all at Q2 = 1 GeV2 Leader et al, PRD 75, 074027 • Additional question for future 500 GeV runs: What is the polarization of the anti-quarks?

7. f: polarized parton distribution functions 10 20 30 pT(GeV) Longitudinally polarized pp collisions at RHIC Partonic fractions in jet production at 200 GeV For most RHIC kinematics, gg and qg dominate, making ALL for inclusive jets and hadrons sensitive to gluon polarization. 0

8. STAR inclusive π0 ALL at various rapidities || < 0.95 • During Run 6, STAR measured ALL for inclusive π0 for three different rapidity regions • Mid-rapidity result excludes large gluon polarization scenarios • Larger rapidity correlates to stronger dominance of qg scattering with larger x quarks and smaller xgluons • Expect ALL to decrease as  increases • Forward rapidity: baseline for future γ andγ-jet measurements 1 <  < 2  = 3.2, 3.7

9. STAR STAR inclusive charged pions • STAR measured ALL for inclusive charged pions during Run 5 • ALL(π+) – ALL(π-) is sensitive to the sign of G • Difficult to trigger on charged pions • Used the EMCal jet patch trigger as a surrogate, which introduces significant trigger bias (dominates syst. error band) π+ π-

10. STAR Lambda longitudinal polarization transfer • STAR can measure the longitudinal polarization transfer from one of the proton beams to outgoing Λ and anti-Λ • Outgoing anti-Λ polarization is particularly sensitive to s • Has some of the same trigger difficulties as charged pions

11. STAR STAR inclusive jet ALL from Run 6 200 GeV -0.7 <  < 0.9 • Confidence level calculations from comparison to the GRSV polarized parton distributions • Also compare to other polarized parton fits • Large gluon polarization scenarios excluded, except for GS-C

12. DSSV – first global analysis with polarized jets de Florian et al., PRL 101, 072001 • The first global NLO analysis to include inclusive DIS, SIDIS, and RHIC pp data on an equal footing • Finds a node in the gluon distribution near x ~ 0.1, but with the opposite phase from GS-C STAR

13. Future inclusive jet ALL sensitivity Projected sensitivities: Run 9 & future 500 GeV running Projected improvement in xg from Run 9 • Goal for the current 200 GeV run: • 50 pb-1 @ 60% pol – reduce ALL uncertainties a factor of ~4 • Will provide much stronger constraints on gluon polarization • Goal for future 500 GeV running: • 300 pb-1 @ 70% pol • Extend precision determination to lower xg

14. STAR Looking beyond inclusive ALL measurements PRL 100, 232003 • Inclusive ALL measurements at fixed pT average over a broad x range. • Can hide considerable structure if Δg(x) has a node • Correlation measurements can constrain the shape of Δg(x)

15. STAR Beyond inclusives: charged pions opposite jets ALL ALL • Making lemons into lemonade • Beat the trigger bias by using it • Trigger and reconstruct a jet, then look for a charged pion on the opposite side • Correlation measurement significantly increases the sensitivity of ALL(π+)

16. Jet+hadron correlations at NLOfrom de Florian, arXiv:0904.4402 • NLO calculations show strong correlation between the real x and z values and LO estimates • NLO calculations verify the increased sensitivity of ALL(π+) • Typical scale dependence ±20% of calculated ALL value

17. Remaining trigger bias in charged pions + jets • Trigger bias will be reduced with the Run 9 data: • Larger acceptance and lower thresholds increase efficiency, reduce bias • Higher statistics will permit finer binning in jet pT

18. STAR Di-jets and Δg(x) 2005 preliminary di-jet distributions • Di-jets provide direct access to parton kinematics at LO Mass Rapidity |cos(*)| Ratio

19. Di-jets and g(x): projected sensitivity for Run 9 • Di-jets provide direct access to initial parton kinematicsat LO • Di-jets in different regions of the STAR detector • Sample different mixtures of qq, qg, gg • Sample different ranges of xg • Goal for the current run: 50 pb-1 @ 60% pol

20. Projected di-jet sensitivity for √s = 500 GeV • Higher energy accesses lower xg • Expect smaller ALL • Assumes 300 pb-1 at 70% polarization

21. Gamma + jet • 90% of gamma+jet yield from quark-gluon Compton scattering • Forward photons + mid-rapidity jets provide clean access to g(x) via scattering off highly polarized valence quarks STAR sensitivity for: pT,γ > 10 GeV/c 1.09 < ηγ < 2 (EEMC) √s = 200 GeV 50 pb-1 @ 60% pol. NOT INCLUDING: Efficiency and background effects

22. Gamma+jet experimental issues • Signal yield is very small compared to di-jet backgrounds • Background suppression is VERY CHALLENGING • Efforts underway for photons in the STAR EEMC • Current status from Run 6: 25-40% purity at 70% efficiency • Have less material in STAR to produce backgrounds in Run 9 • FGT will improve photon isolation beginning in Run 12 • Gamma+jet with photons in the STAR FMS investigates even lower xg • To date, only PYTHIA+GEANT background studies have been performed

23. 500 GeV pp: quark and anti-quark polarizations • Projected sensitivity for parity-violating W asymmetries • With two polarized beams and W+ and W-, canseparate u, d, u, d polarizations • Explores the origin of the sea quarks Will constrain u

