1. 1 Probing the Spin Structure of the Proton at J. Sowinski Indiana University For the STAR Collaboration STAR

2. RHIC and STAR STAR results both transverse and longitudinal Future measurements Collaboration STAR

3. RHIC RUN  s [GeV] L Rec [pb -1 ] Long. * L Rec [pb -1 ] Tran. * Pol. (%)‏ 20022000.30.1515 20032000.30.2530 20042000.4040-45 20052003.10.145-50 20062008.53.4/6.860 2008-9200, 200 + 50025 + 107.845, 55 + 35 Many special devices in RHIC to generate, preserve and measure polarization Development runs First devoted physics run Snakes * Lum. recorde d.at STAR Long prod run 9

4. Detector  =0 Forward Pion Detector Endcap EM Calorimeter Beam-Beam Counters Time Projection Chamber -2<  < 2 Barrel EM Calorimeter -1  1 1  2 -4.1  -3.3 2  5 Solenoidal Magnetic Field 5kG  =2  = -1 Tracking Lum. Monitor Local Polarim. 200320042005 Triggering  = - ln(tan  2)‏ STAR Special interest for spin  =2.5  =4 FMS EM Calorimeter 2008

5. Next: STAR Transverse Results and Prospects for the Future 5 Summary: STAR’s large solid angle coverage allows detection of correlations between particles, jets, two jets etc Significant new data sets at 200 GeV taken in Run 9 First 500 GeV collisions in Run 9

6. The First Spin Results from Transverse Program 6 STAR Runs 2-3 with Forward Pion Detector (FPD) Transverse SSA are Large! Run 6 Sivers effect, Collins and Twist 3 all nominally describe data SSA for  even larger Run 8 First early data with FMS confirms previous results PRL 92, 171801 (2004)‏ PRL 101, 222001 Confirmed in following measurements

7. 7 PRL 101, 222001 E704 Nucl. Phys. B 510 (1998) 3 Star p T dependence does not fall as pQCD calcs (yet)‏ STAR + BRAHMS very similar to E704 at 1/10 cm Energy What is connection to low energy physics?

8. 8 Both Sivers and Collins effects promise access to interesting physics Sivers mechanism: Transverse motion (orbital angular momentum) wrt proton spin + ISI and FSI “deflect” jets SPSP p p SqSq k T, π Analyze correlations within jet on one side Of interest in both forward and mid-rapidity regions! SPSP k T,q p p Reconstruct jets and compare left to right wrt p pol. vector Collins mechanism: Polarization of incident quark (transversity) transferred to scattered quark which self analyzes in spin dependant fragmentation

9. Spin Transfer to Determine Transversity  q The transverse polarization of quarks is preserved in forward scattering d TT Can we measure  q the polarization of scattered q? Fragmentation can be spin dependent! Collins functions Interference functions Leading hadron preference to one side of jet wrt s x p jet Preference for orientation of two leading hadrons wrt s x p jet Measured jet asymmetry =  q x d TT x  (z)‏ Determined in e + /e -. experiment from Belle ~10% effects seen Collins, Heppelman et al.

10. Initially thought could be large and give sensitivity to q and g orbital angular momentum z x y Colliding beams proton spin parton k T x Sivers with di-Jets at Mid-rapidity 10

11. Initially thought could be large and give sensitivity to q and g orbital angular momentum z x y Colliding beams proton spin parton k T x Sivers with di-Jets at Mid-rapidity PRL 99 (2007) 142003 Observe small SSA Much smaller than Sivers effects from SIDIS STAR Analysis Mtg – July 7-11 2009 11

12. Non-Universality in Sivers Functions STAR Analysis Mtg – July 7-11 2009 12 W. Vogelsang and F. Yuan, PRD 72, 054028 ( 2005 ). ISI only Bomhof et al., hep-ph/0701277 31 Jan 2007 u-quark d-quark u+d ISI+FSI SSA requires interference between amplitudes FSI have one sign (SIDIS)‏ ISI have opposite sign (Drell Yan)‏ Di-jets have both - cancellation Forward  -jet also has only ISI Important upcoming FMS meas.

