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0001 ## Basic intro and how to get started
0002
0003 This analysis package is designed to be run over with the main sPHENIX G4 macro, Fun4All_G4_sPHENIX.C. When written, it was made to be run over dedicated PYTHIA direct photon-jet events that were output into HEPMC format, although in principle any input that works with Fun4All_G4_sPHENIX.C should work. The standard "generic" PYTHIA triggers can be used to generate events, i.e. the PHPy8ParticleTrigger set to find photons and the PHPy8JetTrigger set to find jets of your choosing.
0004
0005 In order to run the macro and include the PhotonJet analysis package, the following steps should be taken:
0006
0007 1. Execute the usual `autogen.sh` and `make install` building of the PhotonJet package, per the usual sPHENIX way.
0008
0009 2. Include the following lines of code in the Fun4All_G4_sPHENIX.C macro:
0010 ```
0011 gSystem->Load("libPhotonJet.so");
0012 PhotonJet * photjet = new PhotonJet("outputfilename.root");
0013 se->registerSubsystem(photjet);
0014 ```
0015 where se is the Fun4AllServer instantiated at the beginning of the macro.
0016
0017 The output is then "outputfilename.root" and contains several trees which have the relevant information. The tree names should be reasonably self explanatory. Note that truth information for the photon-jet correlated pairs is located directly in that tree, i.e. in the tree "isophoton-jets" the truth jet pt for a given entry is _truthjetpt and the reconstructed jet pt is _recojetpt.
0018
0019 Of course one can add whatever they would like to the file PhotonJet.C to add leaves to these trees or any other thing they see fit. Happy photon-jetting!
0020
0021
0022
0023 ## More detailed things (will continue adding as code develops)
0024
0025 There are a few public functions that one can set automatically, for example the minimum photon pT cut or the isolation cone size.
0026 The isolation cone algorithm is by default used; one can change this with the function use_isocone_algorithm. The isolation algorithm details are in the code, but it currently checks for reconstructed tracks and clusters within a cone size radius set by the user greater than 200 MeV in pT. One can also change the cone size used for the jets. The default size is R=0.3, and the DSTnodes can accomodate R=0.2 to R=0.8 in increments of 0.1.
0027
0028 There are two different truth trees; one is produced with the node "PHG4TruthInfoContainer" and the other is from the raw HEPMC files, i.e. the tree "truthtree" contains all of the PYTHIA output which includes parton info while the "truth_g4particles" is a more condensed truth tree without the parton information.
0029
0030
0031 Additional functionality has been added to look at forward jets. Tracked jet capabilities were added as well; one can set the use of these in their macro.
0032
0033 One can additionally choose to use or not use the position correction in the CEMC via the flag use_positioncorrection_CEMC. For pure photon measurements this should be used as it improves the resolution on the constant energy term of the photon by 1-2%
0034
0035 ## Addition of HIJING embedding capabilities
0036 In order to produce photon-jets in heavy ion background environments, PYTHIA events can be embedded into HIJING hits files. The code has been updated to include these capabilities; notably the flag "set_AA_collisions" must be set to 1 in order to grab the correct nodes from the node tree. This is because the jets undergo a background subtraction method, and this is put onto a different node. If you want regular pp capabilities, then just set this flag to 0 and it will get the usual tower-jet nodes.
0037
0038
0039 ## Additional code packages
0040 There are two additional code packages, Photons and TruthPhotonJet. Photons is a similar analysis package to PhotonJet with less of the overhead and is intended to be used with a single photon generator. TruthPhotonJet is also similar to PhotonJet but is intended to be used only with truth PYTHIA information. The purpose is to quickly generate and analyze truth PYTHIA events to determine e.g. the pT/eta reach of jets/photons, etc.
