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Recent STAR results and prospects of W +/- boson production in polarized p+p collisions at RHIC

Recent STAR results and prospects of W +/- boson production in polarized p+p collisions at RHIC. Joe Seele (MIT) for the Collaboration SPIN 2010. The Spin Puzzle. Its decomposition is not well understood, especially the sea… needs data. The proton is viewed as being a “bag” of

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Recent STAR results and prospects of W +/- boson production in polarized p+p collisions at RHIC

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  1. Recent STAR results and prospects of W+/- boson production in polarized p+p collisions at RHIC Joe Seele (MIT) for the Collaboration SPIN 2010 J. Seele (MIT) for the STAR Collaboration - SPIN 2010

  2. The Spin Puzzle Its decomposition is not well understood, especially the sea… needs data The proton is viewed as being a “bag” of bound quarks and gluons interacting via QCD Spins + orbital angular momentum need to give the observed spin 1/2 of proton Being measured at RHIC Fairly well measured only ~30% of spin J. Seele (MIT) for the STAR Collaboration - SPIN 2010

  3. Flavor Asymmetry in the Sea arXiv:0904.3821 • E866 results are qualitatively consistent with pion cloud models, instanton models, chiral quark soliton models, etc. • Pauli blocking should contribute to the observed signal, but how much is currently debated • Non-perturbative processes may be needed in generating the sea J. Seele (MIT) for the STAR Collaboration - SPIN 2010

  4. Probing the Sea through Ws • Detect Ws through e+ ande- decay channels • V-A coupling leads to perfect spin separation • Neutrino helicity gives preferred direction in decay Measure parity violating single helicity asymmetry AL (Helicity flip in one beam while averaging over the other) J. Seele (MIT) for the STAR Collaboration - SPIN 2010

  5. RHIC and STAR A polarized proton-proton collider to study spin in QCD up to sqrt(s)=500 GeV BEMC (barrel) TPC (time projection chamber) EEMC (endcap) J. Seele (MIT) for the STAR Collaboration - SPIN 2010

  6. W Algo: Motivation What we want to accept What we want to reject Look for the electron-type events with no energy/momentum on the away side J. Seele (MIT) for the STAR Collaboration - SPIN 2010

  7. W Algo: Lepton Isolation • Lepton Isolation Cuts: • Require TPC track with pT > 10 GeV • Extrapolate track to Barrel Calorimeter • Require highest 2x2 cluster around pointed tower sum ET > 15 GeV • Require excess ET in 4x4 cluster < 5% • Match track to 2x2 cluster position • Get charge sign of lepton 4x4 2x2 TPC track extrapolated to BTOW tower grid Signal region Jets TPC Q+ reco TPC Q-reco J. Seele (MIT) for the STAR Collaboration - SPIN 2010

  8. W Algo: Suppress QCD Background Signal Dijets • Suppress jets with leading hadron • Near side jet-cone veto • Suppress di-jets and multi-jet events • Away side pT sum veto • Require an imbalance in pT of the lepton cluster and any jets reconstructed outside the near side jet cone nearCone delR=0.7 Transverse plane view sum only jets electron ptBalance 4pi - nearCone Signal region Jets Cluster ET / 0.7 Cone ET J. Seele (MIT) for the STAR Collaboration - SPIN 2010

  9. The Raw Signal STAR recorded 13.7pb-1 in the run9 500 GeV running period J. Seele (MIT) for the STAR Collaboration - SPIN 2010

  10. Extracting the W Signal Total Background 50 70 Run analysis with EEMC in veto cuts Run analysis without EEMC in veto cuts Subtract two raw signals Signal EMC Cluster ET (GeV) 50 Dijets EMC Cluster ET (GeV) 50 Run 9 Data Missing Endcap Vetoed QCD Background Normalized at ET<19 GeV Data Driven QCD Bkgd. MC Normalized to L=13.7 pb-1 PYTHIA+GEANT MC J. Seele (MIT) for the STAR Collaboration - SPIN 2010

  11. STAR W Signals J. Seele (MIT) for the STAR Collaboration - SPIN 2010

  12. Data/MC Shape Comparison Monte-Carlo is full PYTHIA+GEANT simulation of W→e+ν events at 500 GeV J. Seele (MIT) for the STAR Collaboration - SPIN 2010

  13. First STAR W Cross Section Run 9 STAR Preliminary (p+p 500 GeV) J. Seele (MIT) for the STAR Collaboration - SPIN 2010

  14. First STAR W AL Then spin sorting, we calculate the AL STAR Run 9 Result Newly released at arXiv:1009.0326 Lepton pseudorapidity J. Seele (MIT) for the STAR Collaboration - SPIN 2010

  15. Future W AL Measurements J. Seele (MIT) for the STAR Collaboration - SPIN 2010

  16. Future Ws at mid-rapidity STAR has shown the capability to detect the W at mid-rapidity. With the expected 300pb-1 for the 500 GeV program STAR will provide strong constraints on the polarized sea pdfs using the mid-rapidity data J. Seele (MIT) for the STAR Collaboration - SPIN 2010

