Purpose:

Pileup event is referred to those recorded event has more than 1 collisions happened. It will effect the correlation study since tracks coming from the different collisions now are associated into the same events with fitting. 

Dihadron correlation such would be effected by the pileup issue. Those pileup events either need to be removed. Or those tracks not belong to the primary vertex should not be used.

Here I try to remove the pileup in d+Au Run8. 

One pile up event feather in dihadron is the 'W' shape on Δη along Δφ . See plot below from Duncan.  
 

Backgroud and some methods:

1. Duncan's Z_first and Z_last (The z position of the first and last hit points)

Cu+Cu 200 GeV

http://www.star.bnl.gov/protected/estruct/prindle/pileup/index.html
http://www.star.bnl.gov/protected/estruct/prindle/pileupFinder/
http://www.star.bnl.gov/protected/estruct/prindle/pileupResults/

2. Prabhat's summary
Vertex ranking, Duncan's method, Tof Matching, Global dca, global vs primary tracks, 

Au+Au 200 GeV: Run10 and Run11

http://www.rhip.utexas.edu/public/prabhatb/Meeting/Run10Pileup.html

http://www.rhip.utexas.edu/public/prabhatb/Meeting/Pileup_Issues_In_Run_10_11.pdf
http://www.rhip.utexas.edu/public/prabhatb/Meeting/JetCorr_Nov_20_2012_PILEUP_RUN_10.pptx
http://www.rhip.utexas.edu/public/prabhatb/Meeting/Berkley_Meeting2013.pptx

3. Matthew's global Dca cuts

p+p 200 GeV Run 9
d+Au 200 GeV Run 8

http://www.star.bnl.gov/protected/heavy/mcc/HFupdates/UpsilonHadron/HF_update13/mcc2010UpsHad_HFupdate13.ppt

4. Elizabeth's number of global vs primary cuts

Au+Au 200 GeV Run 10

http://www.rhip.utexas.edu/public/eoldag/private/AuAu200/AxialPlots/2010/run10_pileup.html

5. Chitrasen's Chi^2 cuts

d+Au 200 GeV Run 8

http://www.star.bnl.gov/protected/lfspectra/cjena/STARAnaMeet_MIT/dAuCentrality_run8_STARAnaMeet_July09.pdf
http://www.star.bnl.gov/protected/lfspectra/cjena/run8_dAu200GeV/dAu_centrality/documentation/centrality_run8_dAu200GeV.html

6. Anthony: run8 some issue

http://www.star.bnl.gov/protected/lfspectra/atimmins/dAu2008/dAu.ppt

QA Plots according to the Method Suggested

1. number of primary vs global tracks:

The TGraph (black dots with error bar) is the 2-sigma region. 

--> May not pick out pileup events.

2. BEMC matching

--> Effectively remove pile-up contribution for TPC tracks