## ROOT usage

Many PYTHIA users wish to use ROOT to produce histograms, or even to run PYTHIA as a plugin to ROOT. This is possible. It is not a task supported by the PYTHIA team, however. All issues involving ROOT usage should be directed to the ROOT team, or to the local support team of your collaboration. Below some helpful hints have been collected. The text is based on contributions by Rene Brun, Andreas Morsch and Axel Naumann. Another example may be found in the VINCIA add-on program for parton showers, but this should also work for a PYTHIA standalone run.
Note that in all that follows, a Linux-type system with a Bash shell and GNU Make is assumed. In particular, for Mac OS X, the `LD_LIBRARY_PATH` should be replaced with `DYLD_LIBRARY_PATH` and the extension for shared libraries `.so` should be replaced with `.dylib`.

### Standalone usage

One can perform the generation and analysis of events in a completely standalone fashion, and only use ROOT to process the completed events. Two example programs are provided in the `examples` directory, with details provided below. The examples assume that ROOT is installed, that you have run

        ./configure --with-root=root-installation-directory

where you have to specify which is the ROOT installation directory, and subsequently run `make`. More fine-grained options are available with `configure`, if need be.

#### Histogramming with ROOT

An example of histogramming with ROOT is provided in `examples/main91.cc`. It may be compiled and run just like the other example programs. After PYTHIA has run, a ROOT histogram of the charged multiplicity in the events will be shown. This is now stored in the `hist.root` file. If you can make this example work, the road should be open to do the same for all other histogramming needs. Specifically, you need to edit the `examples/Makefile` file to add the other programs to link as `main91.cc` currently does.

### Storing partial PYTHIA events in ROOT trees

A common use case is to store partial track information including some overall event information to ROOT trees, relevant for a specific analysis. The resulting ROOT trees will then be what is often referred to as "n-tuples". The advantage of this over for example storing full events, is a significant reduction of disk space used, as well as the possibility to construct trees resembling those familiar from the experiments' central MC production. The `examples/main93` example provides this - among other - functionality. As for the above example, it is split up in several files.

- `main93.cc` is the main example program;
- `main93.cmnd` is a sample input command file;
- `main93LinkDef.h` is used by Makefile to generate the dictionary for only the used PYTHIA classes involved in the IO, for the example; and
- `main93.h` defines a "track" and an "event" class where relevant event -and track information is defined.

#### Compiling the example

The `main93` example is compatible with ROOT v.6 and above. One should have a working installation of ROOT, and then configure PYTHIA with:

        ./configure --with-root=root-installation-directory

One can then compile `main93` with the usual:

        make main93

provided that all ROOT paths are set correctly by eg. running:

        source root-installation-directory/bin/thisroot.sh

#### Running the example

The `main93` example can be run with several command line options. Running:

        ./main93 -h

will display a help text showing these options. To produce events, the user needs to supply a command file with option `-c COMMAND-FILE.cmnd`. The example command file `main93.cmnd` is a good starting point. The crucial command to output ROOT trees is to set `Main:writeROOT = on`. The ROOT file will be named `pythia.root` per default. This can be changed by appending `-o ONAME` on the command line.

#### Changing the event information

The header file `main93.h` defines a simple event class and track class, which in turn defines the information stored to the tree. If a user wants to change this, either by adding more track information or imposing cuts corresponding to detector acceptance (thus reducing the file size), this can be done directly in this header file. Both the track class and the event class has `init` functions returning a boolean value, and by returning `false`, the track/event is rejected. The `main93` example must be recompiled after making any changes to the header file.

### PYTHIA as a plugin to ROOT

In more ROOT-centric applications, PYTHIA can be run as a ROOT plug-in. This requires a version of ROOT that has been installed from source. The reason is that the interfaces depend on PYTHIA header files that are not distributed with ROOT. Installing ROOT is not more difficult than the PYTHIA installation, and some guidelines are provided below.

