1 Spin Physics with STAR at RHIC 徐庆华, 山东大学 威海， Introduction STAR longitudinal spin program: results and future STAR transverse spin program: results and future Summary STAR

2 Quark spin, (~30%)-DIS Gluon spin, Poorly known Orbital Angular Momenta Little known Spin sum rule (longitudinal case): Spin structure of nucleon Little known in the transverse case:  Proton spin  Proton spin    Helicity distribution: Transversity:

3  Detailed knowledge on ∆q(x), ∆g(x) (before RHIC) x

4 RHIC- the first polarized pp collider in the world

5 pp Run Year (200/500) % / 35* L max [ s -1 cm -2 ] / 85* L int [pb -1 ] at STAR (Long./Transverse) 0 / / / 03.1 / / 3.40 /3.122 /10.5* *first 500 GeV run

6 The STAR spin program  Longitudinal spin program: determination of the helicity distributions: Gluon polarization ∆g(x) in the nucleon -- results & status (inclusive jet, hadrons) -- status & future plan (di-jets,  +jet, heavy flavor) Flavor separation: quark & anti-quark polarization -- RHIC 500 GeV program (W  prodction) -- (anti-)hyperons spin transfer  Transverse spin program: Single spin asymmetry A N (SSA) on  0,  QCD mechanisms (Sivers, Collins, high-twist) -- forward  +jet production on Sivers effects

7 MRPC ToF barrel 100% ready for run 10 PMD FPD FMSFMS EMC barrel EMC End Cap DAQ1000 Complete Ongoing TPC FTPC BBC STAR Detector (current)

8 COMPASS, PLB676,31(2009)  g determination from DIS Recent measurements from DIS:

9 f2f2 f1f1 Longitudinal spin asymmetry: Accessing ∆g(x) in pp collision

10 STAR PRL 97, STAR PRL 97, pQCD works at RHIC energies-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

11 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 |  | < <  < 2  = 3.2, 3.7

12 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 |  | < <  < 2  = 3.2, 3.7 PHENIX, arXiv: |  | < 0.35

13 PRL 97, (2006) Results on jet X-section and spin asymmetry PRL 97, (2006) Experimental cross section agrees with NLO pQCD over 7 orders of magnitude

14 Experimental cross section agrees with NLO pQCD over 7 orders of magnitude PRL 97, (2006) PRL 100, (2008) Results on jet X-section and spin asymmetry

15 RHIC constraints Impact of RHIC early results on  g(x) de Florian et al., PRL101(2008) Early RHIC data (2005, 2006) included in a global analysis along with DIS and SIDIS data. Evidence for a small gluon polarization over a limited region of momentum fraction (0.05<x<0.2). STAR

16 Future inclusive jet measurements: Increasing Precision Precision will be significantly improved in future runs. 500 GeV data will reach low x-range for  g with high statistics. Projected improvement in x  g from Run 9 Projected sensitivities: Run 9 & 500 GeV running x T =2p T /  s

17 - Inclusive measurement cover integration of x-gluon. - High p T measurement begin to separate large x, but still suffer from mixture of subprocesses. - Need correlation measurements to constrain the shape of Δg(x) 1020 fraction 30 Inclusive Jets: LO (W. Vogelsang) p T /GeV

18 First correlation study: charged pions opposite jets Trigger and reconstruct a jet, then look for charged pion on the opposite side Correlation measurement significantly increases the sensitivity of A LL ( π + )

19 Probing  g(x) with di-jets production Upcoming Correlation Measurements :  access to partonic kinematics through di-jet production, direct photon+jet production

20 Sensitivity of di-jets measurements Projections with 50 pb -1 provide high sensitity to gluon polarization:

21 Direct Photon - Jet Correlations Direct  +jet dominated by qg-Compton process: 90% from qg Reconstruction of partonic kinematics --> x-dependence of  g ! x2x2 x1x1

22 Anti-quark helicity distribution D. de Florian et al, PRL101(2008) From global fit with DIS data:

23 PRD71,2005 Extrating  q(x) in Semi-inclusive DIS

24 Flavor separation of proton spin Quark polarimetry with W-bosons: Spin measurements: W-detection through high energy lepton -

