Paper strategy discussion of Run 9 W Analysis

 Paper 1: W Asymmetry Analysis from Run 9 Data

Title: Measurement of the parity-violating longitudinal single-spin asymmetry for W^-/W⁺ boson production in polarized p+p collisions at √s=500GeV 

PA: Jan Balewski^*, Ross Corliss, Joe Seele, Justin Stevens, Hal Spinka, Bernd Surrow^* and Scott Wissink

^*Contact and writing team! 

Proposed Target Journal: Physical Review Letters

Remaining analysis tasks:

• Include 2 missing fills (10415, 10508) (7% of data)
• Take into account theoretical Jacobian peak shape for W⁺/W^- in QCD background subtraction
• Remove reconstructed tracks with inconsistent curvature using Q/p_T vs. E_T plot (see plot) 
• Write PN / PRL paper  

Abstract:

We report on the first measurement of the parity-violating longitudinal single-spin asymmetry for W^- / W⁺ boson production in longitudinally polarized p+p collisions at √s=500GeV by the STAR experiment at RHIC. The selection of W boson events is carried out in the electron decay mode at mid-rapidity. The measured asymmetries are large and consistent with theory calculations using helicity distribution functions constrained by inclusive and semi-inclusive polarized DIS measurements emphasizing the expected universality of helicity distribution functions.  

Outline of paper:

1. Introduction - Theory
2. Introduction - Measurement:
   1. A_L measurement in W production - General discussion
   2. Specific discussion for STAR
   3. RHIC Collider
   4. RHIC Polarization measurements (CNI / Polarized-Gas Jet Target)
   5. STAR detector
   6. Data taking / Trigger
      
3. W reconstruction:
   1. Algorithm / Selection criteria
   2. QCD background
   3. Charge separation
4. Spin Asymmetry measurement:
   1. Concept 
   2. Relative luminosity analysis
   3. Systematics of asymmetry measurement
5. Theory evaluations
6. Result presentation:
   1. W⁺/W^- asymmetries
   2. Comparison of asymmetry measurement and theory
7. Summary and outlook

Figures:

Note: The following figures are meant to show explicitly what type information is planned to be conveyed in each of the three figures. It should be understood that all figures are not necessarily optimized yet in terms of their graphical layout.

Figure 1: Event display of a typical reconstructed W candidate event showing a high-p_T track in the STAR TPC pointing to a calorimeter tower with large energy deposition in the STAR BEMC. 

Figure 2: Ratio of reconstructed in TPC charge sign Q and the transverse momentum p_T as a function of the reconstructed EMC lepton transverse energy, E_T.   W candidate events dominate at ET of 30 GeV, whereas QCD background events with leading charge hadron populate the lower ET range. Two well separated regions for positive and negative Q are visible referring to W⁺ and W^- candidate events, respectively.

Figure 3: Reconstructed EMC lepton transverse energy, E_T, for W⁺ (top) and W^- (bottom) events for a total luminosity of ~14pb^-1. The W candidate events are shown by the dark shaded histograms. The QCD background estimates are shown by dashed lines. The W signal events are shown by solid black circles.  

Figure 4: Parity-violating longitudinal single-spin asymmetry, A_L, for W⁺ and W^- events as a function of the leptonic rapidity, η, for a lepton transverse momentum of p_T > 25GeV/c. The W^- AL measurement is positive whereas the W⁺A_L data point is negative. The horizontal error bars indicate the bin width. The vertical error bars indicate the statistical uncertainties. Systematic scale uncertainties are shown as dark-grey bands around the respective data points. Also shown are full resummed (RHICBOS) and NLO (deFlorian / Vogelsang) calculations (See text for further details).  

Summary: 

The measurement presented in this paper establishes a new and direct way to explore the spin structure of the proton using parity-violating weak interactions in polarized p-p collisions. We report the first measurement of the parity-violating longitudinal single-spin asymmetry, A_L,  for W^- / W⁺ boson production at mid-rapidity, -1 <  η < +1, in polarized proton-proton collisions at √s=500GeV by the STAR experiment at RHIC.  The W^- / W⁺ asymmetries have been found to be of opposite sign with a statistical significance in the asymmetry difference of 3.4σ. The difference in the sign of A_L for W^- / W⁺ production at mid-rapdidty is dominated by the negative d-quark polarization and the positive u-quark polarization, respectively. The measured asymmetries agree well with NLO and resummed pQCD calculations using the DSSV set of helicity distribution functions, which have been constrained by earlier data from inclusive and semi-inclusive polarized DIS measurements. This underlines the expected universality of helicity distribution functions which are being probed at RHIC in polarized p-p collisions at much larger scales compared to previous and ongoing polarized DIS experiments.  Future planned high-statistics measurements at RHIC will focus on constraining the polarization of anti-d and anti-u quarks using an upgrade of the STAR forward tracking system to deepen our understanding of the spin structure of the QCD sea. 

Documentation: APS talks 

• Bernd Surrow: Introduction
• Ross Corliss: Luminosity analysis / Vernier scans
• Justin Stevens: Cross-section analysis
• Jan Balewski: Asymmetry analysis

Rough timeline:

• DONE: Tuesday, March 23, 2010: Discussion of PN / PRL paper in W analysis group
• Sunday, March 28, 2010:
  • Presentation of final W result (A_L incl. all above tasks to SPIN PWG)
  • Formal paper proposal presentation to SPIN PWG incl. PN / PRL paper and request to read drafts and convener presentation
• Tuesday, April 06, 2010: Convener presentation and Deadline for PN / PRL paper comments from SPIN PWG
• Thursday, April 08, 2010: Answer comments to PN / PRL paper and approval of paper by SPIN PWG followed by request of GPC formation
• Paper in GPC: April 15-May 13, 2010
• Thursday, May 13, 2010: Release of paper to collaboration
• Paper reading by collaboration: May 17-31, 2010 
• Paper to PRL: First week of June 2010