Update README.md

README.md

@@ -18,7 +18,7 @@ Note: the following instructions are given assuming you are using `bash` as your
 export PREFIX="/path/to/your/prefix"; mkdir -p $PREFIX
 ```
 ## Install all necessary dependencies
-Ensure you have `cmake`, a modern c++ compiler (gcc4.8+ or clang3.5+) and a `gfortran` installed
+Ensure you have `cmake`, ROOT, a modern c++ compiler (gcc4.8+ or clang3.5+) and `gfortran` installed
 ### nanocernlib
 You will need to install a the necessary cernlib routines through the `nanocernlib` library.
 ```bash
@@ -60,4 +60,46 @@ It requires the following arguments:
 - `--conf`: a configuration file, see `$PREFIX/src/pythia6m/program/example-config.json` for an example.
 - `--run`: a run number. This is also used as the random seed for this run
 - `--events`: the number of events to be generated
-- `--out`: the output directory. This is where the output files will be placed
+- `--out`: the output directory. This is where the output files will be placed
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+
+## Output
+The generated events are written to a ROOT TTree. The following variables are written to the tree for each of the generated events:
+```c++
+// event level
+int32_t evnum;   // the event index
+int32_t evgen;   // the total number of generated events sofar. 
+                 // this includes generated events that were somehow cut out
+double xsec;     // the total generated cross section
+                 // the last stored value is the best estimate of the total
+                 // cross section. 
+                 // To obtain the cross section for a subset of n events:
+                 //     xsec_n = n * xsec / evgen;
+int32_t process; // PYTHIA process number
+double s;        // s for this event
+// particle level
+std::vector<int32_t> index;     // The index of this track, equal to the fortran 
+                                // index, so the C-index is (index - 1)
+std::vector<int32_t> status;    // LUND track status (see pythia manual)
+std::vector<int32_t> type;      // LUND particle type
+std::vector<int32_t> parent;    // fortran index of the parent, 0 if initial
+std::vector<int32_t> daughter1; // fortran index of first daughter, 0 if none
+std::vector<int32_t> daughter2; // fortran index of last daughter, 0 if none
+std::vector<int32_t> charge;    // charge
+std::vector<ROOT::Math::XYZTVector> vertex; // vertex 4-vector
+std::vector<ROOT::Math::XYZTVector> mom;    // momentum 4-vector
+std::vector<double> mass;       // mass
+std::vector<bool> init;         // is this an initial particle?
+std::vector<bool> lund;         // is this a final particle produced by PYTHIA?
+```
+
+The following additional variables are written when running with an electron beam:
+```c++
+// event level
+double Q2;          // Generated Q2
+double nu;          // Generated nu
+double x;           // Generated Bjorken-x
+double y;           // Generated y
+double W2;          // Generated W2
+int32_t scat_index; // the C-index of the scattered (NOT THE FORTRAN INDEX)
+                    // of the scattered lepton
+```
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