Measurement of longitudinal spin asymmetries for weak boson production in polarized proton-proton collisions at RHIC

Abstract

We report measurements of single- and double- spin asymmetries for $W^{\pm}$ and $Z/γ^*$ 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 deep inelastic scattering measurements, and show a preference for a sizable, positive up antiquark polarization in the range $0.05<x<0.2$.

Figures (5)

• Figure 1: (color online) $E_T^e$ distribution of $W^-$ (top) and $W^+$ (bottom) candidate events (black), background contributions, and sum of backgrounds and $W^{\pm} \to e^{\pm} \nu_e$ MC signal (red dashed).
• Figure 2: (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: (color online) Distributions of the invariant mass of $Z/\gamma^* \to e^+e^-$ candidate events. The $Z/\gamma^* \to e^+e^-$ MC distribution (filled histogram) is shown for comparison.
• Figure 4: (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: (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).