Measurement of the longitudinal spin asymmetries for weak boson production in proton-proton collisions at $\sqrt{s}=510\mathrm{GeV}$

We report new STAR measurements of the single-spin asymmetries $A_L$ for $W^{+}$ and $W^{-}$ bosons produced in polarized proton-proton collisions at $\sqrt{s}=510\mathrm{GeV}$ as a function of the decay-positron and decay-electron pseudorapidity. The data were obtained in 2013 and correspond to an integrated luminosity of $250{\mathrm{pb}}^{-1}$. The results are combined with previous results obtained with $86{\mathrm{pb}}^{-1}$. A comparison with theoretical expectations based on polarized lepton-nucleon deep-inelastic scattering and prior polarized proton-proton data suggests a difference between the $\overline{u}$ and $\overline{d}$ quark helicity distributions for $0.05<x<0.25$. In addition, we report new results for the double-spin asymmetries $A_{LL}$ for $W^{\pm }$, as well as $A_L$ for $Z/{\gamma }^{*}$ production and subsequent decay into electron-positron pairs.

Figure 1

Distribution of the product of $Q$, the TPC reconstructed charge-sign, and $E_T/p_T$ in the BEMC (a) and EEMC (b) regions. The positron (red) and electron (blue) candidate events have been fitted with double-Gaussian distributions. The excluded regions are marked by hatched shades.

Figure 2

$E_T^e$ distribution of electron (top) and positron (bottom) candidates (black crosses), background contributions, and sum of backgrounds and $W\to e\nu$ MC signal (red-dashed) in the BEMC region.

Figure 3

Signed $p_T$-balance distribution for electron (left) and positron (right) candidates (black crosses), background contributions, and sum of backgrounds and $W\to e\nu$ MC signal (red-dashed) in the EEMC region.

Figure 4

Longitudinal single-spin asymmetries, $A_L$, for $W^{\pm }$ production as a function of the positron or electron pseudorapidity, ${\eta }_e$, separately for the STAR $2011+2012$ (black squares) and 2013 (red diamonds) data samples for $25<E_T^e<50\mathrm{GeV}$. The $2011+2012$ results have been offset to slightly smaller $\eta$ values for clarity. Shown also are the final asymmetries for high-energy decay leptons from $W$ and $Z/{\gamma }^{*}$ production from the PHENIX central arms as a function of ${\eta }_e$ and from the muon-arms as a function of ${\eta }_{\mu }$ with their statistical and systematic uncertainties [6, 7].

Figure 5

Longitudinal single-spin asymmetries, $A_L$, for $W^{\pm }$ production as a function of the positron or electron pseudorapidity, ${\eta }_e$, for the combined STAR $2011+2012$ and 2013 data samples for $25<E_T^e<50\mathrm{GeV}$ (points) in comparison to theory expectations (curves and bands) described in the text.

Figure 6

The difference of the light sea-quark polarizations as a function of $x$ at a scale of $Q^2=10(\mathrm{GeV}/c{)}^2$. The green band shows the NNPDFpol1.1 results [1] and the blue hatched band shows the corresponding distribution after the STAR 2013 $W^{\pm }$ data are included by reweighting.