Title

Transverse Single-spin Asymmetries in Inclusive Jet and Hadron Production at Central Pseudorapidity from Transversely Polarized Proton Collisions at √s = 500 GeV

Abstract

We report the first measurements of transverse single-spin asymmetries from inclusive jet and jet+hadron production in the central pseudorapidity range from transversely polarized proton-proton collisions at √s = 500 GeV. The data were collected in 2011 from 25 pb^-1 integrated luminosity at 53% polarization with the STAR detector. The present data, in particular at lower jet transverse momenta, are sensitive to gluonic subprocesses in the partonic interaction. Presented are measurements of the inclusive-jet azimuthal transverse single-spin asymmetry, sensitive to twist-3 initial-state quark-gluon correlators; the first measurement at √s = 500 GeV of the quark-based Collins asymmetry, sensitive to transversity coupled to the polarized Collins fragmentation function; and the first measurement of the "Collins-like" asymmetry, sensitive to linearly polarized gluons.

Introduction

• Nucleon spin structure at leading twist
  • Three fundamental structures at leading twist: f, Δf, h₁
  • Tool to probe: spin asymmetries
  • f — beautifully constrained by HERA
  • Δf — coming into focus through polarized SIDIS and p+p
  • h₁ — harder to probe
• Inclusive-hadron A_N through the years
  • Very small at collinear leading-twist
  • Measured to be large at forward η with no apparent √s dependence from 20-500 GeV
  • Twist-3 and TMD frameworks developed to explain the large asymmetries
  • Recent experimental indication that forward inclusive hadron asymmetries driven by some other effect
• Twist-3 and TMD approaches to transverse single-spin asymmetries
  • Twist-3 uses initial- and final-state multiparton correlators within the collinear framework
  • Twist-3 requires one scale, well suited for high-p_T
  • TMD uses leading-twist framework without the collinear approximation
  • TMD requires two scales
  • TMD factorization proven for SIDIS and Drell-Yan/W but expected to be broken for p+p, in general
• SIDIS measurements of TMDs
  • Sivers, Collins, etc. measured by HERMES, COMPASS, JLab, and BaBar
  • BELLE measured isolated Collins FF to allow for global analysis to extract Sivers function and transversity
  • Limited kinematic reach
• Opportunities in jet+hadron
  • Probe various physical quantities through azimuthal moments of hadrons within jets
  • Extend kinematic reach of SIDIS measurements
  • Insight into factorization-breaking and universality (e.g. comparison to di-hadrons)
  • Direct sensitivity to gluonic subprocesses
  • Include figure of subprocess fraction
  • Include figure of underlying par tonic kinematics
• Discussion of A_UT for different moments
  • φ_S: azimuthal angle between polarization vector and jet scattering plane
  • φ_H: azimuthal angle of pion relative to the jet axis (in NLS coordinates)
  • sin(φ_S) gives sensitivity to Sivers effect through the Twist-3 PDF (ETQS effect)
  • sin(φ_S-φ_H) gives sensitivity to transversity + Collins FF
  • sin(φ_S-2φ_H) gives sensitivity to gluon linear polarization + Collins-like FF
  • sin(φ_S+φ_H) and sin(φ_S+2φ_H) sensitive to quark and gluon pretzelocity and Boer-Mulders
  • Figure to show angle layout and/or table to list moments and their sensitivities

