Vector boson asymmetry

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Last modified by Sfazio on 23-02-2015

Primary authors: Salvatore Fazio and Dmitri Smirnov

Production of W boson in proton-proton collisions

When protons accelerated by RHIC collide at the Z and W bosons can be produced 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. 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 pair is challenging due to the neutrino completely escaping the detector. The W has never 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 Vbasym.pdf

Technical Details

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


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

$ -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:

TFile *_file0 = TFile::Open("R1210603X_test.Wtree.root")

MC Studies

Embedded MC - the embedded MC is existing for 2009 year geant configuration. To check the file list:

> -keys 'path,filename' -cond 'filename~MuDst,runnumber=2000010010, storage=HPSS' -limit 0 -delim '/'

'runnumber' for the relevand MC-data set can be found on the web page: [1]

The W events generated by Pythia (in pythia format) can be found in


Request for W Monte-Carlo for Run 11

Links to relevant pages:


W, Z and background MC samples for the Run 11 W/Z asymmetry analysis with embedding


  • Essential detectors are: TPC, BTOW, and ETOW
    • Nice to have BSMD, BPRS, ESMD, EPRS
  • Geometry y2011 (latest revision for Run 11)
  • STAR library SL11d
  • Use PPV vertex finder with beam line constraints, allow PPV to find all vertices
  • Physical processes (Can we use latest Pythia version 8.1? What other MC generators available at STAR?)
    • Sample 1: W + Pythia events, 30k
    • Sample 2: W- Pythia events, 10k
    • Sample 3: W(+,-) ->Tau pythia events, 10k
    • Sample 4: Z0->e+e- without interference term, 5k
    • Sample 5: Z0->anything except e+e-, 10k
      • Can we produce samples 4 and 5 with interference term and apply Failed to parse (syntax error): {\displaystyle q^2 > ?\!\,} to get more Z's?
    • Sample 6: QCD, 100k, partonic pt>35 GeV/c, not critical since we plan to use data for background estimation
    • Total: 165k events
  • Embedding events: zero bias events for Pythia samples (high lumi fills? low lumi fills?)
    • -keys runnumber,events -cond 'filename~st_zerobias_%,storage=hpss,trgsetupname=pp500_production_2011||pp500_production_2011_noeemc||pp500_production_2011_fms,trgname=zerobias,createtime~2011-%' -alls -delim "," -limit 0
  • of this sample we selected the transverse production withe 100 < events <1500
    • -keys runnumber,events -cond 'filename~st_zerobias_%,storage=hpss,trgsetupname=pp500_production_2011,trgname=zerobias,createtime~2011-%,events>100 && events<1500' -alls -delim "/" -limit 0
  • Vertex and beam line constraints with the proper parameters resembling the data:
    • z width determined from data
    • z offset determined from data
    • lateral offset to be done with the beam line offset
  • BFC options (the proper one for the relevant run period ?)
  • Magnetic Field = Reversed Full-Field

Zero bias runs for embedding:

  • Two lists available at the moment (runNumber, events)
    • runlists/run11_zerobias: all zero bias runs in Run 11
    • runlists/run11_zerobias_events50+: zero bias runs with more than 50 events per run
    • runlists/run11_zerobias_events500+: zero bias runs with more than 500 events per run

Preliminary Results

Available at DIS 2014 preliminaries

2016 RHIC Projections

Available at 2016 projections plots


  • 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)

Last modified by Sfazio on 23-02-2015