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Probing Proton Spin Structure with Longitudinally Polarized p + p Collisions at PHENIX

Probing Proton Spin Structure with Longitudinally Polarized p + p Collisions at PHENIX. DIFFRACTION 2008 La Londe -les- Maures September 13 th , 2008 Imran Younus. “Proton Spin Crisis”. EMC Experiment at CERN J. Ashman et al ., Nucl. Phus. B 328, 1 (1989).

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Probing Proton Spin Structure with Longitudinally Polarized p + p Collisions at PHENIX

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  1. Probing Proton Spin Structure with Longitudinally Polarized p+p Collisions at PHENIX DIFFRACTION 2008 La Londe-les-Maures September 13th, 2008 ImranYounus

  2. “Proton Spin Crisis” EMC Experiment at CERN J. Ashman et al., Nucl. Phus. B 328, 1 (1989) Violates Ellis-Jaffe sum rule. Proton-spin sum rule: G measurement – main RHIC-spin goal. • Polarized DIS: contribution of quarks to proton spin is amazingly smallDS  0.1- 0.3 • Other candidate to carry proton spin – Gluons • Orbital angular momentum?  = U + D + S G = gluon polarization LZ = orbital angular momentum Probing Proton Spin Structure with Longitudinally Polarized p+p Collisions at PHENIX Imran Younus

  3. Parton Distribution Functions (PDF) Quark distributions (q= u,d,s…) • Unpolarized dist. • Helicity dist. • Transversity • Gluon distributions • Unpolarized dist. • Polarized dist. q(x,Q2) = = g(x,Q2) = Dg(x,Q2) = Dq(x,Q2) = Universality! PDFs extracted from the data in one process, can be used to make predictions for other processes. dq(x,Q2) = • PDFs - probability of scattering off of a parton carrying a particular fraction of the proton’s momentum. • Polarized PDFs - the difference in probability between scattering off of a parton with one spin state vs. the other. Still as a function of the momentum fraction. Probing Proton Spin Structure with Longitudinally Polarized p+p Collisions at PHENIX Imran Younus

  4. Pre-RHIC Era Situation Asymmetry Analysis Collaboration M. Hirai, S. Kumano and N. Saito, PRD (2004) • Valence Dist’s - determined well. • Sea Dist’ - poorly constrained. • Gluon Dist’ can be either >0, =0, <0. Gluon helicity distribution (g(x)) remains poorly constrained. Probing Proton Spin Structure with Longitudinally Polarized p+p Collisions at PHENIX Imran Younus

  5. Along came Polarized p+p Collider • Strongly Interacting Probes • Probes gluons directly • Higher s  clean pQCD interpretation • Explores quark and anti-quark polarizations through W production DIS • Polarized Gluon Distribution Measurements (G(x)): • Use a variety of probes. • Different probes – different systematics. • Use different energies s – Access to different gluon momentum fraction x. p+p Probing Proton Spin Structure with Longitudinally Polarized p+p Collisions at PHENIX Imran Younus

  6. Various Channels to Extract g(x) Probing Proton Spin Structure with Longitudinally Polarized p+p Collisions at PHENIX Imran Younus

  7. RHIC as Polarized Proton Collider RHIC pC Polarimeters Absolute Polarimeter (H jet) BRAHMS & PP2PP PHOBOS Siberian Snakes Siberian Snakes PHENIX STAR Spin Rotators (longitudinal polarization) Spin flipper Spin Rotators (longitudinal polarization) Solenoid Partial Siberian Snake Pol. H- Source LINAC BOOSTER Helical Partial Siberian Snake AGS 200 MeV Polarimeter AGS Internal Polarimeter Rf Dipole AGS pC Polarimeters Strong AGS Snake Lmax = 2 x 1032 s-1 cm-1 70% polarization 50 < s < 500 GeV Probing Proton Spin Structure with Longitudinally Polarized p+p Collisions at PHENIX Imran Younus

  8. PHENIX Detector High resolution at the cost of acceptance, High rate capable DAQ, Excellent trigger capability for rare events. Central Arms: || < 0.35,  = 2900 EM Calorimeter, photon trigger, , o, o detection. Ring Imaging Cherenkov Detector, e Drift Chamber, charged hadrons. Muon Arms: 1.2 < || < 2.4 Panels of Iarocci tubes/absorbers, Muon ID/trigger, Cathode Strip Chambers, Muon tracking, J/,  Global Detector: Beam-Beam Counter (Quartz Cherenkov det.) 3.0 < || < 3.9 Zero Degree Calorimeter. || > 6.6 Collision trigger, Collision vertex characterization, Relative luminosity, Local Polarimetry. Probing Proton Spin Structure with Longitudinally Polarized p+p Collisions at PHENIX Imran Younus

