Measurement of Longitudinal Spin Asymmetries for Weak Boson Production in Polarized Proton-Proton Collisions at RHIC

We report measurements of single- and double-spin asymmetries for $W^{\pm }$ and $Z/{\gamma }^{*}$ boson production in longitudinally polarized $p+p$ collisions at $\sqrt{s}=510\mathrm{GeV}$ 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 polarized deep inelastic scattering measurements, and show a preference for a sizable, positive up antiquark polarization in the range $0.05<x<0.2$.

Figure 1

$E_T^e$ 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

(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

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

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

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).