  17. Future Ws at forward rapidity At forward/backward rapidity the ALs become more sensitive to a single quark flavor The expected uncertainties for the 500 GeV program are shown to the right for the endcap acceptance Measurements rely on the planned Forward GEM Tracker upgrade J. Seele (MIT) for the STAR Collaboration - SPIN 2010

  18. Conclusions • Measurements of the W in polarized p+p collisions provide needed information about the polarized sea in the proton. • STAR has shown a first measurement of the cross section and single helicity asymmetry of the W signal in polarized p+p collisions at sqrt(s)=500 GeV which agree with expectations. • Planned STAR measurements will provide strong constraints on the polarized sea of the proton. J. Seele (MIT) for the STAR Collaboration - SPIN 2010

  19. Backup Slides J. Seele (MIT) for the STAR Collaboration - SPIN 2010

  20. 2009 500 GeV Data Set STAR recorded 13.7pb-1 in the 500 GeV running period leptons from Ws should appear here Required a high tower trigger (ET > 7.3 GeV) and a high ET 2x2 clusters (ET > 13 GeV) 7.3 GeV 13 GeV EMC tower ADC J. Seele (MIT) for the STAR Collaboration - SPIN 2010

  21. Predictions for AL STAR Barrel EMC LO interpretation for x1=x2 J. Seele (MIT) for the STAR Collaboration - SPIN 2010

  22. Charge Separation at High pT shown electron & positron PT= 5 GeV/c vertex infinite PT +/- distance D ~ 1/PT PT=5 GeV/c : D~15 cm PT=40 GeV/c : D ~2 cm 200 cm of tracking TPC Q+ reco TPC Q-reco J. Seele (MIT) for the STAR Collaboration - SPIN 2010

  23. Event Rejection Run 9 Data Pythia+Geant W+ MC Track pT > 10 GeV Lepton Isolation Cut Near Side pT Veto Away Side pT & ptBalance Veto Ws J. Seele (MIT) for the STAR Collaboration - SPIN 2010

  24. Example Lego Plots W event Dijet event π π J. Seele (MIT) for the STAR Collaboration - SPIN 2010

  25. Cross Section Formula Efficiencies Calculated from full PYTHIA + GEANT simulations J. Seele (MIT) for the STAR Collaboration - SPIN 2010

  26. Cross Section Uncertainties J. Seele (MIT) for the STAR Collaboration - SPIN 2010 • W Reconstruction Systematic • Track Reconstruction: 15-20% • Vertex Reconstruction: 4% • Energy Scale: < 1% • Normalization/Luminosity Systematic • Vernier scan absolute cross section: 23% • Background Systematic • Vary data driven QCD background shape and normalization region

  27. Helicity of beams at STAR spin rotator R R R R Blue helicity: - - + + ... Yellow helicity: + - + - ... + helicity - helicity STAR sees 4 helicity states STAR runs 4 parallel measurements RHIC measured polarization Run 9 @ 2x250 GeV Pol yellow 0.40 Pol blue 0.38 syst. pol (blue+yellow)=9.2% J. Seele (MIT) for the STAR Collaboration - SPIN 2010

  28. Monitor spin dependent luminosity 4x4 relative luminosity monitor 2x2 Counts Counts pi0’s jets Ws B + Y - B + Y + B - Y - B - Y + helicities of beams colliding at STAR relative luminosities of 4 states monitored to ~1% TPC track extrapolated to BTOW tower grid J. Seele (MIT) for the STAR Collaboration - SPIN 2010

  29. Up quark pol. seen by “naked eye” W+ yield integrated over |eta|<1 negative helicity unpolarized Counts negative ALP statistically significant Null test passed B + Y - B + Y + B - Y - B - Y + J. Seele (MIT) for the STAR Collaboration - SPIN 2010

  30. Spin dependent xsec for long. Pol. AN x residual transverse pol Q neglected because STAR is phi-symmetric yields integrated over |eta|<1 P-V AL ( the goal ) ALL J. Seele (MIT) for the STAR Collaboration - SPIN 2010

  31. Long. spin asymmetries for Ws STAR has measured 4 independent yields for the physics process selected 3 asymmetries are independent ( 6 were investigated) yields integrated over |eta|<1 AL ALL Null test J. Seele (MIT) for the STAR Collaboration - SPIN 2010

  32. 6 measured spin asymmetries for Ws W+ W- ALL P-V AL Null test Null test P-V AL ALL Physics asymmetries corrected for unpolarized background J. Seele (MIT) for the STAR Collaboration - SPIN 2010

  33. Systematic errors for AL Following effects were considered and their contribution set to zero * dilution of AL due to swap of W+/W- charge - the Q/PT cut prevents it * * Full list of accounted systematic errors in Run 9 J. Seele (MIT) for the STAR Collaboration - SPIN 2010

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