#### Installation

Define an environment variable for the path to your PYTHIA installation directory

        export PYTHIA8=path_to_PYTHIA8_installation

Before compiling ROOT, configure ROOT by running the `configure` command including the following options

        --enable-pythia8
        --with-pythia8-incdir=$PYTHIA8/include/Pythia8
        --with-pythia8-libdir=$PYTHIA8/lib

In case ROOT has already been compiled before, it will only recompile the PYTHIA module and build the library `libEGPythia8`.

#### Interfaces

When running PYTHIA as a plugin, the exact interface structure becomes very relevant. ROOT provides two simple interfaces (wrappers) for PYTHIA 8. The code for these interfaces are located in

        path_to_ROOT_source/montecarlo/pythia8

The two interfaces are

- `TPythia8` is an implementation of the `TGenerator` interface for PYTHIA 8.
  It allows you to use PYTHIA within a ROOT macro or as a plug-in for a general-purpose particle generator based on this interface. The main methods of the interface are
  - `GenerateEvent()` which triggers the generation of the next event, and
  - `ImportParticles(TClonesArray* particles)` which copies the native PYTHIA stack into a `TClonesArray` of `TParticles`.
  In addition, some methods that are directly related to corresponding PYTHIA methods are implemented
  - `ReadString(const char* string)` → `readString(...)`
  - `ReadConfigFile(const char* string)` → `readFile(...)`
  - `Initialize(int idAin, int idBin, double ecms)` → `init()`
    Warning: this method will have to be updated for the 8.2 version!
  - `EventListing()` → `event.list()`
  - `PrintStatistic()` → `stat()`
    Warning: this method will have to be updated for the 8.2 version!
  These methods provide already the basic PYTHIA functionality interactively from the ROOT command line. However, this does not mean that the usage of PYTHIA from within ROOT is restricted to these methods. In compiled code, one can always obtain a pointer to the `Pythia` instance e.g.

          TPythia8        *tp = new TPythia8();
          Pythia8::Pythia *p  = tp->Pythia8();

  giving access to the full PYTHIA functionality. To access this functionality in the CINT interpreter see the "Advanced usage" section below.
- `TPythia8Decayer` is an implementation of the `TVirtualMCDecayer` interface.
  It allows you to use PYTHIA as a plug-in decayer for simulation frameworks based on the Virtual Monte Carlo (VMC) interface classes. The main methods of the interface are
  - `TPythia8Decayer::Init()` for initialisation,
  - `TPythia8Decayer::Decay(Int_t pdg, TLorentzVector* p)` to decay a particle with PDG code `pdg` and 4-momentum `p`, and
  - `ImportParticles(TClonesArray* particles)` to retrieve the decay products as `TParticles` in the `TClonesArray particles`.

#### An example

A basic example for generating minimum-bias events with PYTHIA 8 inside a ROOT macro, and filling some histograms with the kinematics of the final-state particles is provided in either of the locations below

        /path_to_ROOT_source/tutorials/pythia/pythia8.C
        /path_to_ROOT_installation/share/doc/root/tutorials/pythia/pythia8.C

Note that before executing this script

- the environment variables `PYTHIA8` and `PYTHIA8DATA` must be setup correctly e.g.

          export PYTHIA8=/path_to_PYTHIA_installation
          export PYTHIA8DATA=$PYTHIA8/share/Pythia8/xmldoc

- your LD_LIBRARY_PATH must contain the location of the PYTHIA 8 shared library, e.g.

          export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:path_to_PYTHIA8_installation/lib

The script can then be run with ROOT

        root pythia8.C

After execution, ROOT will display some histograms from the event generation.

#### Advanced usage

To access the full PYTHIA functionality from the CLING interpreter, a ROOT dictionary must be created. Currently that option has not been implemented as a standard option for PYTHIA 8.3, but it should be in the same spirit as what can be found in the 8.1 `rootexamples` directory. Also note that one dictionary is found in the `examples/main93LinkDef.h` file. This may then be loaded in ROOT giving full access to the full PYTHIA 8 functionality, e.g. in an interactive session

        gSystem->Load("path_to_PYTHIA8_installation/examples/pythiaDict");
        Pythia8::Pythia *p = new Pythia8::Pythia();
        p->readString("SoftQCD:nonDiffractive = on");