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Proton spin structure from longitudinally polarized pp collisions from PHENIXat RHIC. Alexander Bazilevsky BNL The 6 th Circum-Pan-Pacific Symposium on High Energy Spin Physics July 30 – August 2, 2007 Vancouver BC, Canada.  Gluons are polarized ( G )  Sea quarks are polarized:.

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Proton spin structure from longitudinally polarized pp collisions from PHENIXat RHIC


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    1. Proton spin structure from longitudinally polarized pp collisions from PHENIXat RHIC Alexander Bazilevsky BNL The 6th Circum-Pan-Pacific Symposium on High Energy Spin Physics July 30 – August 2, 2007 Vancouver BC, Canada

    2.  Gluons are polarized (G)  Sea quarks are polarized: For complete description include parton orbital angular momentum LZ: 1989 EMC (CERN): =0.120.090.14  Spin Crisis Nucleon Spin Structure Naïve parton model: Determination of G and q-bar is the main goal of longitudinal spin program at RHIC

    3. Gluon Distributions g(x,Q2)= Dg(x,Q2)= No Transverse Gluon Distribution in 1/2 Parton Distribution Functions (PDF) • Quark Distribution unpolarised distribution q(x,Q2)= = helicity distribution Dq(x,Q2)= transversity distribution dq(x,Q2)=

    4. Polarized PDF from DISAsymmetry Analysis Collaboration M. Hirai, S. Kumano and N. Saito, PRD (2004) • Valence distributions well determined • Sea Distribution poorly constrained • Gluon can be either positive, 0, negative!

    5. … To polarized pp collider Utilizes strongly interacting probes • Probes gluon directly • Higher s  clean pQCD interpretation • Elegant way to explore guark and anti-quark polarizations through W production • Polarized Gluon Distribution Measurements (G(x)): • Use a variety of probes • Access to different gluon momentum fraction x • Different probes – different systematics • Use different energies s • Access to different gluon momentum fraction x

    6. BRAHMS & PP2PP (p) PHENIX (p) STAR (p) RHIC as polarized proton collider Absolute Polarimeter (H jet) RHIC pC Polarimeters Siberian Snakes Spin Rotators 2  1011 Pol. Protons / Bunch e = 20 p mm mrad Partial Siberian Snake LINAC BOOSTER Pol. Proton Source 500 mA, 300 ms AGS AGS Internal Polarimeter 200 MeV Polarimeter Rf Dipoles

    7. Philosophy (initial design): • High rate capability & granularity • Good mass resolution & particle ID • Sacrifice acceptance PHENIX for Spin • p0/g/h • Electromagnetic Calorimeter • p+/p- • Drift Chamber • Ring Imaging Cherenkov Counter • J/y • Muon Id/Muon Tracker • Relative Luminosity • Beam Beam Counter (BBC) • Zero Degree Calorimeter (ZDC) • Local Polarimetry - ZDC

    8. PHENIX Long. Spin runs

    9. Unpol. Cross Section in pp pp0 X : hep-ex-0704.3599 pp X: PRL 98, 012002 ||<0.35 Good agreement between NLO pQCD calculations and data  confirmation that pQCD can be used to extract spin dependent pdf’s from RHIC data. • Same comparison fails at lower energies

    10. Double longitudinal spin asymmetry ALL is sensitive to G Probing G in pol. pp collisions pp  hX

    11. Measuring ALL • (N) Yield • (R) Relative Luminosity • BBC vs ZDC • (P) Polarization • RHIC Polarimeter (at 12 o’clock) • Local Polarimeters (SMD&ZDC) • Bunch spin configuration alternates every 106 ns • Data for all bunch spin configurations are collected at the same time •  Possibility for false asymmetries are greatly reduced

    12. ALL: 0 PHENIX Preliminary Run6 (s=200 GeV) 5 10 pT(GeV) GRSV model: “G = 0”: G(Q2=1GeV2)=0.1 “G = std”: G(Q2=1GeV2)=0.4 Stat. uncertainties are on level to distinguish “std” and “0” scenarios? … Run3,4,5: PRL 93, 202002; PRD 73, 091102; hep-ex-0704.3599

    13. From soft to hard hep-ex-0704.3599 • Exponent (e-pT) describes our pion cross section data perfectly well at pT<1 GeV/c (dominated by soft physics): • =5.560.02 (GeV/c)-1 • 2/NDF=6.2/3 • Assume that exponent describes soft physics contribution also at higher pTs  soft physics contribution at pT>2 GeV/c is <10% exponential fit For G constrain use pi0 ALL data at pT>2 GeV/c

    14. From pT to xgluon • NLO pQCD: 0 pT=29 GeV/c  xgluon=0.020.3 • GRSV model: G(xgluon=0.020.3) ~ 0.6G(xgluon =01 ) • Each pT bin corresponds to a wide range in xgluon, heavily overlapping with other pT bins • These data is not much sensitive to variation of G(xgluon) within our x range • Any quantitative analysis should assume some G(xgluon) shape Log10(xgluon)

    15. From ALL to G (with GRSV) Calc. by W.Vogelsang and M.Stratmann  • “std” scenario, G(Q2=1GeV2)=0.4, is excluded by data on >3 sigma level: 2(std)2min>9 • Only exp. stat. uncertainties are included (the effect of syst. uncertainties is expected to be small in the final results) • Theoretical uncertainties are not included

