Simulation - a generator for the hadronic background

[gen (Eugene Chudakov) <gen@jlab.org>]

The new generator does the following:
1) Generated the photon energy in a given energy range. The coherent 
   Bremsstrahlung spectrum multiplied by the total photoproduction cross 
   section is used. The energy range can be set within an interval
   0.15-12 GeV 
2) For E>3 GeV generates a PYTHIA event.
3) For E<3 GeV generates an event from a list of 10 processes dominating
   in the given energy range. The cross sections and kinematical
   distributions have been tuned to match the existing data.

The generated events are written out in the HDDM format in order to feed them
to HDGeant and as an NTUPLE to make quick studies of various spectra.

Most of the input parameters can be set/adjusted using several external files.
Normally, there is no need to change the code unless one wants to add
another process.

More info can be found in a README file in the top directory.

I have committed this generator into the software repository (directory 
Simulation/bggen). One can check it out:
svn list https://halldsvn.jlab.org/repos/trunk/src/programs/Simulation/bggen
cd (Your location)/Simulation/
svn checkout https://halldsvn.jlab.org/repos/trunk/src/programs/Simulation/bggen

  Make sure that you have the standard env. variables initialized
  (see http://www.jlab.org/Hall-D/software/wiki/index.php/Getting_started_with_GlueX_Software)

cd bggen
cd code
make

Now, you can run the code, using the default control parameters.
cd ../run/
../code/.bin/(your_platform)/bggen

It will simulate 395k events in a range 0.15-12. GeV 
(the expected hadronic rate is 395kHz at high luminosity). 
It will take a few minutes.

The final printout should look like:

 ==========================================================================
 Events Simulated:    395000     Reference interaction rate:      394.67 kHz
  process                    events     fraction   range
 --------------------------------------------------------------------------
      0  PYTHIA               82679      20.9 %     3.00<E<12.00 GeV 
 --------------------------------------------------------------------------
      1  p pi0                87584      22.2 %     0.15<E< 3.00 GeV 
      2  n pi+               116372      29.5 %                      
      3  p pi+ pi-            13232       3.3 %                      
      4  p rho0                9988       2.5 %                      
      5  Delta++ pi-          26903       6.8 %                      
      6  p pi0 pi0             6802       1.7 %                      
      7  n pi+ pi0            31892       8.1 %                      
      8  p eta                 2422       0.6 %                      
      9  p pi+ pi- pi0        12633       3.2 %                      
     10  n pi+ pi+ pi-         4493       1.1 %                      
 --------------------------------------------------------------------------

Eugene