25 Sensitivity of W measurements Strong impact on constraining the sea quark polarizations with 300 pb -1 :

26 Clear need to measure. Can we do it with hyperons at RHIC? - hyperons contain at least one strange quark and their polarization can be determined via their weak decay. D. de Florian et al, PRL101(2008)  S~ from inclusive DIS under SU(3)_f symmetry Strange quark polarization SDIS results at HERMES: PLB666(2008)

27 Q. X, E. Sichtermann, Z. Liang, PRD 73(2006) Expectations at LO show sensitivity of D LL for anti-Lambda to : GRSV00-M.Gluck et al, Phys.Rev.D63(2001) Typ. range at RHIC D LL -Longitudinal spin transfer at RHIC - D LL of  is less sensitive to  s, due to larger u and d quark frag. contributions. Pol. frag. func. models - Promising measurements---effects potentially large enough to be observed.

28 Spin transfer for Lambda hyperons (anti-)Lambda reconstruction using TPC tracks: p V0_vertex V0_DCA  First proof-of-principle measurement; ~10% precision with p T up to 4 GeV. - not yet to discriminate pol. pdfs, - extend p T with specific trigger D LL extraction:

29 Transverse spin program Single transverse-spin asymmetry STAR, Phys. Rev. Lett. 92 (2004) Basic QCD calculations (leading- twist, zero quark mass) predict A N ~0 ---A N ~0.4 for  + in pp at E704 (1991) Understanding transverse spin effect:  Qiu and Sterman (initial-state) / Koike (final-state) twist-3 pQCD calculations  Sivers: spin and k  correlation in initial state (related to orbital angular momentum)  Collins: spin and k  correlation in fragmentation process (related to transversity) Twist-3 correlation and the k  dependent distribution/fragmentation in intermediate p T generate the same physics. Ji-Qiu-Vogelsang-Yuan,PRL97,2006

30 Recent results on SSA A N increase with x F, in agreement with pQCD model calculation. X-section reproduced with pQCD STAR, PRL97,152302(2006)

31 pQCD based models predicted decreasing A N with p T, which Is not consistent with data. A N increase with x F, in agreement with pQCD model calculation. Recent results on SSA STAR, PRL97,152302(2006) X-section reproduced with pQCD STAR, Phys. Rev. Lett. 101 (2008)222001

32 A N for the η mass region is much larger at high x F >0.55 STAR 2006 PRELIMINARY η ~ 3.66 Run 6 inclusive  A N at large x F  = /  = /- 0.02

33 E704 Nucl. Phys. B 510 (1998) 3 ++ -- 200 GeV 62.4 GeV BRAHMS,PRL101(2008) Large SSA of different hadrons in different experiments

34 Separating Sivers and Collins effect in pp collisions  Collins effect: spin and k  correlation in fragmentation process (related to transversity) For hadron SSA, both Sivers and Collins effects can contribute. Forward jets and photon may provide separation of them. SPSP k ,q p p  Sivers effect: spin and k  correlation in initial state (related to orbital angular momentum) SPSP p p SqSq k,πk,π Sensitive to transversity Sensitive to orbital angular momentum

35 Mid-rapidity jet A N ~0, different as the conventional calculations with Sivers function fitted from SDIS. Sivers distribution, is process dependent (not universal), A N of jet production - Sivers effect STAR, PRL99,142003(2007) A N of mid-rapidity consistent with zero: An example: attractive color interactionrepulsive color interaction

36 Probing Sivers effect with  + mid-rapidity jet Bacchetta et al., PRL 99, Conventional calculations predict the asymmetry to have the same sign in SIDIS and γ+jet Calculations that account for the repulsive interactions between like color charges predict opposite sign Critical test of our basic theoretical understanding

37 STAR Detector - future MRPC ToF barrel 100% ready for run 10 FMSFMS FGT Ongoing MTD R&D HFT TPC FHC Forward jet reconstruction with FMS+FHC  =2.8 FPD

