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Spin Physics with STAR at RHIC

STAR. Spin Physics with STAR at RHIC. 徐庆华 , 山东大学 威海， 2009.8.11. Introduction STAR longitudinal spin program: results and future STAR transverse spin program: results and future Summary. Spin structure of nucleon. Spin sum rule (longitudinal case) :. Gluon spin , Poorly known.

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Spin Physics with STAR at RHIC

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  1. STAR Spin Physics with STAR at RHIC 徐庆华, 山东大学 威海，2009.8.11 • Introduction • STAR longitudinal spin program: results and future • STAR transverse spin program: results and future • Summary

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

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

  4. 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 first polarized pp collider in the world

  5. 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 first polarized pp collider in the world *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 AN (SSA) on 0,  • QCD mechanisms (Sivers, Collins, high-twist) -- forward +jet production on Sivers effects

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

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

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

  10. STAR STAR pQCD works at RHIC energies-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

  11. STAR inclusive π0 ALL at various rapidities || < 0.95 1 <  < 2  = 3.2, 3.7 • 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

  12. STAR inclusive π0 ALL at various rapidities || < 0.95 1 <  < 2  = 3.2, 3.7 • 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 || < 0.35 PHENIX, arXiv:0810.0694

  13. Results on jet X-section and spin asymmetry Experimental cross section agrees with NLO pQCD over 7 orders of magnitude PRL 97, 252001 (2006) PRL 97, 252002 (2006)

  14. Results on jet X-section and spin asymmetry Experimental cross section agrees with NLO pQCD over 7 orders of magnitude PRL 100, 232003 (2008) 2005 PRL 97, 252002 (2006) 2006

  15. RHIC constraints Impact of RHIC early results on g(x) de Florian et al., PRL101(2008) STAR • 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).

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

  17. Inclusive Jets: LO (W. Vogelsang) 10 20 fraction 30 pT/GeV • Inclusive measurement cover • integration of x-gluon. • High pT measurement begin to • separate large x, but still suffer • from mixture of subprocesses. • - Need correlation measurements • to constrain the shape of Δg(x) 17

  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 ALL(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 x2 x1 Reconstruction of partonic kinematics --> x-dependence of g !

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

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

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

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

  26. Strange quark polarization • S~ -0.08 from inclusive DIS • under SU(3)_f symmetry D. de Florian et al, PRL101(2008) • SDIS results at HERMES: PLB666(2008) • 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.

  27. DLL-Longitudinal spin transfer at RHIC • Expectations at LO show sensitivity of DLL for anti-Lambda to : GRSV00-M.Gluck et al, Phys.Rev.D63(2001)094005 Pol. frag. func. models Typ. range at RHIC Q. X, E. Sichtermann, Z. Liang, PRD 73(2006)077503 - Promising measurements---effects potentially large enough to be observed. - DLL of  is less sensitive to s, due to larger u and d quark frag. contributions.

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

  29. Transverse spin program • Basic QCD calculations (leading- • twist, zero quark mass) predict AN~0 • ---AN~0.4 for + in pp at E704 (1991) • Understanding transverse spin effect: • Single transverse-spin asymmetry • 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 pT generate the same physics. STAR, Phys. Rev. Lett. 92 (2004)171801 Ji-Qiu-Vogelsang-Yuan,PRL97,2006

  30. Recent results on SSA STAR, PRL97,152302(2006) • X-section reproduced with pQCD • AN increase with xF, in agreement • with pQCD model calculation.

  31. Recent results on SSA STAR, PRL97,152302(2006) • X-section reproduced with pQCD • AN increase with xF, in agreement • with pQCD model calculation. • pQCD based models predicted • decreasing AN with pT , which • Is not consistent with data. STAR, Phys. Rev. Lett. 101 (2008)222001

  32. STAR 2006 PRELIMINARY Run 6 inclusive  AN at large xF • AN for the η mass region is much larger at high xF>0.55 η ~ 3.66 <AN>η = 0.36 +/- 0.06 <A>π = 0.08 +/- 0.02

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

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

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

  36. Probing Sivers effect with + mid-rapidity jet Bacchetta et al., PRL 99, 212002 • 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. Forward jet reconstruction with FMS+FHC STAR Detector - future MTD MRPC ToF barrel 100% ready for run 10 FMS =2.8 FPD TPC FHC Ongoing R&D HFT FGT

  38. SSA with forward jets and photons Jet energy profile from FHC+FMS: Projected precision of AN for p+pjet + X : • Collins effects(spin and k correlationinfragmentation 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 L ->n+p0 : transversity distribution : f(x) = f (x) - f(x) Transversely polarized fragmentation function : Measurement at BELLE ? pQCD - Transverse spin transfer can provide access to transversity, which is still poorly known so far.

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

  41. Summary & Outlook - I 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.

  42. Summary & Outlook -II 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 . • STARtransverse γ+jet measurements will provide a direct illustration ofattractive vs. repulsive color-charge interactions • Transverse hyperon polarization at forward region at STAR

  43. 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

  44. What is the FHC? • Two identical 9x12 enclosures of E864 hadron calorimeter detectors • ---100X100X117 cm3 • Refurbished and used by PHOBOS collaboration as forward hadron multiplicity detectors for run-3 d+Au Recycle

  45. PHENIX, arXiv:0810.0694

  46. World efforts for spin physics Finished experiments: SLAC, EMC, SMC, HERMES RHIC@BNL pp@200&500GeV • Current running • Lepton-nucleon scattering: COMPASS, JLAB • Polarized proton-proton scattering, RHIC • Future facilities • EIC (BNL) • JPARC (Japan) • GSI-FAIR (Germany) HERMES@ DESY e+-p @27GeV SLAC E142-155 EMC@CERN COMPASS@CERN p@160GeV Jefferson Lab e-p@6,12GeV All these experiments have their unique coverage on q, g, Lq,g, and they are complementary as well

  47. Hyperon spin transfer at forward region Forward hyperons, L reconstructed via n+p0 channel, and polarization can be determined through decay product, i.e, dN/dcos* = N0(1+aLPLcos*) • Longitudinal spin transfer DLL: Provide access to pol.p.d.f. and fragmentation functions Model evaluation shows DLL provide sensitivity to pol. parton distributions. s(x) models

  48. Jet Finding in STAR • Jet reconstructed with TPC tracks and EMC energy deposits, using midpoint Cone Algorithm:

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