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5. generate events

chapter four

5.1 generating events in the interactive mode

5.1.1 prepare for the actual event generation

After issuing the command of generating a process, one can directly issue the event generation, with the command launch:

./bin/mg5_aMC
MG5_aMC> generate p p > z
launch

The command launch automatically creates an output folder. From the output folder, the configuration files are then read:

INFO: load configuration from /.../MGTutorial/MG5_aMC_v2_7_2/PROC_sm_0/Cards/me5_configuration.txt  
INFO: load configuration from /.../MGTutorial/MG5_aMC_v2_7_2/input/mg5_configuration.txt  
INFO: load configuration from /.../MGTutorial/MG5_aMC_v2_7_2/PROC_sm_0/Cards/me5_configuration.txt

And the current generation configuration is shown to the user:

The following switches determine which programs are run:
/=================== Description ===================|============= values ==============|======== other options ========\
| 1. Choose the shower/hadronization program        |        shower = Not Avail.        |     Please install module     |
| 2. Choose the detector simulation program         |      detector = Not Avail.        |     Please install module     |
| 3. Choose an analysis package (plot/convert)      |      analysis = Not Avail.        |     Please install module     |
| 4. Decay onshell particles                        |       madspin = OFF               |     ON|onshell                |
| 5. Add weights to events for new hypp.            |      reweight = OFF               |     ON                        |
\=======================================================================================================================/

5.1.2 parameters setting and event generation

  • Before triggering the actual event generation, one can change the values of parameters: 
    > set ebeam1 4000
> set ebeam2 4000
> done
  • The instruction done will let the event generation begin. The first phase will be compiling the code for the event generation, the second phase will be the actual production of events (as many as marked in the run_card.dat file).
    • In cases of sophisticated processes, MG may not be able to produce many events in a single run. In this case, multiple runs with smaller events is the way to go.
  • At the end of the generation, the total cross section of the process in displayed: 
    === Results Summary for run: run_01 tag: tag_1 ===

   Cross-section :   2.437e+04 +- 23.46 pb
   Nb of events :  10000
  • All the information is summarised in the index.html file present in the generation folder, as also prompted by MG: 
    more information in /.../MGTutorial/MG5_aMC_v2_7_2/PROC_sm_0/index.html

5.1.3 datacards editing in interactive mode

  • in interactive mode, after issuing the instruction launch, it is possible to directly edit the run_card.dat and param_card.dat files: 
    Do you want to edit a card (press enter to bypass editing)?
/------------------------------------------------------------\
|  1. param : param_card.dat                                 |
|  2. run   : run_card.dat                                   |
\------------------------------------------------------------/
  • By typing 1 or 2, the corresponding datacard is opened in the editor of choice for the user to modify it. Saving the file propagates any modifications.
  • The instruction done will let the event generation begin.

5.2 generating events with the command line

5.2.1 launching the generation

The folder created by MG for the process generation contains an executable that runs the event production, with the input parameters read from the datacards present in the Cards subfolder:

cd test_Z
./bin/generate_events

This command will prompt the user with the same interface of the interactive event generation.

5.2.2 running the generation without answering questions

The answers to the interactive questions may be passed as input to the generate_events program:

printf "0\n done\n" | ./bin/generate_events

Additional instructions may be added:

printf "0\n set ebeam1 4000\n set ebeam2 4000\n done\n" | ./bin/generate_events

5.3 the output of the event generation

  • Every time a generation run is executed, a subfolder in the Events folder is created:
ls Events/
run_01  run_02  run_03
  • In each folder, the events compressed (unweighted_events.lhe.gz) and the summary of the running conditions (run_01_tag_1_banner.txt) are saved:
ls Events/run_01/
run_01_tag_1_banner.txt  unweighted_events.lhe.gz
  • At the end of the *banner.txt file, the resulting cross-section is reported:
<MGGenerationInfo>
#  Number of Events        :       10000
#  Integrated weight (pb)  :       42107.0
</MGGenerationInfo>
  • The value of the cross-section (42107) and its numerical uncertainty (39.4) in pb can also be found in the HTML folder, for each run:
cat HTML/run_01/results.html  | grep "(pb)"
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<b>s= 42107 &#177 39.4 (pb)</b><br><br>

chapter six