13. Correlation Studies Have Begun with 2008 Data 13 STAR Preliminary First high x F J/Ψ measurement at a collider Correlation of particles in jet cone Triple clusters to construct the  STAR Analysis Mtg – July 7-11 2009

14. Including Correlations within and between “jets” 14 Forward – mid rapidity correlations FMS  0 – TPC h +- p+p-> π 0 +h ± +X Forward correlations Two FMS  0 ‘s

15. 15 Transverse Summary: Large  0 asymmetries at forward rapidity observed in first runs and reproduced with higher precision and in  meson Sivers effect would indicate orbital angular momentum Collins effect would provide sensitivity to transversity Origin of large SSA to be illuminated via future correlation measurements with FMS Investigate extension to 500 GeV in future runs Sivers in  -jet and Drell-Yan should be opposite in sign to SIDIS – future measurement with FMS Next: Results on  G

16. 16 STAR inclusive π 0 A LL at various rapidities Run 6 we measured A LL for inclusive π 0 for three different rapidity regions Mid-rapidity result excludes large gluon polarization scenarios While statistics similar signal decreases with  Forward rapidity: baseline for future γ and γ -jet measurements It remains important to confirm results in multiple channels |  | < 0.951 <  < 2  = 3.2, 3.7

17. 17 Correlations at mid-rapidity used in  G program Trigger on jet and analyze awayside charged pions to avoid trigger bias NLO calculations indicate that LO reconstructed x 1, x 2 and z are good approximations to NLO quantities

18. 18 To date inclusive jets have been the work horse at 2003-4 Disfavor extreme PDFs offered as fix to proton spin puzzle 2005 data add strong contraints on large positive  G for 0.02<x<0.3 2006 data best fit now upper limit. Constrains large neg. values STAR

19. 19 2006 Preliminary Results STAR incl. jets Existing data, STAR and others, have placed strong constraints on  G STAR

20. 20 Additional A LL predictions for 2005 A LL Small range is allowed by current measurements Implications for many previous PDFs

21. 21 Global fit D. de Florian, R. Sassot, M. Stratmann and W. Vogelsang arXiv:0804.0422 [hep-ph] Significant constraints in RHIC range 0.05<x<0.2 Uncertainty at low x prevents a constraint on the full integral  G Shape and low x important STAR Analysis Mtg – July 7-11 2009

22. 22 Message for new measurements x Δ g(x) at Q 2 = 10 GeV 2 Remaining PDFs either have small  G or Nodes, large contributions at low x are not ruled out

23. 23  G Results Summary: Large contributions to  G for x>0.05 unlikely Full integral not constrained due to uncertain low x contribution Shape  g(x) important Next: Future Directions to constrain  G

24. 24 Increased statistics at high p T just begin to separate large x from wide integration range at low p T but complicated with subprocess mix 1020 fraction 30 Inclusive Jets: LO (W. Vogelsang)‏ p T /GeV

25. Projected statistics based on measured yields and sys. error floors Inclusive Jet Projections Accumulated FoM at 200 GeV from run 9 was ~1/3 that assumed in fig. below Substantial future running at 500 GeV gives sensitivity to lower x Higher √s at same p T gives lower x g, lower x q and hence less q polarization, but better statistical precision. x T = 2 p T / √s

26. 2 (GeV/c) 2 4 (GeV/c) 2 10 (GeV/c) 2 Spin measurements, A LL ~  g/g Need to measure small gluon polarization at low x Negative polarizations evolve toward 0 40 (GeV/c) 2 100 (GeV/c) 2 Gehrmann and Stirling - C Small contribution at large x Large contribution at low x  g/g(x=0.02) ~ 0.04 for high scales

27. Di-jets require large coincident solid angle High yields allow near triple differential distributions Select kinematics for x g dependence Select kinematics for valence quarks and favorable a LL Di –Jets analyzed in 2005 data Pythia based full detector MC vs. 2005 data ½(  3 +  4 )‏ ½(  3 -  4 )‏ M

28. Each panel gives an x dependence eg. X=0.08, 0.10, 0.13  g/g~0.02-0.03 Two body kinematics Left two panels same η 1 +η 2 (x 1 /x 2 ) but different η 1 -η 2 (cosθ*) so smaller A LL in upper Same M X=0.06 X=0.16 200 GeV projections 50 pb -1 P=60% NLO and LO reasonable agreement NLO scale variations small, p T >7,10 GeV M>20 GeV Detector regions select x 1 /x 2 Also â LL Strong sensitivity in range of curr. allowed PDFs Run 9 ~ 1/3 rd FoM: expect 2x error bars

29. 500 GeV projections 300 pb -1 P=70% Access to lower x Asymmetries smaller But uncertainties smaller as well No significant data so far De Florian, Frixione, Signer and Vogelsang NPB 539 (1999) 455 and PC for present calc.