  9. Beam Polarization CNI Polarimeter: Measures Left-Right Asymmetry in elastic p+C collisions. Provides fast relative measurement of beam polarization. Hydrogen Jet Polarimeter: Measures Left-Right Asymmetry in elastic p+p collision. Provides absolute measurement of beam polarization. At PHENIX Interaction Point: Bunch spin configuration alternates every 106 ns. 111 Bunches max. Transverse to Longitudinal at IP. Spin Pattern: Blue: + +  Yellow: +  +  Provides all possible combinations. Greatly reduces possibilities of false asymmetries and systematics due to time dependent det. efficiencies. Probing Proton Spin Structure with Longitudinally Polarized p+p Collisions at PHENIX Imran Younus

  10. Luminosity and Polarization Probing Proton Spin Structure with Longitudinally Polarized p+p Collisions at PHENIX Imran Younus

  11. QCD Factorization f1p(x1) One can write cross-section as convolution of PDFs and partonic sub process cross-section, e.g.,  production,  Df p1 f1 f f2 Unpolarized case: p2 X f2p(x2) Polarized case: Unpolarized, polarized parton distribution functions, determined from experimental data. Partonic level hard-scattering cross section. Calculated by perturbative QCD. Process dependant. Fragmentation function, determined from experiment. Probing Proton Spin Structure with Longitudinally Polarized p+p Collisions at PHENIX Imran Younus

  12. Unpolarized Cross-Sections in p+p Measured un-polarized cross-sections at s = 200 GeV at RHIC are well described by NLO pQCD calculations. Necessary Confirmation that pQCD can be used successfully at RHIC to extract PDFs. pp  o X ; PRD76, 051106 (2007) pp  X ; PRL 98, 012002 (2007) Probing Proton Spin Structure with Longitudinally Polarized p+p Collisions at PHENIX Imran Younus

  13. Double Longitudinal Spin Asymmetries + + f1p(x1)  Df p1 f1 f f2 p2 X f2p(x2) ALL provides access to G. At LO, pQCD give nonzero aLL for all sub processes. Probing Proton Spin Structure with Longitudinally Polarized p+p Collisions at PHENIX ImranYounus

  14. Measuring ALL P1,P2– Polarization of the colliding beams. N ++(N +) – experimental yields for same (opposite) helicity collisions. R – Relative luminosity, determined at PHENIX using Minimum Bias trigger counts. R is measured using BBC and ZDC. Two independent measurements are compared to determine uncertainty on R. Preliminary values for Run6. Probing Proton Spin Structure with Longitudinally Polarized p+p Collisions at PHENIX Imran Younus

  15. Local Polarimetry At PHENIX • Left-Right Asymmetry of Neutrons in transversely polarized p+p was discovered at IP12. • Use neutron asymmetry to estimate longitudinal and transverse components of polarization. • ZDC/SMD make local polarimetry measurement at PHENIX. • Allows us to measure transverse component of the longitudinally polarized beam. Transverse Pol. For Run 5, the fraction of transverse component is (PT/P)B= 0.10  0.02 (PT/P)Y= 0.14  0.02 PTBPTY/ PBPY <(PT/P)B(PT/P)Y =0.014  0.003 PBPY = 0.24 Longitudinal Pol. Probing Proton Spin Structure with Longitudinally Polarized p+p Collisions at PHENIX Imran Younus

  16. ALL(o) at s = 200 GeV PHENIX measures odecays using a highly segmented (    0.01  0.01) electromagnetic calorimeter (EMCal), at mid rapidity. Fractional contribution to pp X at s=200 GeV at mid-rapidity W. Vogelsang et al. o + BG region:±25 MeV around opeak BG region: Two 50 MeV regions Background contribution less than 10% for pT > 3.5 GeV. ALL(BG) consistent with zero for both Run5 and Run6. Probing Proton Spin Structure with Longitudinally Polarized p+p Collisions at PHENIX ImranYounus

  17. ALL(o) Results From Run5 and Run6 • NLO pQCD: • pT (o) = 29 GeV/c xgluon= 0.020.3 • This range represents 60% of the full integral of G(x) (GRSV). • Each pT bin corresponds to a wide range in xgluon, heavily overlapping with other pT bins. • The data is not very sensitive to variation of G(xgluon) within our x range. • Any quantitative analysis should assume some G(xgluon) shape. Run3,4,5: PRL 93, 202002; PRD 73, 091102; PRD 76, 051106 (2007) GRSV models: “G = 0”: G(Q2=1GeV2) = 0.1 “G = std”: G(Q2=1GeV2) = 0.4 GRSV std fits better to DIS data (as of 2000). Log10(xgluon) Probing Proton Spin Structure with Longitudinally Polarized p+p Collisions at PHENIX Imran Younus

