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Showering and hadronization
To perform simulation of TMD parton showers and/or hadronization one
has to process the PEGASUS output in Les Houches Event format (*.lhe
) with Monte-Carlo event generator CASCADE.
In the collinear factorization scenario, one can use a conventional showering program as implemented into the PYTHIA 8.2.
NOTE: to reconstruct the TMD initial state parton showers, the TMD gluon density from CCFM or PB families should be chosen.
To produce an Les Houches Event file with PEGASUS please follow the manual. Shortly,
one should choose the process by double-clicking, limit the phase space in order to record only necessary events, and check the “Generate Les Houches Event file” box in the lower left corner. One then starts the calculation with the Start button (or F5). The produced *.lhe
file will be created in the PEGASUS folder
and is ready to use with the CASCADE.
Here we describe the procedure for 2.4 version.
As a basic file one can take the steer_pp-LHEin
file, provided with the distributive. Just check, that the following lines are set as needed:
NEVENT -1 'IPRO' 1 0 -1 ! read in LHE file
Other parameters define, in particular, whether you turn the fragmentation and showers on or off. You can set them as you need. Importantly, one has to set the IGLU
parameter corresponding to the TMD gluon density, used in the *.lhe
file calculation. The TMD number can be found in the *.lhe
file, it corresponds to the TMDLib numbering scheme. PEGASUS uses the following of them (see Overview). The ICCF
parameter should be defined according to the used TMD gluon distribution.
PEGASUS | CASCADE/TMDLib | IGLU | ICCF |
---|---|---|---|
LLM'2022 (CCFM) | LLM-2022 | 101601 | 1 |
A0 (CCFM) | ccfm-setA0 | 101010 | 1 |
B0 (CCFM) | ccfm-setB0 | 101020 | 1 |
JH'2013 set 1 (CCFM) | ccfm-JH-2013-set1 | 101201 | 1 |
JH'2013 set 2 (CCFM) | ccfm-JH-2013-set2 | 101301 | 1 |
PB NLO set 1 | PB-NLO-HERAI+II-2018-set1 | 102100 | 2 |
PB NLO set 2 | PB-NLO-HERAI+II-2018-set2 | 102200 | 2 |
The path to the *.lhe
file to be processes is defined by CLHE
.
NOTE:
ILHE
parameter should be set to be 0 for the off-shell partons.
Typically one produces a HepMC output file with CASCADE. One defines the name of the file with
export HEPMCOUT=name_of_the_output_file
Then one starts the CASCADE with
export CASEED=some_random_number ./cascade < steer_pp-LHEin
The produced HepMC file can be then processed to form necessary cross sections. Note that the weighted events from PEGASUS are normalized by CASCADE, so one should multiply the weights by the total cross section during processing. The total cross section can be found in the CASCADE output or in the *.lhe
file.
In the collinear QCD factorization scenario, we successfully tested the compatibility of
generated *.lhe
file with PYTHIA 8.2.
One can use the following page as a guide to link *.lhe
file with PYTHIA 8.2.