38 SSA with forward jets and photons Projected precision of A N for p  +p  jet + X : Jet energy profile from FHC+FMS: Collins effects(spin and k  correlation in fragmentation process ):  Accessed via spin-dependent correlations of hadrons within forward jet Sivers effect(spin and k  correlation in initial state):  Accessed by symmetric azimuthal integration of hadrons from forward jet  Accessed by forward direct photons

39 Transverse spin transfer of hyperons and  q(x) Transverse spin transfer of hyperons transverse spin can provide access to transversity, via channel  ->n+   : transversity distribution :  f(x) = f  (x) - f  (x) pQCD Transversely polarized fragmentation function : Measurement at BELLE ? - Transverse spin transfer can provide access to transversity, which is still poorly known so far.

40 Large polarization with unpolarized beam p + p    + X, observed in different experiments. Still not fully understood. target produced  production plane Transverse hyperons polarization in unpolarized pp How about at RHIC energy? ( = 2p L /  s)

41 Longitudinal spin physics at STAR:  Determination of gluon polarization  G :  Currently inclusive probes with jets, are providing important constraints on  G. Early results have been included in global analysis.  Near future probes:  Increased statistics and higher energy for inclusive jets will provides additional constraints with better precision and wider x-range.  Correlation measurements (di-jet, photon-jet) with access to partonic kinematics will provide better resolution in x and direct probe to  G.  Determination of sea quark polarization:  With 500 GeV collisions, W-production provide unique tool to study the anti-quark polarization.  Spin transfer of hyperons provides sensitivity to strange quark polarization. Summary & Outlook - I

42 Transverse spin physics at STAR:  STAR has observed large transverse single-spin asymmetries for forward particle production.  Study Collins and Sivers effects in pp reaction with Single-spin asymmetry with forward jet.  STAR transverse γ +jet measurements will provide a direct illustration of attractive vs. repulsive color-charge interactions  Transverse hyperon polarization at forward region at STAR Summary & Outlook -II

43 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

44 What is the FHC? Two identical 9x12 enclosures of E864 hadron calorimeter detectors X100X117 cm 3 Refurbished and used by PHOBOS collaboration as forward hadron multiplicity detectors for run- 3 d+Au Recycle

45 PHENIX, arXiv:

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47 World efforts for spin physics Current running –Lepton-nucleon scattering: COMPASS, JLAB –Polarized proton-proton scattering, RHIC Future facilities – EIC (BNL) –JPARC (Japan) –GSI-FAIR (Germany) DESY e +  Jefferson Lab SLAC E CERN Finished experiments: SLAC, EMC, SMC, HERMES All these experiments have their unique coverage on  q,  g, Lq,g, and they are complementary as well

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49 Hyperon spin transfer at forward region Forward hyperons,  reconstructed via n+   channel, and polarization can be determined through decay product, i.e, dN/dcos  * = N 0 (1+   P  cos  *) Provide access to pol.p.d.f. and fragmentation functions Longitudinal spin transfer D LL : Model evaluation shows D LL provide sensitivity to pol. parton distributions.  s(x) models

50 Jet Finding in STAR  Jet reconstructed with TPC tracks and EMC energy deposits, using midpoint Cone Algorithm:

51 The STAR Detector Magnet 0.5 T Solenoid Triggering & Luminosity Monitor Beam-Beam Counters –3.4 < |  | < 5.0 Zero Degree Calorimeters Central Tracking Large-volume TPC –|  | < 1.3 Calorimetry Barrel EMC (Pb/Scintilator) –|  | < 1.0 –Shower-Maximum Detector Endcap EMC (Pb/Scintillator) –1.0 <  < 2.0

52 Transverse spin asymmetry - spin structure of nucleon Large single transverse-spin asymmetry observed at RHIC: STAR, Phys. Rev. Lett. 92 (2004)171801STAR, Phys. Rev. Lett. 97 (2006) Basic QCD calculations (leading- twist, zero quark mass) predict A N ~0, while cross sections are found to be in agreement with pQCD evaluations.