surrow's blog

Abstract:

We report new measurements of single- and double-spin asymmetries for W⁺ and W^-boson production in polarized proton+proton collisions at √s = 510 GeV as a function of the decay lepton pseudorapidity. These new measurements based on 256 pb^-1 of data are combined with previous results based on 86 pb^-1. The combined results are compared to theoretical predictions constrained by polarized deep-inelastic scattering measurements and prior results in polarized p+p collisions suggesting an asymmetry in the helicity distributions of anti-u and anti-d quarks for 0.05<x<0.2.

Figures:

Figure 1:

Caption: (color online) $E_T^e$ distributions of $W^-$ (top) and $W^+$ (bottom) candidate events (black), background contributions, and sum of backgrounds and $W \rightarrow e\nu$ MC signal (red-dashed) in the BEMC region.

Figure 2:

Caption: (color online) Signed $p_{T}$-balance distributions for $W^+$ (left) and $W^-$ (right) candidate events (black), background contributions, and sum of backgrounds and $W \rightarrow e\nu$ MC signal (red-dashed) in the EEMC region.

Figure 3:

Caption: (color online) Distributions of the product of the TPC-reconstructed charge sign and $E_T/p_T$ in the BEMC region (left) and the EEMC region (right). The electron (blue) and positron (red) candidate events have been fitted with double-Gaussian distributions. The excluded regions are marked by hatched shades.

Figure 4:

Caption: (color online) Longitudinal single-spin asymmetry, $A_L$, for $W^\pm$ production as a function of the lepton pseudorapidity, $\eta_e$, separately for the 2011+2012 (black squares) and 2013 (red circles) STAR data samples for 25 < $E_T^e$ < 50GeV in comparison to theory predictions (See text for details).

Figure 5:

Caption: (color online) Longitudinal single-spin asymmetry, $A_L$, for $W^\pm$ production as a function of the lepton pseudorapidity, $\eta_e$, for the combined 2011+2012 and 2013 (red circles) STAR data samples for 25 < $E_T^e$ < 50GeV in comparison to theory predictions (See text for details).

Tables:

Table 1:

Caption: Longitudinal single- and double-spin asymmetries, $A_L$ and $A_{LL}$ for $W^\pm$ production for the 2013 STAR data sample. The longitudinal single-spin asymmetry is measured over six pseudorapidity bins. The longitudinal double-spin asymmetry is measured over the same bins combining bins belonging to the same absolute value.

Summary:

In summary, we report new measurements of single- and double-spin asymmetries for W⁺ and W^-boson production in polarized proton+proton collisions at √s = 510 GeV as a function of the decay lepton pseudorapidity. The production of W bosons in these collisions and their subsequent leptonic decay is a unique process to delineate the quark and anti-quark polarizations of the proton by flavor. The data, combined with previously published STAR results, suggest a preference for an anti-u quark helicity distribution that is larger than the anti-d quark helicity distribution for the fractional momentum range of 0.05 < x < 0.2 at a scale of Q² = 10 GeV², opposite to the spin-averaged quark-sea distributions.

Additional analysis tasks:

Embedding incl. background estimation and determination of beta values

Prepare all updated theory curves

Update "money plots" with new theory curves

Check all systematic uncertainties and cuts

Drafts and Review: