1. Carl Gagliardi – STAR Spin: Recent Results, Future Directions 1 Spin Recent Results, Future Directions Carl A. Gagliardi Texas A&M University for the Collaboration STAR STAR

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

3. Carl Gagliardi – STAR Spin: Recent Results, Future Directions 3 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. Carl Gagliardi – STAR Spin: Recent Results, Future Directions 4 STAR detector in cross section BEMC EEMC FMS FPD TPC BBC

5. Carl Gagliardi – STAR Spin: Recent Results, Future Directions 5 Essential benchmark – unpolarized cross sections Mid-rapidity jet cross section is consistent with NLO pQCD over 7 orders of magnitude Forward rapidity π 0 cross section also consistent with NLO pQCD Many other examples pQCD works over a very broad kinematic range at RHIC energies STAR PRL 97, 252001 STAR PRL 97, 152302

6. Carl Gagliardi – STAR Spin: Recent Results, Future Directions 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 Q 2 = 1 GeV 2 Leader et al, PRD 75, 074027 Additional question for future 500 GeV runs: What is the polarization of the anti-quarks?

7. Carl Gagliardi – STAR Spin: Recent Results, Future Directions 7  f: polarized parton distribution functions For most RHIC kinematics, gg and qg dominate, making A LL for inclusive jets and hadrons sensitive to gluon polarization. 10 20 30 pT(GeV) Partonic fractions in jet production at 200 GeV 0 Longitudinally polarized pp collisions at RHIC

8. Carl Gagliardi – STAR Spin: Recent Results, Future Directions 8 STAR inclusive π 0 A LL at various rapidities During Run 6, STAR measured A LL for inclusive π 0 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 x gluons Expect A LL to decrease as  increases Forward rapidity: baseline for future γ and γ -jet measurements |  | < 0.95 1 <  < 2  = 3.2, 3.7

9. Carl Gagliardi – STAR Spin: Recent Results, Future Directions 9 STAR inclusive charged pions STAR measured A LL for inclusive charged pions during Run 5 A LL ( π + ) – A LL ( 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) STAR π+π+ π-π-

10. Carl Gagliardi – STAR Spin: Recent Results, Future Directions 10 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 STAR

11. Carl Gagliardi – STAR Spin: Recent Results, Future Directions 11 STAR inclusive jet A LL from Run 6 200 GeV -0.7 <  < 0.9 STAR 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. Carl Gagliardi – STAR Spin: Recent Results, Future Directions 12 DSSV – first global analysis with polarized jets 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 de Florian et al., PRL 101, 072001 STAR

13. Carl Gagliardi – STAR Spin: Recent Results, Future Directions 13 Future inclusive jet A LL sensitivity Goal for the current 200 GeV run: –50 pb -1 @ 60% pol – reduce A LL 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 x g Projected improvement in x  g from Run 9 Projected sensitivities: Run 9 & future 500 GeV running

14. Carl Gagliardi – STAR Spin: Recent Results, Future Directions 14 Looking beyond inclusive A LL measurements Inclusive A LL measurements at fixed p T 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) STAR PRL 100, 232003

15. Carl Gagliardi – STAR Spin: Recent Results, Future Directions 15 Beyond inclusives: charged pions opposite jets STAR 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 A LL ( π + ) A LL

16. Carl Gagliardi – STAR Spin: Recent Results, Future Directions 16 Jet+hadron correlations at NLO from 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 A LL ( π + ) Typical scale dependence ±20% of calculated A LL value

17. Carl Gagliardi – STAR Spin: Recent Results, Future Directions 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 p T

18. Carl Gagliardi – STAR Spin: Recent Results, Future Directions 18 Di-jets and Δg(x) Di-jets provide direct access to parton kinematics at LO MassRapidity |cos(  * )| Ratio 2005 preliminary di-jet distributions STAR

19. Carl Gagliardi – STAR Spin: Recent Results, Future Directions 19 Di-jets and  g(x): projected sensitivity for Run 9 Di-jets provide direct access to initial parton kinematics at LO Di-jets in different regions of the STAR detector –Sample different mixtures of qq, qg, gg –Sample different ranges of x g Goal for the current run: 50 pb -1 @ 60% pol

20. Carl Gagliardi – STAR Spin: Recent Results, Future Directions 20 Projected di-jet sensitivity for √s = 500 GeV Higher energy accesses lower x g Expect smaller A LL Assumes 300 pb -1 at 70% polarization

21. Carl Gagliardi – STAR Spin: Recent Results, Future Directions 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: p T, γ > 10 GeV/c 1.09 < η γ < 2 (EEMC) √s = 200 GeV 50 pb -1 @ 60% pol. NOT INCLUDING: Efficiency and background effects

22. Carl Gagliardi – STAR Spin: Recent Results, Future Directions 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 x g –To date, only PYTHIA+GEANT background studies have been performed

23. Carl Gagliardi – STAR Spin: Recent Results, Future Directions 23 500 GeV pp: quark and anti-quark polarizations Projected sensitivity for parity-violating W asymmetries With two polarized beams and W + and W -, can separate u, d, u, d polarizations Explores the origin of the sea quarks Will constrain u