  18. 2 Test Based on 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 • G = +G, Gare rejected. • The results are more consistent with G = 0. • 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. Probing Proton Spin Structure with Longitudinally Polarized p+p Collisions at PHENIX Imran Younus

  19. Accessing Different xgluon Range • The most likely xgluon for PHENIX o data in each pT point is xT /0.7, xT = 2pT /s • Can access higher xgluon with higher xT. • s = 62.4 GeV gives access to higher xgluon. • s = 500 GeV gives access to lower xgluon. • At fixed xT, the cross section is almost 2 orders of magnitude higher at 62.4 GeV than at 200 GeV. xT = 2pT /s Probing Proton Spin Structure with Longitudinally Polarized p+p Collisions at PHENIX Imran Younus

  20. Cross section and ALL(o) ats=62.4GeV s=62.4 GeV o cross section described well by NLO/NLL pQCD calculations within theoretical uncertainties. Sensitivity of Run6 s=62.4 GeV data collected in one week is comparable to Run5 s=200 GeV data collected in two months, for the same xT = 2pT/s. D. de Florian, W. Vogelsang, and F. Wagner Phys.Rev.D76,094021(2007), arXiv:0708.3060 Probing Proton Spin Structure with Longitudinally Polarized p+p Collisions at PHENIX Imran Younus

  21. Global Analysis Global NLO QCD analysis of DIS, SIDIS, and RHIC data in terms of the parton helicity distributions. Ref. D. de Florian et al. arXiv:0804.0422v1 [hep-ph] Includes PHENIX Run5 200 GeV, Run6 200 GeV preliminary and Run6 62.4 GeV preliminary results Also, STAR ALL(jets) data is included. “.. RHIC data set significant constraints on the gluon helicity distribution, providing evidence that g(x,Q2) is small in the accessible range of momentum fraction.” Probing Proton Spin Structure with Longitudinally Polarized p+p Collisions at PHENIX Imran Younus

  22. ALL() at s = 200 GeV • No fragmentation functions (FFs) in the literature! • Preliminary extraction of fragmentation functions using e+e- data and ppX data. • Extraction uses method from DSS (deFlorian, Sassot, Stratmann, PRD75, 2007). • Produced ~1/2 as much as πo , needs more statistics. PHENIX Run-05 Preliminary s = 200 GeV  has slightly enhanced sensitivity to qg (when compared to o) ALL() excludes GRSV min and max scenarios. Probing Proton Spin Structure with Longitudinally Polarized p+p Collisions at PHENIX Imran Younus

  23. ALL(+-) at s = 200 GeV • Produced in large quantities. • No real trigger at PHENIX. • Charged pions above 4.7 GeV identified with RICH. • At higher pT, qg interactions become dominant: qg term. • Different sensitivities of charged pions to u and d (combined with opposite signs of u and d) provide more sensitivity to sign of G through qg scattering. • Needs more statistics. W. Vogelsang et al. Probing Proton Spin Structure with Longitudinally Polarized p+p Collisions at PHENIX Imran Younus

  24. ALL() • p+p + jet • Theoretically cleanest mode for extracting G. No fragmentation function at LO. • Gluon Compton Dominates. Sensitive to sign and magnitude of G. • Small (15%) contamination from annihilation. • NLO pQCD describes cross-section well  can be used to interpret ALL(). • Rare probe, needs substantial statistics. Fractional contribution of different channels to the cross section. p1 x1 x2 p2 Probing Proton Spin Structure with Longitudinally Polarized p+p Collisions at PHENIX Imran Younus

  25. ALL() Probing Proton Spin Structure with Longitudinally Polarized p+p Collisions at PHENIX Imran Younus

  26. Jet kT Asymmetry Net transverse momentum of a dilepton or dijet pair kT jet pT • intrinsic – fermi motion of the confined quarks or gluons • NLO – radiation of an initial state or final state hard gluon • soft – Gaussian like distribution observed as pTpair → 0; resummation. Another Possibility: • Spin-Correlated transverse momentum – partonic orbital angular momentum. • Assume other contributions have no spin dependence… • We can perhaps measure using jet kT spin asymmetry. • Possible Effect in double longitudinal spin • Idea proposed for the Drell-Yan process by M.Ta-chung et. al. (Phys. Rev. D40 p.769, 1989) • Same idea for jets suggested by Douglas Fields. Probing Proton Spin Structure with Longitudinally Polarized p+p Collisions at PHENIX Imran Younus

