Vector boson asymmetry

Primary authors: Salvatore Fazio, Dmitri Smirnov, and Elke C. Aschenauer

When protons accelerated by RHIC collide at the Z and W bosons can be produced and their decay products measured at STAR. In this analysis we measure the asymmetry of the W (and Z) bosons produced in pp collisions with the spin of the protons perpendicular to the beam (transverse polarization). During the 2011 run STAR has collected such data for the first time. The bosons cannot be detected directly but their kinematics can be reconstructed by detecting the decay products. The Z bosons can be easily reconstructed in the di-electron channel. However, the reconstruction of the W's decaying into a eν_e pair is challenging due to the undetected neutrino. The W has never before been reconstructed with the STAR detector. This analysis is the first attempt to reconstruct the kinematics of the W bosons at STAR.

The latest version of the analysis note is available from File:Vbasym.pdf


 

Technical Details

Getting the Code

The code for this analysis can be pulled from a CVS repository as:

cvs co -P -d WeakBosonTSSA offline/paper/psn0624

alternatively the code can be cecked from a github repository at https://github.com/plexoos/vbasym as:

$git clone git@github.com:plexoos/vbasym.git

The code heavily depends on the STAR libraries. It is verified to compile on the STAR cluster with the STAR environment. Before you can build and run the program, all the enviroment variables must be properly set. Please refer to the README file within the code's main directory or to the analysis note for more information on how to do it.
To compile the code one can do:

$cd vbasym
$starver SL12c
$cons
$make stana
$make vbana

The make command will produce standalone executables stana and vbana.

Running the Code

First one needs to reconstruct jets with stana as

$./stana -f filelist.lis -j

If the jet files are in the current directory one can run the analysis code as

$./stana -f filelist.lis

There is a job template available for running jobs on the farm. We use the following script to submit jobs

$./scripts/submit_jobs.sh

to check out transversely polarised pp2pp runs for the 2011 data taking year:

$get_file_list.pl -keys 'path,filename' -cond 'path!~long,filename~st_W,production=P11id,filetype=daq_reco_mudst,storage=hpss ' -limit 0 -delim '/' 

Reading output files

The root file with the tree can be navigated after loading the appropriate libraries:

gROOT->LoadMacro("$STAR/StRoot/StMuDSTMaker/COMMON/macros/loadSharedLibraries.C");
loadSharedLibraries();
gSystem->Load(".sl53_gcc432/lib/libutils.so");
gSystem->Load(".sl53_gcc432/lib/libStVecBosAna.so");
TFile *_file0 = TFile::Open("R1210603X_test.Wtree.root")

Preliminary Results

Available at DIS 2014 preliminaries

2017 RHIC Projections

Available at 2017 projections plots

Paper proposal

Available at Paper Proposal page

Analysis Note

The current version of the analysis note is available here

Collection of latest results for the asymmetry

A collection of the latest results for the asymmetry including blue and yellow beam asymmetries extracted separately, asymmetries from the geometry and luminosity effects, asymmetries calculated from the decay lepton and all the sin(phi) modulations fits can be downloaded here
Results can also be easily accessed via a web interface at the following page:
http://www.star.bnl.gov/~fazio/vbasym/webview/

References

• Theoretical paper relevant for this analysis
  • "Test the time-reversal modified universality of the Sivers function", http://arxiv.org/abs/0903.3629v1
  • "Helicity Parton Distributions from Spin Asymmetries in W-Boson Production at RHIC", http://arxiv.org/pdf/1003.4533.pdf

• Two internal STAR notes on a similar W analysis
  • "Longitudinal single-spin asymmetry A_L for W+ and W- production in polarized p+p collisions at \sqrt{s}=500~GeV, Run 9 data", http://drupal.star.bnl.gov/STAR/starnotes/private/psn0516
  • "Measurement of the W and Z Production Cross Sections at Mid-rapidity in Proton-Proton Collisions at \sqrt{s} = 500 GeV in Run 9", http://drupal.star.bnl.gov/STAR/starnotes/private/psn0546

• Another paper from CDF on W reconstruction (CDF RunI data)
  • "Measurement of the polar-angle distribution of leptons from W boson decay as a function of the W transverse momentum in p \bar{p} collisions at \sqrt{s}=1.8TeV"; PRD 70, 032004 (2004)