24. Mid-rapidity W asymmetries Will constrain d • W cross section is much larger at mid-rapidity than at forward and backward rapidity • Mid-rapidity W asymmetries mix quark and anti-quark sensitivities • Provide high precision information about anti-quarks when quark polarizations are well known

25. World Data on g1p as of 2005 All fixed-target data World DIS database with DGLAP fits Extending the Q2 reach • W asymmetry measurements will extend the reach of polarized data to Q2 ~ 6500 GeV2 • Significant test of the evolution when combined with our g(x) measurements

26. STAR STAR transverse spin program PRL 92, 171801 • Large rapidity transverse single-spin asymmetries at 200 GeV are BIG • May arise from the Sivers effect, Collins effect, or a combination Parton orbital motion Transversity

27. STAR Run 6 inclusive π0 AN at forward rapidity PRL 101, 222001 • Large transverse single-spin asymmetries at large xF • xF dependence matches Sivers effect expectations qualitatively (but not quantitatively) • pT dependence at fixed xF does not follow ~1/pT expectation of pQCD-based calculations

28. STAR STAR 2006 PRELIMINARY Run 6 inclusive  AN at large xF • To date, the η meson has looked like a “high-mass, low-yield π0” in all measurements at RHIC • AN for the η mass region is much larger at high xF η ~ 3.66

29. Separating Sivers and Collins effects in pp collisions Sivers mechanism:asymmetry in the forward jet or γ production Collins mechanism:asymmetry in the forward jet fragmentation SP SP kT,q p p p p Sq kT,π Sensitive to proton spin – parton transverse motion correlations Sensitive to transversity • Need to go beyond inclusive hadrons to measurements of jets or direct γ

30. STAR Sivers di-jet measurementPRL 99, 142003 • Observed asymmetries are an order of magnitude smaller than seen in semi-inclusive DIS by HERMES • Detailed cancellations of initial vs. final state effects and u vs. d quark effects?

31. STAR FMS: expanding STAR’s forward acceptance STAR Forward Meson Spectrometer 2.5 < η < 4.0 • Expanded pT range for inclusive π0 AN during Run 8

32. More than just π0 out there ηγγ ωπ0γγγγ • FMS acceptance is large enough to provide good efficiency for higher mass hadrons over a broad kinematic range STAR Preliminary Pythia+GEANT (3γ background only)

33. STAR Run 8 forward π0 + hadron correlation measurements pp  π0 (FMS) + π0 (FMS) + X pp  π0 (FMS) + h± (TPC) + X • Trigger on a π0 in the FMS with pT > 2.5 GeV/c • Correlate with hadrons with pT > 1.5 GeV/c • FMS-TPC and FMS-FMS back-to-back correlations enable di-hadron / di-jet Sivers effect measurements • FMS-FMS near-side correlations sensitive to transversity • FMS-TPC: key step toward future transverse spin γ+ jet study Uncorrected Coincidence Probability (radian-1)

34. First look at jet-like events with the STAR FMS • Comparisons of the jet profile and effective mass in data vs. PYTHIA + GEANT simulations • If we find: • AN for jets must arise from Sivers effect • Azimuthal variation in jet shape must arise from Collins effect

35. Proposed addition: Forward Hadron Calorimeter Estimated sensitivity for FMS+FHC • Install hadronic calorimetry behind the FMS left and right of the beam • Forward Lambdas • Improved FMS direct photon isolation • Forward jets

36. Future: transverse spin forward g + mid-rapidity jet Bacchetta et al., PRL 99, 212002 • Conventional calculations predict the asymmetry to have the same sign in SIDIS and +jet • Calculations that account for the repulsive interactions between like color charges predict opposite sign • Critical test of our basic theoretical understanding

37. STAR Transverse spin possibilities with mid-rapidity jets -0.9 <  < -0.5 -0.5 <  < 0 AN STAR Preliminary STAR Preliminary • Run 6 transverse spin measurements • Measured AN, AΣ while investigating systematics in ALL • Study of Collins effect in jet fragmentation underway (please calculate) • Will measure ATT for inclusive jets during next extended transverse spin run (please calculate) AΣ STAR Preliminary 0 <  < 0.5 0.5 <  < 0.9 PT (GeV/c) PT (GeV/c) STAR Preliminary STAR Preliminary PT (GeV/c) PT (GeV/c)

38. Conclusions • STAR is making significant contributions to three poorly constrained pieces of the spin puzzle • Gluon polarization • Flavor-separated quark and anti-quark polarizations • Parton orbital motion and transversity • STAR in embarking on precision explorations of these phenomena through correlation measurements • STAR is generating a wealth of new data regarding the spin structure of the proton

40. STAR detector E-M Calorimeter Projection           Chamber Time of    Flight

41. Other global analyses • There have been many global analyses of the polarized DIS data xΔg(x) at Q2 = 10 GeV2

42. Full set of DSSV polarized distributions de Florian et al, PRL 101, 072001 and arXiv:0904.3821

43. Current status on transversity Anselmino et al, arXiv:0812.4366 • Global analysis combining Collins effect measurements in SIDIS from HERMES and COMPASS with measurements of the Collins fragmentation function by BELLE