Requirements:

A sample of events of the type gamma p -> pi+ pi- pi+ n that has been generated
and reconstructed in the earlier stages of the analysis workshop.  Three
files are needed.

threepi_data.root : the mock data after reconstruction and analysis
threepi_gen.root : an MC sample generated flat in invariant mass with no physics
threepi_acc.root : the MC sample above subjected to the same reconstruction and analysis as data

These files (or symbolic links to them) must be in the directory containing this README file
before proceeding.

The goals of this example are:

A. Perform a fit to extract the production amplitudes for several resonances starting
   with output data files.
B. Use the AmpPlotter library to view projections of the fit

<><><><><><><> Quick recipe for doing example <><><><><><><><>
fit -c fit_3pi.cfg
threepi_plotter threepi.fit
<><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><>

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A. Perform fit
-------------------------------------------------

1.  In this fit we are simply going to try to fit for what was input to the
    the generator.  We will perform a "mass dependent fit" under the assumption
    that the resonances in 3 pi are described by the simple Breit-Wigner 
    shapes that we generated.  The file fit_3pi.cfg is provided to configure
    the fit.  Note the only difference between it and the gen_3pi.cfg file
    used to generate the sample is that the locations of the input and
    MC files are specified and one of the fit parameters is fixed to be real.
    (There is always an arbitrary overall phase when performing a fit that
    needs to be removed by fixing a parameter to be real.)
    
    Perform the fit by executing the command:
    
    fit -c fit_3pi.cfg
    
    You can examine the output on the screen.  The results of the fit will
    be written to the file threepi.fit.  This file contains information
    about the configuration of the fit as well as the parameter values,
    covariances, and information about the convergence of MINUIT.


-------------------------------------------------
C. View fit results using AmpPlotter GUI package
-------------------------------------------------

1.  To view the results of the fit we will use the threepi_plotter
    application.  The application generates plots of data and
    the Monte Carlo that has been weighted by the intensity
    determined by the fit.  First examine the source code of
    threepi_plotter.  The only relevant items are the definition
    of the PlotGenerator (in the example ThreePiPlotGenerator) and
    the registration of the amplitudes and data readers used in
    the fit.
    
2.  The definition of ThreePiPlotGenerator can be found in the library
    AMPTOOLS_DATAIO.  The source for this class specifies which 1D
    histograms to create and how to fill these histograms for a single
    event.  It is useful to look at distributions that can be used to
    distinguish the various physics amplitudes like invariant masses
    and angles.
    
3.  Start up the plotter GUI, passing in the fit results on the command
    line:

    threepi_plotter  threepi.fit

    The GUI allows you to turn off and on amplitudes, select a projection
    and generate plots of data and/or MC.  Access to both the accepted
    and generated MC is provided so the acceptance can be studied in
    the variables of interest.