Motivation:

For Aj study, we first cut pT > 2 GeV/c to minimize the background. Then find the dijet pair with leading jet > 20 GeV/c and subheading jet > 10 GeV/c. Once we have the dijet pair, we do the geometric matching and find the dijet pair at the similar locations with particle pT cut 0.2 GeV/c. The ideal is 1. first we can easily find where is the dijet. 2. second the finial matched jet are the same jet pairs staring from.  

Test whehter they are the same jet. 

Dataset:

Au+Au 200 GeV run11

trigger:

NPE_18 (350503, 350513)

Centrality 0-20% from StRefMultCorrr

Method:

fastjet 
anti-kt algorithm
jetmedian for background subtruction
Constituents cuts pT>2 GeV/c to find dijet pair with leading jet pT>20 GeV/c and subleading jet pT>10 GeV/c. Rerun the jet finding with particles pT>0.2 GeV/c for Aj calculation.

The code provided by Kolja Kauder github.com/kkauder/BasicAj branch master (~ Apr 20, 2015).


Result:

1. Leading jet pT distribution with 2 GeV/c constituent pT cut.


Clearly see our 20 GeV/c jet cut


2. SubLeading jet pT distribution with 2 GeV/c constituent pT cut.


There is 10 GeV/c Jet pT cut


3. Matched Leading jet pT distribution with 0.2 GeV/c constituent pT cut.


There is ~10% Jet pT < 20 GeV/c. Not as we would expect.



4. Matched SubLeading jet pT distribution with 0.2 GeV/c constituent pT cut.

There is ~ 7% Jet pT < 10 GeV/c






Now, here is the counts of number of 2GeV/c particles shared by the 2 GeV/c constituent pT jet & 0.2 GeV/c constituent pT jets:



There are 230 dijet paris found in 1 million events. 
93% for leading jets has all 2 GeV/c particles matched (100%) in matched jet (0.2 GeV/c consituents pT cuts).  
83% for subleading jets has all 2 GeV/c particles matched (100%) in matched jet (0.2 GeV/c consituents pT cuts).  

and the size (number of particles in 2 GeV/c consituents jets)


The average 2 GeV/c particles in the jet with 2GeV/c constituents pT cut is ~ 4 particles. 



Discussion:

Would it be a code problem? Here it the same result with 1 GeV/c pt cut instead 2 GeV/c.

Aj distribution:

This is Cent 5-20% && refmult<470 cut.

1. 40 times more dijet pairs with 1GeV/c constituents pT cut compared to 2GeV/c 
To check how much jets are fake. We rotate the dijet Δφ to be 90 degree instead of 180 degreee. So we are searching at the background area. In this way, the program finds < 50 dijet pairs for 2 GeV/c. Therefore, the the fake jet (background fluctuation) seems to be small (1%) for 2 GeV/c. The fake jet is more than half for 1 GeV/c. 
2. There is peak at Aj = 1


For Jet pT before and after matching:

1. Leading jet pT distribution with 1 GeV/c constituent pT cut.



2. SubLeading jet pT distribution with 1 GeV/c constituent pT cut.



3. Matched Leading jet pT distribution with 0.2 GeV/c constituent pT cut.


4. Matched SubLeading jet pT distribution with 0.2 GeV/c constituent pT cut.


There is peak at 0 pT.