  27. Measuring jet kT– di-hadron correlation o- h±azimuthal correlation Trigger on a particle, e.g. o, with transverse momentum pTt. Measure azimuthal angular distribution w.r.t. the azimuth of associated (charged) particle with transverse momentum pTa. The strong same and awayside peaks in p+p collisions indicate di-jet origin from hard scattering partons. Probing Proton Spin Structure with Longitudinally Polarized p+p Collisions at PHENIX Imran Younus

  28. Measuring jet kT– di-hadron correlation Intra-jet pairs angular width : near jT Inter-jet pairs angular width : far jT ^ ^ pTa pTt • Zero kT pTa pTt jTy Probing Proton Spin Structure with Longitudinally Polarized p+p Collisions at PHENIX Imran Younus

  29. Measuring jet kT– di-hadron correlation Intra-jet pairs angular width : near jT Inter-jet pairs angular width : far jT  kT pTa pTt ^ ^ pTa pTt jTy • Zero kT • Non-Zero kT kTy Probing Proton Spin Structure with Longitudinally Polarized p+p Collisions at PHENIX Imran Younus

  30. Jet Kinematics pTa pTt ^ ^ pTa pTt pout jTy kTy pTt For details, see PRD 74, 072002 (2006) Probing Proton Spin Structure with Longitudinally Polarized p+p Collisions at PHENIX Imran Younus

  31. Fitting the Correlation Probing Proton Spin Structure with Longitudinally Polarized p+p Collisions at PHENIX Imran Younus

  32. kT Results for Unpolarized Case jT2 = 585 ± 6(stat) ± 15(syst) MeV kT2 = 2.68 ± 0.07(stat) ± 0.34 (syst) GeV Phys. Rev. D 74, 072002 (2006) Probing Proton Spin Structure with Longitudinally Polarized p+p Collisions at PHENIX Imran Younus

  33. Helicity Sorted kT • Take the difference Like – Unlike helicities • Normalized by beam polarizations • j2T asymmetry (32 ± 24 MeV out of ~580 MeV for unpolarized) • k2Tasymmetry (672 ± 387 MeV out of ~2.7 GeV for unpolarized) • Final results soon to be published. • Final uncertainty on kT asymmetry is reduced by almost a factor of 2, and the average is consistent with zero within one sigma. Probing Proton Spin Structure with Longitudinally Polarized p+p Collisions at PHENIX Imran Younus

  34. ALL(“quasi – jets”) at s=200GeV • ALL of jet-like clusters. Even with a limited acceptance in PHENIX central arm, we can capture most of a jet. • Tag one high energy photon and sum energy of all nearby photons & charge hadrons. No jet trigger at PHENIX. • Not statistically separate from o and . Definition of "pT cone“ Sum of pT measured by EMCal & Tracker with Real pT of jet is evaluated by modified PYTHIA. Probing Proton Spin Structure with Longitudinally Polarized p+p Collisions at PHENIX Imran Younus

  35. ALL(J/) at s=200GeV Heavy quark production - Gluon Fusion dominates at LO. PYTHIA estimate: Model dependant. Rare probe, needs substantial statistics. Probing Proton Spin Structure with Longitudinally Polarized p+p Collisions at PHENIX Imran Younus

  36. Summary • RHIC is a unique facility, which provides high energy polarized proton-proton collsions. • Allows to directly use strongly interacting probes (parton interactions) to study parton densities. • High s make NLO pQCD applicable. • Inclusive o data for ALL has reached high statistical significance to constrain G in limited xgluon range (0.02 – 0.30). • G is consistent with zero. • Global Analysis of many channels together with DIS, SIDIS data will give us a more accurate picture of G(x). • Need more statistics to explore different rare channels to study different gluon kinematics. • Di-hadron correlations provide an analysis tool to measure jet kTin p+p collisions. • Measuring jet kT in longitudinally polarized collisions may provide access to spin-dependant coherent component of intrinsic kT , which one expects due to the orbital motion of partons inside the proton. • Future planned and ongoing detector upgrades (silicon vertex detector, forward muon trigger upgrade, etc..) will provide access to even rarer probes. • W measurements for flavor decomposition. • DY measurements for possible access to orbital angular momentum. Probing Proton Spin Structure with Longitudinally Polarized p+p Collisions at PHENIX Imran Younus

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