W+/W- Paper Proposal: (2011-2013)

Title:  Measurement of  W and  cross-sections and cross-section ratios in p+p Collisions at RHIC 

M. Posik (Temple University), S. Fazio (BNL) , and B. Surrow (Temple University)    

Proposed Target Journal:
PRD (18 pgs, 15 figures, and 9 tables)



 We report on the $W$ and $Z/\gamma^*$ differential and total cross sections as well as the $W^+$/$W^-$ and $(W^+ + W^-)$/$(Z/\gamma^*)$ cross-section ratios measured by the STAR experiment at RHIC in $p+p$ collisions at $\sqrt{s} = 500$ GeV and $510$ GeV. The cross sections and their ratios are sensitive to quark and antiquark parton distribution functions. In particular, at leading order, the $W$ cross-section ratio is sensitive to the $\bar{d}/\bar{u}$ ratio. These measurements were taken at high $Q^2=M_W^2,M_Z^2$ and can serve as input into global analyses to provide constraints on the sea quark distributions. The results presented here combine three STAR data sets from 2011, 2012, and 2013, accumulating an integrated luminosity of 350 pb$^{-1}$. We also assess the expected impact that our $W^+/W^-$ cross-section ratios will have on various quark distributions, and find sensitivity to the $\bar{u}-\bar{d}$ and $\bar{d}/\bar{u}$ distributions.

  1. Introduction
  2. Experimental Setup
  3. Data and Simulation
  4. W and Z Reconstruction
  5. Background Estimates
  6. Efficiencies
  7. W and Z Cross-Sections
  8. Cross-Section Ratios
  9. Summary



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STAR has measured the $W$ and $Z$ total and differential cross sections, along with the $W^+/W^-$ and $(W^+ + W^-)/Z$ cross-section ratios measured in $p+p$ collisions at a center of mass energy of $500$ GeV and $510$ GeV at RHIC. These results included data sets from the 2011, 2012, and 2103 $p+p$ running periods at RHIC accumulating $350$ pb$^{-1}$ of data.

These measurements will not only provide additional high $Q^2$ data to be used in future global analyses to help constrain PDFs, but also serve as complementary measurements to other experiments. In particular, our total and differential $W$ and $Z$ cross sections along with the $(W^+ + W^-)/Z$ cross-section ratio, will complement LHC's $W$ and $Z$ production program by providing data at lower $\sqrt{s}$ and sensitivity at larger $x$. Our $W^+/W^-$ cross-section ratio measurement, which is particularly sensitive to the $\bar{d}/\bar{u}$ sea quark distribution~\cite{Nadolsky:Private} (Eq.~\ref{eq:RW-Theory}), provides an alternative method to study the $\bar{d}/\bar{u}$ distribution which is complementary to the measurements performed by the NuSea and SeaQuest experiments.

Using our pseudorapidity dependent $W^+/W^-$ cross-section ratio results in a PDF reweighting study, we find sensitivity to the sea quark distributions. Our study shows modest improvement in the uncertainties of several distributions, in particular the $\bar{d}/\bar{u}$ and $\bar{u}-\bar{d}$ distributions, as well as a change in the central values.

Overall we find good agreement between our measurements and the current PDF distributions. Inclusion of these data into future global fits will help to constrain the PDFs.

STAR Analysis Note PSN0734  

GPC #288



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