Measurement of the Parity-Violating Longitudinal Single-Spin Asymmetry for $W^{\pm }$ Boson Production in Polarized Proton-Proton Collisions at $\sqrt{s}=500\mathrm{GeV}$

We report the first measurement of the parity-violating single-spin asymmetries for midrapidity decay positrons and electrons from $W^{+}$ and $W^{-}$ boson production in longitudinally polarized proton-proton collisions at $\sqrt{s}=500\mathrm{GeV}$ by the STAR experiment at RHIC. The measured asymmetries, $A_L^{W^{+}}=-0.27\pm 0.10(\mathrm{stat}.)\pm 0.02(\mathrm{syst}.)\pm 0.03(\mathrm{norm}.)$ and $A_L^{W^{-}}=0.14\pm 0.19(\mathrm{stat}.)\pm 0.02(\mathrm{syst}.)\pm 0.01(\mathrm{norm}.)$, are consistent with theory predictions, which are large and of opposite sign. These predictions are based on polarized quark and antiquark distribution functions constrained by polarized deep-inelastic scattering measurements.

Figure 1

(a) Ratios of $E_T^e$ with respect to the $4\times 4$ BEMC $E_T$ sum, $E_T^e/E_T^{X=4\times 4}$, (b) the near-cone BEMC, EEMC, and TPC $E_T$ sum, $E_T^e/E_T^{X=R<0.7}$, and (c) correlation of the signed $p_T$ balance variable and $E_T^e$. MC shape distributions (arbitrary normalization) are shown in (a) and (b) for $W^{\pm }\to e^{\pm }+X$ as filled histograms in comparison to both data distributions.

Figure 2

$E_T^e$ as a function of the ratio of the TPC reconstructed charge sign to the transverse momentum $p_T$. The black solid and red dashed lines indicate the selected kinematic region used for the asymmetry analysis.

Figure 3

$E_T^e$ for $W^{+}$ (bottom panel) and $W^{-}$ (top panel) events showing the candidate events as solid line histograms, the full background estimates as dashed line histograms, and the signal distributions as shaded histograms.

Figure 4

Longitudinal single-spin asymmetry, $A_L$, for $W^{\pm }$ events as a function of the leptonic pseudorapidity, ${\eta }_e$, for $25<E_T^e<50\mathrm{GeV}$ in comparison to theory predictions (see text for details).