24. Carl Gagliardi – STAR Spin: Recent Results, Future Directions 24 Mid-rapidity W asymmetries 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 Will constrain d

25. Carl Gagliardi – STAR Spin: Recent Results, Future Directions 25 Extending the Q 2 reach W asymmetry measurements will extend the reach of polarized data to Q 2 ~ 6500 GeV 2 Significant test of the evolution when combined with our  g(x) measurements World DIS database with DGLAP fits All fixed- target data World Data on g 1 p as of 2005

26. Carl Gagliardi – STAR Spin: Recent Results, Future Directions 26 STAR transverse spin program Large rapidity transverse single-spin asymmetries at 200 GeV are BIG May arise from the Sivers effect, Collins effect, or a combination PRL 92, 171801 STAR Parton orbital motion Transversity

27. Carl Gagliardi – STAR Spin: Recent Results, Future Directions 27 Run 6 inclusive π 0 A N at forward rapidity Large transverse single-spin asymmetries at large x F x F dependence matches Sivers effect expectations qualitatively (but not quantitatively) p T dependence at fixed x F does not follow ~1/p T expectation of pQCD-based calculations PRL 101, 222001 STAR

28. Carl Gagliardi – STAR Spin: Recent Results, Future Directions 28 To date, the η meson has looked like a “high-mass, low-yield π 0 ” in all measurements at RHIC A N for the η mass region is much larger at high x F STAR 2006 PRELIMINARY η ~ 3.66 STAR Run 6 inclusive  A N at large x F

29. Carl Gagliardi – STAR Spin: Recent Results, Future Directions 29 Separating Sivers and Collins effects in pp collisions Collins mechanism: asymmetry in the forward jet fragmentation Sivers mechanism: asymmetry in the forward jet or γ production SPSP k T,q p p SPSP p p SqSq k T, 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. Carl Gagliardi – STAR Spin: Recent Results, Future Directions 30 Sivers di-jet measurement PRL 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? STAR

31. Carl Gagliardi – STAR Spin: Recent Results, Future Directions 31 FMS: expanding STAR’s forward acceptance Expanded p T range for inclusive π 0 A N during Run 8 STAR Forward Meson Spectrometer 2.5 < η < 4.0 STAR

32. Carl Gagliardi – STAR Spin: Recent Results, Future Directions 32 More than just π 0 out there FMS acceptance is large enough to provide good efficiency for higher mass hadrons over a broad kinematic range Pythia+GEANT (3 γ background only) STAR Preliminary η  γγ ω  π 0 γ  γγγ

33. Carl Gagliardi – STAR Spin: Recent Results, Future Directions 33 Run 8 forward π 0 + hadron correlation measurements Trigger on a π 0 in the FMS with p T > 2.5 GeV/c Correlate with hadrons with p T > 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 ) pp  π 0 (FMS) + h ± (TPC) + X STAR pp  π 0 (FMS) + π 0 (FMS) + X

34. Carl Gagliardi – STAR Spin: Recent Results, Future Directions 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: –A N for jets must arise from Sivers effect –Azimuthal variation in jet shape must arise from Collins effect

35. Carl Gagliardi – STAR Spin: Recent Results, Future Directions 35 Proposed addition: Forward Hadron Calorimeter Install hadronic calorimetry behind the FMS left and right of the beam –Forward Lambdas –Improved FMS direct photon isolation –Forward jets Estimated sensitivity for FMS+FHC

36. Carl Gagliardi – STAR Spin: Recent Results, Future Directions 36 Future: transverse spin forward  + 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. Carl Gagliardi – STAR Spin: Recent Results, Future Directions 37 Transverse spin possibilities with mid-rapidity jets Run 6 transverse spin measurements –Measured A N, A Σ while investigating systematics in A LL –Study of Collins effect in jet fragmentation underway (please calculate) Will measure A TT for inclusive jets during next extended transverse spin run (please calculate) STAR Preliminary STAR Preliminary P T (GeV/c) -0.9 <  < -0.5 STAR Preliminary 0 <  < 0.5 STAR Preliminary 0.5 <  < 0.9 STAR Preliminary -0.5 <  < 0 AΣAΣ ANAN STAR

38. Carl Gagliardi – STAR Spin: Recent Results, Future Directions 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

39. Carl Gagliardi – STAR Spin: Recent Results, Future Directions 39

40. Carl Gagliardi – STAR Spin: Recent Results, Future Directions 40 E-M Calorimeter Time of Flight Projection Chamber STAR detector

41. Carl Gagliardi – STAR Spin: Recent Results, Future Directions 41 Other global analyses There have been many global analyses of the polarized DIS data x Δ g(x) at Q 2 = 10 GeV 2

42. Carl Gagliardi – STAR Spin: Recent Results, Future Directions 42 Full set of DSSV polarized distributions de Florian et al, PRL 101, 072001 and arXiv:0904.3821

43. Carl Gagliardi – STAR Spin: Recent Results, Future Directions 43 Current status on transversity Global analysis combining Collins effect measurements in SIDIS from HERMES and COMPASS with measurements of the Collins fragmentation function by BELLE Anselmino et al, arXiv:0812.4366