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Running gevgen_fnal

The gevgen_fnal GENIE standalone application is a means of generating events in a realistic geometry with a realistic flux (this variant supports GSimpleNtpFlux, GNuMIFlux, GDk2NuFlux, and simple 1-d flavor specific flux histograms).

Setup

# do some UPS setups here

Using the UPS products for genie and genie_xsec make using GENIE easy. The use of genie_xsec in particular helps to ensure consistency in the spline files and the (possible) relevant modified configuration files

Identifying Relevant Global Parameters

export GEOMETRY="mylardetector.gdml"       # or whatever (.root is okay too)
export TOPVOL="vWorld"                     # top volume in which to generate
export GPREFIX="xyz"                       # file naming prefix

# GENIEXSECFILE is defined by UPS  setup genie_xsec ...

export GENLIST="Default"       # default generator list (no MEC) see below
                               # but might be "Default+CCMEC+NCMEC" or some such

export TUNE_FLAG="" 
# for GENIE v3 and beyond you  must also specify a tune
# if you setup a genie_xsec product you can use
export TUNE_FLAG="--tune ${GENIE_XSEC_TUNE}" 

# quiet GENIE down ... by default it is _quite_ verbose
# this file should either be in a genie_xsec or genie_phyopt added path
# as part of $GXMLPATH cascade of locations
export MSGLVL=Messenger_production.xml
# or
 export MSGLVL=Messenger_laconic.xml
# or
export MSGLVL=Messenger_whisper.xml

use of GSimpleNtpFlux

export FLUXFILE="myfakeflux.gsimple.root"  #
export DETLOC="NOTHINGSPECIAL"             # not relevant for gsimple flux files
                                           #   but needs to be non-blank

use of GDk2NuFlux (and GNuMIFlux)

export FLUXFILE="myfakeflux.dk2nu.root"  #   or "myflugg.root" or "myg4numi.root" 
export DETLOC="MyParamSetName"           # <param_set> name in first GNuMINtuple.xml file found

Run Specific Parameters

export run=12345   # arbitrary run number

# one can either run for 
#   a fixed # of protons-on-target (for a sensible flux file)
#   a fixed # of generated events
# but not both 

# export EXPOSURE="-e 7.5e13"   # POT exposure
export EXPOSURE="-n 1000"     # number of events

General, little known but important, note:

The ROOT TTree gtree in the "gntp.*.ghep.root" file contains within it a critical bit of information:
gtree->GetWeight() returns the POTs used in generating the file's events
irrespective of whether then stopping condition was number of events or a POTs limit.

Running the Executable to Explore the Geometry

To do event generation one must "explore" the geometry in order to determine an upper-limit on the material pathlengths. This is used to scale the cross-sections to make generation efficient. One can do this each individual run, but it is most efficient to do this once and write out the result for input to actual generation.

The following will generate a file named mysetup.maxpl.xml by running 25000 flux rays through the geometry. This will then be used as input for all further generation.

gevgen_fnal \
   -f ${FLUXFILE},${DETLOC} \
   -g ${GEOMETRY} \
   -t ${TOPVOL} \
   -m "+mysetup.maxpl.xml" \
   -S "+25000" \
   -L "cm" -D "g_cm3" \
   -n 1 \
   --seed ${run} \
   -r 9999 \
   -o xyzzy \
   --cross-sections ${GENIEXSECFILE} \
   --event-generator-list ${GENLIST} ${TUNE_FLAG} \
   --message-thresholds ${MSGLVL} 

Running the Executable to Generate Event Sample

gevgen_fnal \
   -f ${FLUXFILE},${DETLOC} \
   -g ${GEOMETRY} \
   -t ${TOPVOL} \
   -m "mysetup.maxpl.xml" \
   -L "cm" -D "g_cm3" \
   ${EXPOSURE} \
   --seed ${run} \
   -r ${run} \
   -o ${GPREFIX} \
   --cross-sections ${GENIEXSECFILE} \
   --event-generator-list ${GENLIST} ${TUNE_FLAG} \
   --message-thresholds ${MSGLVL} \
   --event-record-print-level 0

