Run 11+12 W AL Paper Proposal Webpage






PAs: Jan Balewski, Justin Stevens, and Jinlong Zhang

Proposed Target Journal: Physical Review Letters (published)

Table with results (CSV format,html format

Paper Drafts:
     Version 0.0
     Version 0.1 
     Version 1.0
     Version 1.2
     Version 2.0
     Version 2.1
     Version 2.2
     Version 2.2.1 (same as Version 2.2 + STAR author list)
     Version 2.4 
     Version 2.5
     Version 2.6 (submitted to PRL)
     Version 3.0 (Referee response for GPC review)
     Version 3.1 (Referee response for Collaboration review) and Difference beteween Version 2.6 and 3.1
     Version 3.2 (Second PRL referee response) and Difference between Version 3.1 and 3.2

STAR Notes
     #590 : Profile Likelihood Method
     #597 : Asymmetry Analysis Note

Analysis code: CVS repository


Spin PWG Review:

GPC Review:

  • Hal Response 1 and 2
  • Anselm Response 1 and 2
  • Zhenyu Response 1 and 2

Collaboration Review:

PRL Referee Review:

Previous STAR W papers: 2009 Asymmetry PRL and Cross section PRD


We report measurements of single and double spin asymmetries for $W^{\pm}$ and $Z/\gamma^*$ boson production in longitudinally polarized $p+p$ collisions at $\sqrt{s} = 510$ GeV by the STAR experiment at RHIC. The asymmetries for $W^{\pm}$ were measured as a function of the decay lepton pseudorapidity, which provides a theoretically clean probe of the proton's polarized quark distributions at the scale of the $W$ mass. The results are compared to theoretical predictions, constrained by recent polarized DIS measurements, and show a preference for a sizable, positive up antiquark polarization in the range $0.05<x<0.2$.

Figure 1:

Caption: (color online) $E_T^e$ distribution of  $W^-$ (top) and $W^+$ (bottom) candidate events (black), background contributions, and sum of backgrounds and $W \rightarrow e\nu$ MC signal (red-dashed).

Figure 2:

Caption: (color online) (a) Signed $P_{T}$-balance distribution for $e^\pm$ candidates reconstructed in the EEMC and (b) distribution of the product of the TPC reconstructed charge sign and $E_T/p_T$.

Figure 3:

Caption: (color online) Distribution of the invariant mass of $Z/\gamma^* \rightarrow e^+ e^-$ candidate events.  The $Z/\gamma^* \rightarrow e^+ e^-$ MC distribution (filled histogram) is shown for comparison.

Figure 4:

Caption: (color online) Longitudinal single-spin asymmetry $A_L$ for $W^\pm$ production as a function of lepton pseudorapidity, $\eta_e$, in comparison to theory predictions (see text for details).

Figure 5:

Caption: (color online) Longitudinal double-spin asymmetry $A_{LL}$ for $W^\pm$ production as a function of lepton pseudorapidity, $|\eta_e|$, in comparison to theory predictions (see text for details).


In summary, we report new measurements of the parity-violating single-spin asymmetry, $A_L$, and parity-conserving double-spin asymmetry, $A_{LL}$, for $W^\pm$ production as well as a first measurement of $A_L$ for $Z/\gamma^*$ production in longitudinally polarized proton collisions by the STAR experiment at RHIC.  The dependence of $A_L^{W^\pm}$ on the decay lepton pseudorapidity probes the flavor-separated quark and antiquark helicity-dependent PDFs at the $W$ mass scale.  A comparison to theoretical predictions based on different helicity-dependent PDFs suggests a positive up antiquark polarization in the range $0.05<x<0.2$.  The inclusion of this measurement in global analyses of RHIC and DIS data should significantly improve the determination of the polarization of up and down antiquarks in the proton and provide new input on the flavor symmetry of the proton's antiquark distributions.

Presentations after 2012 W AL preliminary result using final 2011 and 2012 data: