THEORY

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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 [[https://arxiv.org/abs/hep-ph/0609017|Les Houches Event format]] (''*.lhe'') with  Monte-Carlo event generator [[https://cascade.hepforge.org|CASCADE]].
In the collinear factorization scenario, one can use a conventional showering program as implemented into the [[http://home.thep.lu.se/Pythia|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 [[https://arxiv.org/abs/hep-ph/0609017|Les Houches Event]] file with PEGASUS please follow the [[pegasus:manual|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 [[https://cascade.hepforge.org|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:

<code>
NEVENT -1
'IPRO'     1       0        -1      ! read in LHE file 
</code>

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 [[https://tmdlib.hepforge.org/doxy/html/pdfsets.html|TMDLib numbering scheme]]. PEGASUS uses the following of them (see [[pegasus:overview|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 [[https://www.sciencedirect.com/science/article/pii/S0010465500001892?via%3Dihub | HepMC]] output file with CASCADE. One defines the name of the file with 

<code>
export HEPMCOUT=name_of_the_output_file
</code>

Then one starts the CASCADE with
<code>
export CASEED=some_random_number
./cascade < steer_pp-LHEin
</code>

The produced [[https://www.sciencedirect.com/science/article/pii/S0010465500001892?via%3Dihub | 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 [[http://home.thep.lu.se/Pythia|PYTHIA 8.2]]. 
One can use the [[http://home.thep.lu.se/~torbjorn/pythia82html/Welcome.html|following page]] as a guide to link ''*.lhe'' file with [[http://home.thep.lu.se/Pythia|PYTHIA 8.2]].