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Jet Production in Polarized pp Collisions at RHIC

This presentation outlines the production of jets in polarized proton collisions at RHIC, focusing on determining the gluon polarization distribution. It discusses results from 2005 and 2006, future directions, and the quest to uncover the origin of proton spin.

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Jet Production in Polarized pp Collisions at RHIC

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  1. STAR Jet Production in Polarized pp Collisions at RHIC Carl A. Gagliardi Texas A&M University for the Collaboration Outline • Introduction • 2005 results • 2006 results • Looking ahead

  2. Origin of the proton spin? Polarized DIS: 0.2~0.3 Poorly Constrained Three recent fits of equal quality: • ΔG = 0.13 ± 0.16 • ΔG ~ 0.006 • ΔG = -0.20 ± 0.41 all at Q2 = 1 GeV2 Leader et al, PRD 75, 074027 • A primary goal of the RHIC Spin program is to determine the gluon polarization distribution

  3. f: polarized parton distribution functions 10 20 30 pT(GeV) Polarized pp collisions at RHIC Partonic fractions in jet production at 200 GeV For most RHIC kinematics, gg and qg dominate, making ALL for jets sensitive to gluon polarization. 0

  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 world’s first polarized hadron collider • Spin varies from rf bucket to rf bucket (9.4 MHz) • Spin pattern changes from fill to fill • Spin rotators provide choice of spin orientation • “Billions” of spin reversals during a fill with little if any depolarization “Thousands of millions”

  5. STAR detector E-M Calorimeter Projection           Chamber Time of    Flight

  6. Jet reconstruction in STAR Data jets MC jets • Midpoint cone algorithm • (Adapted from Tevatron II - hep-ex/0005012) • Seed energy = 0.5 GeV • Cone radius in - • R=0.4 with 0.2 <  < 0.8 (2005) • R=0.7 with -0.7 <  < 0.9(2006) • Splitting/merging fraction f=0.5 GEANT Detector Particle PYTHIA Use PYTHIA + GEANT to quantify detector response

  7. STAR Jet production at RHIC • Jet structure at 200 GeV is well understood • Mid-rapidity jet cross section is well described by pQCD over 7 orders of magnitude PRL 97, 252001

  8. 2005 inclusive jets ALL systematics • Reconstruction bias: combination of steeply falling spectrum and ~25% jet ET resolution • Trigger bias: non-uniform sampling of gg, qg, and qq processes • Will illustrate impact on ALL with 2006 data Neutral Energy Fraction

  9. STAR 2005 inclusive jets ALL Model calcs from: Jager et al, PRD 70, 034010 • Comparison to predictions within the GRSV framework with various input values of ΔG demonstrate sensitivity hep-ex arXiv:0710.2048 200 GeV 0.2 < η < 0.8

  10. GRSV DIS STAR Limits on ΔG from 2005 jet results hep-ex arXiv:0710.2048 • Significant new constraints on ΔG when compared to predictions derived from one global fit to DIS data

  11. Other global analyses • There are many other global analyses of the polarized DIS data xΔg(x) at Q2 = 10 GeV2

  12. Additional ALL predictions for 2005 • Many predictions for ALL vs. pT • Is the sensitivity unique to GRSV?

  13. STAR Comparison to other global analyses • The STAR data exclude a broad range of models that have ΔG larger than that in GRSV-std • The counterexample is GS-C, which is negative at large x, has a node near x ~ 0.1, and has a large first moment at small x

  14. 2006 data • Improved figure of merit • Luminosity : 2  4.7 pb-1 • Polarization: 50%  60% (online polarization) • Barrel EMC  coverage: [0,1]  [-1,1] • In addition • Jet cone radius: 0.4  0.7 • -0.7 < jet axis < 0.9 • Neutral energy fraction < 0.85 • Increased trigger thresholds • Inclusion of Endcap EMC towers • Improved tracking at large || • Shielding from backgrounds

  15. Jet reconstruction and trigger bias for 2006 data Step 1: Use PYTHIA+GEANT to estimate the pT shift from detector jets to particle jets Step 2: Simulate the difference in ALL between particle and detector jets for various gluon polarization scenarios Particle jets Shifted detector jets Detector jets • Maximum deviation determines ALL systematic • Confidence level calculations take account of any residual

  16. STAR 2006 inclusive jets ALL Model curves calculated with cone radius 0.7 and -0.7 <  < 0.9 • Statistical uncertainties are 3-4 times smaller than in ’05 data for pT > 13 GeV/c 200 GeV -0.7 <  < 0.9

  17. GRSV DIS STAR Limits on ΔG from 2006 jet results • Within the GRSV framework: • GRSV-std excluded with 99% CL • ΔG < -0.7 excluded with 90% CL

  18. Looking beyond inclusive ALL measurements • Inclusive ALL measurements at fixed pT average over a broad x range. • Need a global analysis to determine the implications • Can hide considerable structure if Δg(x) has a node

  19. DSSV – first global analysis with polarized jets de Florian et al., arXiv:0804.0422 [hep-ph] • The first global NLO analysis to include inclusive DIS, SIDIS, and RHIC pp data on an equal footing • Finds a node in the gluon distribution near x ~ 0.1, but with the opposite phase from GS-C

  20. STAR The next few years: di-jets and Δg(x) 2005 preliminary di-jet distributions • Di-jets provide direct access to parton kinematics at LO Mass Rapidity |cos(*)| Ratio

  21. Projected di-jet sensitivity for the 2009 run x1 ~ 0.33 x2 ~ 0.044 Rel FOM ~ 70% • Assumes 50 pb-1 of 200 GeV pp collisions with 60% polarization GS-C x1 ~ 0.45 x2 ~ 0.16 x1 ~ 0.27 x2 ~ 0.27

  22. Conclusions • STAR inclusive jet ALL measurements have excluded extreme gluon polarization scenarios • Over the next few years, STARdi-jet measurements will provide direct information about Δg(x) • STAR will also obtain complementary information about Δg(x) from γ + jet • Stay tuned!

  23. Double longitudinal spin asymmetry, ALL N : Spin dependent yields (# of reconstructed jets) P : Beam polarization (measured by RHIC Polarimeter) R : Relative luminosities between different spin states

  24. STAR detectors for jet measurements • Time Projection Chamber ||<1.4tracking • Barrel EM Calorimeter ||<1triggering & calorimetry • Endcap EM Calorimeter 1.09<<2 triggering & calorimetry • Beam-Beam Counters 3.4<||<5triggering, luminosity, local polarimetry

  25. Other global analyses xΔg(x) at Q2 = 10 GeV2

  26. Additional ALL predictions for 2005 ALL

  27. Estimating the non-jet background Data Monte Carlo Charged Energy Fraction Charged Energy Fraction • Non-jet backgrounds stand out in the data as single-jet events that have a small charged energy fraction • Isolate the background in the ratio of mono-jet to di-jet events vs. charged energy fraction Data Charged Energy Fraction

  28. ALL vs. run index for 2006 data STAR Preliminary

  29. False asymmetries in 2006 data STAR Preliminary STAR Preliminary • Longitudinal single-spin (parity-violating) asymmetries are consistent with zero, as expected at this precision

  30. Inclusive jet ALL for fixed values of ΔG • Calculations by Stratmann and Vogelsang within the GRSV framework

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