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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 a 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:

'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.

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.

pegasus/showering.txt · Last modified: 11/09/2023 09:13 by lipatov