    16. GSC: G(xgluon= 01) = 1 G(xgluon= 0.020.3) ~ 0 GRSV-0: G(xgluon= 01) = 0 G(xgluon= 0.020.3) ~ 0 GRSV-std: G(xgluon= 01) = 0.4 G(xgluon= 0.020.3) ~ 0.25 Extending x range is crucial! Gehrmann-Stirling models GSC: G(xgluon= 01) = 1 GRSV-0: G(xgluon= 01) = 0 GRSV-std: G(xgluon= 01) = 0.4 Current data is sensitive to G for xgluon= 0.020.3

    17. Different channels • Different systematics • Different x • gqg sensitive to G sign G: what’s next • Improve exp. (stat.) uncertainties and move to higher pT • More precise G constrain in probed x range • Probe higher x and constrain G vs x • Different s • Different x

    18. Improve exp. uncertainties Need more FOM=P4 L (stat. uncertainty ~ FOM) 0: expectations from Run-8 Higher pT measurements  probe higher x  constrain G vs x

    19. Different channels Need more FOM=P4 L (stat. uncertainty ~ FOM) • Different sensitivities of charged pions to u and d provide more sensitivity to sign of G through qg scattering • Predictions are sensitive to fragmentation functions

    20. Different channels Need more FOM=P4 L (stat. uncertainty ~ FOM) • Complementary to 0 measurements • Need  fragmentation functions • Probe G with heavy quarks • Open charm will come soon • Need more theoretical input

    21. pp  + jet PHENIX Projection • Theoretically clean (no fragmentation at LO) • Gluon Compton dominates  sensitive to sign of G • Requires substantial FOM=P4 L

    22. Different s s=62 GeV 0 cross section described by NLO pQCD within theoretical uncertainties Sensitivity of Run6 s=62 GeV data collected in one week is comparable to Run5 s=200 GeV data collected in two months, for the same xT=2pT/s s=500 GeV will give access to lower x; starts in 2009

    23. Flavor decomposition Measured through longitudinal single spin asymmetry AL in W production at s=500 GeV First data expected in 2009-2010

    24. Other measurements • Helicity correlated kT from PHENIX • May be sensitive to orbital angular momentum

    25. PHENIX Upgrades See talk by I.Nakagawa • Silicon Tracking • VTX (barrel) by 2009 • FVTX (forward) by 2011 • Electromagnetic Calorimetry • NCC by 2011 • MPC, already installed! • Muon trigger upgrade • By 2009 • Momentum selectivity in the LVL-1 trigger rapidity • G from heavy flavor, photon-tagged jets • Expanded reach in x • Flavor separation of spin asymmetries • W physics at 500GeV • Transverse Spin Physics (see talk by M.Liu)

    26. Summary • RHIC is the world’s first and the only facility which provides collisions of high energy polarized protons • Allows to directly use strongly interacting probes (parton collisions) • High s  NLO pQCD is applicable • Inclusive 0 accumulated data for ALL has reached high statistical significance to constrain G in the limited x range (~0.020.3) • G is consistent with zero • Theoretical uncertainties might be significant • Extending x coverage is crucial • Other channels from high luminosity and polarization • Different s • PHENIX upgrades strengthen its capability in nucleon spin structure study • Larger x-range and new channels (e.g. heavy flavor) • W measurements for flavor decomposition

    27. Beam polarization in Run6: P ~ 60-65% Polarization measurements in Run5: P/P~6% and (PBPY)/(PBPY)~9% Polarimetry Utilizes small angle elastic scattering in the coulomb-nuclear interference (CNI) region p or C12 p • Fast relative polarization measurements with proton-Carbon polarimeter Single measurement for a few seconds • (Relatively) slow absolute polarization measurements with polarized atomic hydrogen jet target polarimeter Used to normalize pC measurements 

    28. Backup: Rel. Lum. In PHENIX

    29. Backup: s=62 vs 200 GeV

    30. Jet 2 w/<kT>=0 Peripheral Collisions Larger Peripheral Collisions Larger Jet 2 Jet 1 Integrate over b, left with some residual kT Jet 2 w/<kT>=0 Central Collisions Smaller Jet 2 Jet 1 Partonic Orbital Angular Momentum : Beam momenta Like helicities • Partonic orbital angular momentum leads to rotation of partons correlated with the proton spin vector • This leads to different pT imbalances (pT-kicks) of jet pairs in semiclassical models • Can be measured by measuring helicity dependence of <kT2> Net pT kick

    31. Jet 2 w/<kT>=0 Jet 2 Peripheral Collisions Smaller Jet 1 Integrate over b, left with different residual kT Jet 2 w/<kT>=0 Central Collisions Larger Jet 2 Jet 1 Partonic Orbital Angular Momentum : Beam momenta Unlike helicities • Partonic orbital angular momentum leads to rotation of partons correlated with the proton spin vector • This leads to different pT imbalances (pT-kicks) of jet pairs in semiclassical models • Can be measured by measuring helicity dependence of <kT2> Net pT kick

    32. Backup: W • W production • Produced in parity violating V-A process • Chirality / helicity of quarks defined • Couples to weak charge • Flavor almost fixed xa>>xb: AL(W+) → u/u(x) - - xb>>xa: AL(W+) → d/d(x)

    33. HERMES preliminary Backup: SIDIS for G

    34. Backup: GFrom M. Stratmann

    35. Backup: from G to ALL By Marco&Werner GRSV: G(Q2=1GeV2)= 1.76  +1.89