The output file will take the form ${GPREFIX}.gntp.${run}.ghep.root

Other links

https://cdcvs.fnal.gov/redmine/projects/nutools/wiki
https://cdcvs.fnal.gov/redmine/projects/nutools/wiki/GENIE_Configuration_Files


Documentation Snippets from Source Code

          -f 
              Input 'neutrino flux'.
              This option can be used to specify any of:
              1 > A g[4]numi[_flugg] beam simulation output file 
                  and the detector location
                  The general sytax is:
                      -f /full/path/flux_file.root,detector,flavor1,flavor2...
                  [Notes] 
                  - For more information on the flux ntuples, see the gNuMI doc.
                  - The original hbook ntuples need to be converted to a ROOT 
                    format using the h2root ROOT utility.   
                  - If flavors aren't specified then use default (12,-12,14,-14)
                  - See GNuMIFlux.xml or Dk2Nu.xml for all supported detector 
                    locations
                  - The g[4]NuMI[_flugg] flux ntuples are read via GENIE's 
                    GNuMIFlux driver, and dk2nu file via an external 
                    product w/ the GDk2NuFlux driver (if it can be found).
                    This customized GENIE event generation driver passes-through
                    the complete gNuMI input flux information (eg parent decay 
                    kinematics / position etc) for each neutrino event it 
                    generates (as an additional 'flux' branch is added on the 
                    output event tree).
                  [Examples] 
                  - To use the gNuMI flux ntuple flux.root at MINOS near 
                    detector location, type:
                     '-f /path/flux.root,MINOS-NearDet'
              1a> Similar to 1 above, but filename contains "dk2nu", then use 
                  the GDk2NuFlux driver
              1b> Similar to 1 above, but filename contains "gsimple", then
                  use GSimpleNtpFlux driver
              2 > A set of histograms stored in a ROOT file.
                  The general syntax is:
                      -f /path/histogram_file.root,neutrino_code[histo_name],...
                  [Notes] 
                  - The neutrino codes are the PDG ones.
                  - The 'neutrino_code[histogram_name]' part of the option can
                    be repeated multiple times (separated by commas), once for
                    each flux neutrino species you want to consider, eg
                    '-f somefile.root,12[nuehst],-12[nuebarhst],14[numuhst]'
                  - When using flux from histograms then there is no point in
                    using a 'detailed detector geometry description' as your
                    flux input contains no directional information for those
                    flux neutrinos.
                    The neutrino direction is conventionally set to be 
                      +z {x=0,y=0}.
                    So, when using this option you must be using a simple 
                    'target mix'
                    See the -g option for possible geometry settings.
                    If you want to use the detailed detector geometry 
                    description then you should be using a driver that
                    supplies a detailed simulated beam flux.
                  - When using flux from histograms there is no branch with 
                    neutrino parent information added in the output event 
                    tree as your flux input contains no such information. 
                  - Note that the relative normalization of the flux histograms
                    is taken into account and is reflected in the relative 
                    frequency of flux neutrinos thrown by the flux driver
                  [Examples] 
                  - To use the histogram 'h100' (representing the nu_mu flux)
                    and the histogram 'h300' (representing the nu_e flux) and 
                    the histogram 'h301' (representing the nu_e_bar flux) from 
                    the flux.root file in /path/ type:
                      '-f /path/flux.root,14[h100],12[h300],-12[h301]

           -F 
              Apply a fiducial cut 
              Only used with ROOTGeomAnalyzer
              if string starts with "-" then reverses sense (ie. anti-fiducial)
           -S
              Number of rays to use to scan geometry for max path length
              Only used with ROOTGeomAnalyzer & GNuMIFlux/GDk2NuFlux
              +N  Use flux to scan geometry for max path length
              -N  Use N rays x N points on each face of a box
           -z 
              Z from which to start flux ray in user world coordinates
              Only use with ROOTGeomAnalyzer & GNuMIFlux/GDk2NuFlux
              If left unset then flux originates on the flux window
              [No longer attempts to determine z from geometry, generally
              got this wrong]
           -o 
              Sets the prefix of the output event file. 
              The output filename is built as: 
              [prefix].[run_number].[event_tree_format].[file_format]
              The default output filename is: 
              gntp.[run_number].ghep.root
              This cmd line arguments lets you override 'gntp'