The goal of this session is to generate two sets of simulated data for the reaction
gamma p -> pi+ pi- pi+ n

A) The first set will have resonance in pi+ pi- pi+ invariant mass and this set, after it has
been passed through the detector simulation, will serve as the mock signal data sample for
further simulation.

B) The second set will not have any resonances in it and will be used in the amplitude analysis
procedure to account for the detector acceptance.

This example uses the generator gen_3pi, which is based on the AmpTools package and has
been checked into the GlueX repository at programs/Simulation/gen_3pi.  The configuration
files in this directory are copied from those in that directory.


Steps:

1) First examine the configuration file gen_3pi.cfg.  You can see how each of the amplitudes 
are constructed as a product of amplitudes.  The amplitude definitions themselves are coded as 
classes in the GlueX repository:  libraries/AMPTOOLS_AMPS

2) Generate 50K events of sample A (with resonances) by executing the following command:

gen_3pi -c gen_3pi.cfg -o threepi_data_gen.root -hd threepi_data_gen.hddm -l 0.7 -u 2.0 -n 50000

The arguments specify the configuration file, the output file, the (optional) HDDM formatted copy
of the output, the lower limit in 3 pi invariant mass (GeV), the upper limit in 3 pi invariant
mass (GeV) and the number of events.

[In addition a file called gen_3pi_diagnostic.root is created.  This is created everytime that 
gen_3pi is run and contains some diagnostic histograms.  This behavior can be altered by editing
the gen_3pi.cc source code.]

3) A ROOT script is provided to make some simple plots of the output:  plotInvariantMasses.C
Start ROOT, run the script, and interpret the plots.

4) To complete this exercise you will also need the sample B that does not have resonances.
Generate 200K events of this type -- adding the -f flag to the generation generates the
events flat in 3 pi invaraint mass:

gen_3pi -c gen_3pi.cfg -o threepi_gen.root -hd threepi_gen.hddm -f -l 0.7 -u 2.0 -n 200000


Additional Exercises 
(be sure you maintain the original files for the remainder of the workshop)

* modify plotInvariantMasses.C to read and plot sample B
* comment out/edit some amplitudes in the configuration file, regenerate sample A, and plot
* comment out 'permute' commands in the coniguration file, regenerate sample A, and plot
* modify plotInvariantMasses to plot missing mass, momentum transfer, or other variables

* modify polarization:

  The generation is done with pure x polarization.  A component of y polarization can be added
  by removing the comment in front of the line in gen_3pi.cfg

   include gen_3pi_ypol.cfg

  The fraction of x polarization can be adjusted by modifying the line

   define polFrac 0.0