Measurement of the Transverse Single-Spin Asymmetry in $p^{\uparrow }+p\to W^{\pm }/Z^0$ at RHIC

We present the measurement of the transverse single-spin asymmetry of weak boson production in transversely polarized proton-proton collisions at $\sqrt{s}=500\mathrm{GeV}$ by the STAR experiment at RHIC. The measured observable is sensitive to the Sivers function, one of the transverse-momentum-dependent parton distribution functions, which is predicted to have the opposite sign in proton-proton collisions from that observed in deep inelastic lepton-proton scattering. These data provide the first experimental investigation of the nonuniversality of the Sivers function, fundamental to our understanding of QCD.

Figure 1

$P_T^{\text{recoil}}$ distribution of events simulated with pythia 6.4 and reconstructed before (yellow) and after (hatched green) the $P_T$ correction has been applied is compared with predictions from RhicBOS (dashed blue).

Figure 2

Estimated background contributions are shown for the $W^{+}$ (upper panel) and the $W^{-}$ (lower panel) data samples, respectively. A vertical line marks the minimum $P_T^e$ value in this analysis.

Figure 3

The amplitude of the transverse single-spin asymmetry for $W^{\pm }$ and $Z^0$ boson production measured by STAR in proton-proton collisions at $\sqrt{s}=500\mathrm{GeV}$, with a recorded luminosity of $25{\mathrm{pb}}^{-1}$. The solid gray bands represent the uncertainty on the KQ [11] model due to the unknown sea quark Sivers function. The crosshatched region indicates the current uncertainty in the theoretical predictions due to TMD evolution.

Figure 4

Transverse single-spin asymmetry amplitude for $W^{+}$ (left plot) and $W^{-}$ (right plot) versus $y^W$ compared with the non-TMD-evolved KQ [11] model, assuming (solid line) or excluding (dashed line) a sign change in the Sivers function.