Motivation:

Followed the pp 500 GeV near-side Dphi distribution shows a bump with |Deta|>1.3 in dihadron correlation

See: drupal.star.bnl.gov/STAR/blog/yili/pp-500-gev-year-2013-dihadron-correlation-eemc-dijet-data        
       drupal.star.bnl.gov/STAR/blog/yili/pp-500-gev-year-2012-dihadron-correlation-tofmult4tofmult3vpd-data

       drupal.star.bnl.gov/STAR/blog/yili/pp-500-gev-year-2012-dihadron-correlation-vpdmb-nosmd-0

Here is pythia simulation with Hard QCD only.

I also checked SoftQCD (MB) one, but it needs too much statistics. 

See master page: drupal.star.bnl.gov/STAR/blog/yili/master-page-dihadron-pp-500-gev

Dataset:
 
pythia8->SetFrame("CMS", 510.0);
pythia8->SetBlue("proton");
pythia8->SetYell("proton");

pythia8->Set("HardQCD:all = on");
pythia8->Set("PhaseSpace:mHatMin = 10");

Only pion, kaon, proton are used for refmult calcuation and dihadron correlations. 

MPI ON and OFF will be compared. 
Pythia default is MPI ON. 

Code:
See attachment. 

Result:

1. refmult distribution:
(events < 2 tracks are not included in histogram)

MPI ON	MPI OFF
	ppp

2. Delta eta - Delta phi distribution

MPI ON	MPI OFF

3. Delta eta distribution with |Delta phi| < 1

MPI ON	MPI OFF

4. Delta phi distribution with 1.3 < |Delta eta|<1.7

MPI ON	MPI OFF

MPI ON	MPI OFF