Data Analysis

• RHIC
  • World’s only polarized p+p collider
  • Long. and Trans. polarization
  • Range of collision energy
  • Flip polarization state bunch-to-bunch in varied fill-patterns for very low luminosity asymmetries
  • 2011 polarization of 53%
• STAR
  • VPD — min-bias detector
  • EMCs — jet-trigger detector
  • TPC — charged-particle tracking and PID
  • Full azimuthal coverage allows full jet reconstruction and low acceptance asymmetries
• Trigger
  • VPDMB through VPD timing condition
  • JP0, JP1, JP2, AJP through EMC patch ADC
• Jet reconstruction
  • Use EMC towers + TPC tracks
  • Good track flag
  • Tracks with 12 or more TPC hits
  • N_hits/N_poss > 0.51
  • p_T-dependent DCA
  • Track p_T > 0.2
  • Track |η| < 2.5
  • Last point = 125
  • Tower status
  • Hot tower cut
  • EMC (ADC - ped) > 4 and (ADC - ped) > 3*RMS
  • EMC E_T > 0.2 GeV
  • Anti-k_T jet-finding algorithm with R = 0.5
  • E_scheme recombination
  • “Best” strategy
  • jet p_T > 5 GeV/c
• Cuts
  • Event Cuts
    • VDPMB or JP0 or JP1 or JP2 or AJP trigger in hardware and software (JP-triggers only)
    • Vertex Rank > 0
    • VPDMB: |z_vtx| < 30 cm; XJPX: |z_vtx| < 90 cm
    • Good spin bit
  • Jet Cuts
    • No jets with hadron p_T > 30 GeV/c
    • R_T < 0.94
    • ∑Track p_T > 0.5 GeV/c
    • |η_{det, jet} - 0.1| < 0.8
    • |η_jet| < 1.0
    • "Could fire" JP-trigger patch with associated p_T condition or VDPMB trigger with p_T > 6 GeV/c
      • "Could fire" means |η_jet-η_patch| < 0.6 and |φ_jet-φ_patch| < 0.6
  • Pion Cuts
    • ΔR > 0.1
    • -1.5 < n_σ(π) < 2.5
    • N_{hits dE/dx} > 5
• Embedded Monte Carlo simulations
  • PYTHIA 6.4 with “Perugia 0” tune
  • GEANT 3.21 for detector response
  • Simulated ADC mixed with real zero-bias data
  • Events reweighted to mock up k_T = 1 GeV/c
  • Simulated thresholds raised by 3 counts to mock-up 0-pedestal channels in data
  • Figure of either p_T spectrum or z/j_T comparisons
• Spin asymmetry analysis
  • Weight events with P in num. and P² in denom. using polarization-decay information from CNI group
  • “Cross-ratio” formulation
    • Divide detector into top/bottom halves to mitigate complications from competing effects+acceptance
    • Combine yields for spin-up(down)+top(bottom)-half with spin-down(up)+bottom(top)-half
  • Utilize 6 bins of φ_S and 12 of φ_{Collins(-like)}
  • Extract asymmetry trigger-by-trigger, correct for azimuthal resolution (relative to particle jet), calculate weighted average of triggers
  • Allow p₀ to float and use as check for systematics
  • χ² for systematics

Systematic Uncertainties

• parton/particle-jet matching
  • Reconstructed jets associated in η-φ space with jet at the particle-jet and parton-jet level
  • Systematic calculated as |A×(1-1/f_match)|
• p_T, z, j_T shifts
  • "Statistical" uncertainty from Monte Carlo
  • Jet energy scale uncertainties (for p_T and z)
  • Difference between ADC+3 and nominal
  • Difference between nominal and 7% track rejection
• Azimuthal resolution
  • Correction
  • Calculated by convoluting distribution of φ_reco-φ_true with sinusoid
• "Leak-through" systematic
  • Calculated using event weighting for "unpolarized" physics sample
  • Not corrected; base systematic on measured effects/uncertainties
• Trigger bias
  • Data-driven method
  • Look for signatures trigger-by-trigger and in data "pulls" 
• Pion contamination
  • Use three-Gaussian fit to n_σ(π) distribution
  • If I can resolve the issues with fitting I will apply this as a correction
• Polarization orientation
  • Basically negligible

Results and Discussion

• Inclusive jet asymmetry
  • Consistent with zero
  • Further constraints on ETQS twist-3 PDF (and gluon Sivers function)?
• Collins asymmetry
  • No large effects within statistical precision
  • Set up the question of what we learn from combining with IFFs at 500 GeV and Collins at 200 GeV
• Collins-like asymmetry
  • First-ever constraints on gluon "Collins-like" asymmetry
  • May want to allude to unpolarized gluon linear polarization modulation
• "Other" modulations
  • Check that they are zero in data
  • Perhaps they can place limits on other effects (e.g. pretzelocity)

Summary

We have reported the first measurements of transverse single-spin asymmetries from inclusive jet and jet+hadron production in the central pseudorapidity range from p^↑+p at √s = 500 GeV. The data were collected in 2011 with the STAR detector. As in previous measurements at 200 GeV, the inclusive jet asymmetry is consistent with zero. The first measurement at √s = 500 GeV of the quark-based Collins asymmetry is also found to be small and provides an opportunity for future comparison to Collins asymmetries at 200 GeV as well as those from di-hadrons at 500 GeV and 200 GeV. These comparisons may provide experimental insight into theoretical questions concerning factorization-breaking, universality, and TMD evolution. The first-ever measurement of the "Collins-like" asymmetry, sensitive to linearly polarized gluons, is found to be small and should provide the first constraints upon recent model calculations.