Motivation:

It is observed that the near-side jet peak distribution differs from central and peripheral d+Au collisions.

Here the jet centrality dependence is studied by Pythia simulation.

Two processes are analyzed in Pythia:

  - Soft QCD Minbias with Multiple Parton Interaction (MPI) on

  - Soft QCD Minbias with Multiple Parton Interaction (MPI) off

 

Method

Dihadron correlations are projected on Δη for near-side (Δφ~0) and away-side (Δφ~π) separately. The near-side distribution is fitted with a Gaussian + Pedestal.

The fit parameters are: Gaussian area Y, Gaussian width σ, Pedestal C.

Uncorrelated seed = N_trig/(1+N_assoc)  for same pT trigger and associated particles.
N_trig: number of trigger particles per event.
N_assoc: number of associated particle yield (away+near).
 

The dihadron correlation below shown is ZYAM subtracted.

Both trigger and associate particles are global tracks in TPC |η|<1 acceptance. 1<pT<3 GeV/c.

The centrality classes are categorized by:
1. number of particles in the FTPC East acceptance -3.8<η<-2.8.
2. number of particles in the TPC |η|<1.

The dN/dη used in X-axis is the number of particles in the |η|<0.5 with pT>0.15 GeV/c
 

More about process in PYTHIA:  http://home.thep.lu.se/~torbjorn/pythia81html/QCDProcesses.html.
 
Also see 200 GeV (same data) in drupal.star.bnl.gov/STAR/blog/yili/pythia-multiplicity-selection-effect-jetlike-near-side-yield


ALICE papers on multiple parton interaction and uncorrelated seed:
arxiv.org/abs/arXiv:1407.3628  (from pp to pPb)
arxiv.org/abs/arXiv:1307.1249  (pp)
TPC multiplicity as event selection
arxiv.org/abs/arXiv:1406.5463  (pPb)
Forward detector multiplicity as event selection

The idea is:
1. The uncorrelated seed is related to number of multiple parton interaction.
2. The per trigger yield is related with the fragmentation.

Data set:

PYTHIA simulation used starsim.C.

  - Soft QCD Minbias with Multiple Parton Interaction (MPI) on

SoftQCD:minBias = on
(default: PartonLevel:MPI = on)

  - Soft QCD Minbias with Multiple Parton Interaction (MPI) off

SoftQCD:minBias = on
PartonLevel:MPI = off


Trigger and associated particles:

1<pT<3

|η|<1

pion, kaon and proton(antiproton) are used.

Event selection: mutliplicity in -3.8<η<-2.8.
 

Results:


1. 200 GeV

TPC mutliplicity |η|<0.5 vs FTPC multiplicity -3.8<η<-2.8.

TPC centrality	FTPC centrality

 
  MPI=ON: positive correlation between TPC and FTPC multiplicity
  MPI=OFF: weak correlation between TPC and FTPC multiplicity
 

Gaussian area Y_jetlike
 

TPC centrality	FTPC centrality

(the zoomed in plot of FTPC centrality MPI off can be found here.)
(The FTPC centrality near-side jetlike yield vs FTPC multiplicity can be found here. )


With TPC event selection, increase near-side jetlike yield as event TPC activity increases.
With FTPC event selection, weak dependence.


Pedestal C

TPC centrality	FTPC centrality

Uncorrelated Seed

TPC centrality	FTPC centrality

FTPC centrality MPI=ON Positive correlation between TPC multilplicty and Uncorrelated seed
FTPC centrality MPI=OFF Uncorrelated seed fluctuates as TPC multiplicity increases.

2. 7TeV

TPC mutliplicity |η|<0.5 vs FTPC multiplicity -3.8<η<-2.8.

TPC centrality	FTPC centrality

  MPI=ON: positive correlation between TPC and FTPC multiplicity
  MPI=OFF: weak correlation between TPC and FTPC multiplicity


Gaussian area Y_jetlike

TPC centrality	FTPC centrality

Near-side jetlike yield increases as TPC multiplicity for all cases.


Pedestal C

TPC centrality	FTPC centrality

Uncorrelated Seed

TPC centrality	FTPC centrality

MPI=ON positive correlation between TPC multiplicity and Uncorrelated seed.
MPI=OFF Uncorrelated seed appears independent of TPC multiplicity.