30. Direct photon – jet from qg Compton 90% from qg process LO estimate of statistical errors for 50 pb -1 at 200 GeV with 100% eff. and no background and p T > 10 GeV. More stats at lower p T  /  0 ratio at mid- rapidity -challenging backgrounds from inclusive hadronic channels

31. 31  G Future measurements: Significant step in statistics at 200 GeV with run 9 Enhanced statistics for inclusives Shape  g(x) to be constrained by di-jet and  - jet correlations Lower x from 500 GeV and forward detectors Next: The Polarized Sea

32. 32 NMC and Gottfried Sum Rule in DIS DIS on nuclei SIDIS Drell-Yan Quantitative calculations of Pauli blocking not conclusive Non-perturbative processes seem to be needed in generating the sea Phys.Rev.Lett. 80 (1998) 3715 Flavor Asymmetry of the Sea

33. 33 B. Dressler et al., Chiral Quark Soliton Model Predictions m 2 = (5 GeV) 2 x(d-u)‏ ¯ ¯ x(  u-  d)‏ ¯ ¯ d(x)-u(x) and  u(x)-  d(x) better for Q 2 evolution E866 Results are qualitatively consistent with pion cloud models, instanton models, chiral quark soliton models, etc. ¯ ¯ ¯ ¯ E866 Results Polarized q Flavor Asymmetry _ pQCD motivated models predict [  u(x)-  d(x)]dx  [d(x)-u(x)]dx Chiral motivated models tend to disagree ¯ ¯¯ ¯ ∫ 0 1 ∫ 0 1 D. De Florian et al. Phys.Rev.D80: 034030,2009 Global fit

34. 34 V-A coupling –only LH u and RH d couple to W + –Likewise LH d and RH u to W - –Only LH W’s produced Neutrino helicity gives preferential directionality in decay _ _ Parity violating single spin asymmetry A L (Helicity flip in one beam while averaging over other)‏ A L W - ~ u(x 1 )  d(x 2 )+d(x 1 )  u(x 2 )‏ _ _ Allows kinematic separation especially for W - in EEMC Dressler et al. predict large sensitivity W +(-) Production in p-p at  s = 500 GeV/c 2

35. 09/09/09 35 Ws at STAR (mid-rapidity)‏ The simulations use full detector response and realistic QCD background. The main source of background is hadrons so good e/h separation is necessary. In preparation for analysis of the 500 GeV data from run 9, STAR has been studying the reconstruction of the W. The current analysis uses a combination of tracking, shower shape, isolation style, missing energy style, and event shape cuts. Run9 W Algorithm Simulation Results Sufficient data was taken in run 9 to observe this Jacobian peak

36. J. Sowinski Spin 2008 Charlottesville VA 36 STAR Need to detect e+ e- and measure p T Need to find in huge hadronic background Need to separate e+ and e- charge Large solid angle of STAR Forward GEM Tracker EM Calorimetry h e

37. Projections Sensitivity 500 GeV 300 pb-1 J. Sowinski Spin 2008 Charlottesville VA 37 Realistic BG subtraction Recent PDFs ~represent current allowed  u/  d range  d and  u isolated in forward region  d and  u sensitivity spread over 

38. 38 Conclusions RHIC run 9 new level of statistics at 200 GeV First 500 GeV collisions STAR transverse results Surprisingly large SSA Led to predictions for new measurements based on QCD color interactions Sivers and Collins effects provide windows to orbital angular momentum and transversity (respectively)‏ STAR has made important constraints on the gluon spin contribution to that of the proton Importance of x dependence and low x for  G Di-jets and  -jets will address these W production offers opportunities to constrain the flavor asymmetry of the anti-quarks

39. 39 Backup slides

40. Forward  – jet at mid-rapidity gives access to few 10 -3 < x < few 10 -2 Narrow fiducial volume and E  > 35 GeV gives sufficient rates Rest of FMS as isol. veto gives S/B ~2:1 Benefits from back angle cross section rise a LL ~ 1 Large x valence quarks ^

41. Model for high p T scattering in pp collisions Jets and  0 s Partonic Process Cross Section Calc. NLO pQCD Parton Distribution Functions g(x,Q 2 ),q(x,Q 2 ) - measured Fragmentation Function - measured Partonic Process Spin Dependence Calc. NLO pQCD pp 41 Assumptions: Asymptotic freedom Factorization Universality Evolution Our tool for determining the spin of partons in the proton But a possibly bigger question: How well and when does this all work as a precise quantitative tool?