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== Documentation on PYTHIA: ==
This page is now maintained at [https://eic.github.io/software/pythia6.html https://eic.github.io/software/pythia6.html].
* for ep only versions of PYTHIA 6.4 can be used
* its www-page http://home.thep.lu.se/~torbjorn/pythiaaux/recent.html
* the manual http://home.thep.lu.se/~torbjorn/pythia/lutp0613man2.pdf
==== Pythia processes important in ep ====
{| class="wikitable" style="text-align:left" border="1" cellpadding="3" cellspacing="0"
! style="background:#ffdead;" |Subprocess
! style="background:#ffdead;" |#
! style="background:#ffdead;" |Description
|colspan="3" align="center"| soft VMD
!V N → V N
!elastic VMD
!V N → V X
!single-diffractive VMD
!V N → XN
!single-diffractive VMD
!V N → XX
!double-diffractive VMD
!V N → X
!soft non-diffractive VMD low-pT
|colspan="3" align="center" | QCD 2→2
!semihard QCD 2→2
|colspan="3" align="center" | RESOLVED (hard VMD and anomalous)
!qq → qq
!QCD 2 → 2(q)
!q qbar → q qbar
!q qbar → gg
!gq → gq
!qg → qg
!QCD 2 → 2(g)
!gg → q qbar
!gg → gg
|colspan="3" align="center" | DIRECT
!γ∗q → q
!γ∗T q → qg
!(transverse) QCDC
!γ∗L q → qg
!(longitudinal) QCDC
!γ∗T g → q qbar
!(transverse) PGF
!γ∗L g → q qbar
!(longitudinal) PGF
VMD: Vector Meson Dominance, describing the elastic diffractive production of Vector Mesons<br>
QCDC: QCD-Compton, radiation of a gluon from incoming or outgoing quark lines<br>
PGF: Photon Gluon Fusion<br>
== Running PYTHIA ==
the code can be found on the afs directory for EIC at BNL
this is code is based on PYTHIA 6.4.13 and was modified to include radiative corrections using [[#Documentation on Radiative Corrections: | RadGen]].<br>
The main program is in the same directory and called pyMaineRHIC_v1.f, several other routines are needed which are in the same directory.<br>
The executable is in the same directory and called pythiaeRHIC<br>
There are several steer files (named: input.data. XXXXX.eic) provided in this directory to run PYTHIA and get reasonable output
=== How to Run the Code ===
==== without radiative corrections ====
  pythiaeRHIC < input.data_noradcor.eic > XXX.log 
input.data_noradcor.eic is one of the steer file examples in the directory to run PYTHIA with settings tuned for Hermes, and/or H1 and ZEUS.<br>
If you want to run only elastic vector meson production the example steer file is "input.data_noradcor.VM.eic"
==== with radiative corrections ====
* create a directory called radgen in the area you want to run the code
* you either need to generate the lookup table for your cuts and beam energy settings first <br>
    pythiaeRHIC < input.data_make-radcor.eic
* or you can use one of the files already generated<br>
the directories are in
and called radgen(ebeamB)x(pbeamE), like  radgen10x100 or radgen4x100 or .......
* to run the code than with radiative corrections simply change the steer file to either
    input.data_radcor.eic or input.data_radcor.VM.eic and type pythiaeRHIC < input.data_radcor.eic > XXX.log
=== Output file structure ===
the output file is in a text format which has the following structure.<br>
* 1st line:  PYTHIA EVENT FILE
* 2nd line: "============================================"
* 3rd line: information which is valid for the event <br>
| I: || 0 (line index)
| ievent: || eventnumber running from 1 to XXX
| genevent: || trials to generate this event
| subprocess: || pythia subprocess (MSTI(1)), for details see table above
| nucleon: || hadron beam type (MSTI(12))               
| targetparton: || parton hit in the target (MSTI(16))
| xtargparton: || x of target parton (PARI(34))
| beamparton: || in case of resolved photon processes and soft VMD the virtual photon has a hadronic structure. This gives the info which parton interacted with the target parton (MSTI(15))
| xbeamparton: || x of beam parton (PARI(33))             
| thetabeamparton: || theta of beam parton (PARI(53))
| truey, trueQ2, truex, trueW2, trueNu: || are the kinematic variables of the event.
| || If radiative corrections are turned on they are different from what is calculated from the scattered lepton.
| || If radiative corrections are turned off they are the same as what is calculated from the scattered lepton
| leptonphi:|| phi of the lepton (VINT(313)) 
| s_hat: || shat of the process (PARI(14))
| t_hat: || Mandelstam t (PARI(15))
| u_hat: || Mandelstm u (PARI(16))
| pt2_hat: || pthat^2 of the hard scattering (PARI(18))
| Q2_hat: || Q2hat of the hard scattering (PARI(22)),
| F2, F1, R, sigma_rad, SigRadCor: || information used and needed in the radiative correction code
| EBrems: || energy of the radiative photon in the nuclear rest frame
| photonflux: || flux factor from PYTHIA (VINT(319))
| nrTracks: || number of tracks in this event, includes also virtual particles
* 4th line: "============================================"
* 5th line: now start the track wise information.
  I: line index, runs from 1 to nrTracks 
  K(I,1): status code KS (1: stable particles      11: particles which decay        55; radiative photon)
  K(I,2): particle KF code (211: pion, 2112:n, ....)
  K(I,3): line number of parent particle
  K(I,4): normally the line number of the first daughter; it is 0 for an undecayed particle or unfragmented parton
  K(I,5): normally the line number of the last daughter; it is 0 for an undecayed particle or unfragmented parton.       
  P(I,1): px of particle 
  P(I,2): py of particle 
  P(I,3): pz of particle
  P(I,4): Energy of particle
  P(I,5): mass of particle 
  V(I,1): x vertex information 
  V(I,2): y vertex information 
  V(I,3): z vertex information
* Xth line "=============== Event finished ==============="
the information from line 3 to X repeats for each event.
=== How to analyze events ===
* create a root tree
there are root macros available to convert the output txt-files into root trees.<br>
Details how to run the macros can be found [https://wiki.bnl.gov/eic/index.php/ROOT [here]]
===== MC normalization=====
to normalize your counts to cross section you need two informations
* the total number of trials (NGEN(0,3)), it is printed to the screen/logfile if PYTHIA finishes
* the total integrated cross section (PARI(1)), the unit is microbarn (10^-6), it is printed to the screen/logfile if PYTHIA finishes
==> count * total integrated cross section /total number of trials
to calculate the corresponding luminosity
==> total number of trials/ total integrated cross section
== Documentation on Radiative Corrections: ==
the code implemented in PTHIA to calculate radiative corrections is called RADGEN<br>
The writeup on it can be found here [http://arXiv.org/pdf/hep-ph/9906408 [hep-ph/9906408]]<br>
The following steps have been done to implement it in PYTHIA:
* change the subroutine pygaga.f so it calls radgen after you have thrown y and Q2
* get the true y and Q2 from radgen and the radiated photon
* Pythia will continue to now generate an event based on this y and Q2
* Pythia still operates under accept reject, the extra weigt from the radiative corrections is absorbed in the flux factor
=== Additional Info on radiative corrections ===
www-pages with codes
* [http://www.jlab.org/RC/ [http://www.jlab.org/RC/]]
* [http://www.desy.de/~heramc/mclist.html [http://www.desy.de/~heramc/mclist.html]]
* Nuclear beams in HERA.
M. Arneodo, (Turin U. & INFN, Cosenza) , A. Bialas, (Jagiellonian U.) , M.W. Krasny, (Paris U., VI-VII & Paris U., VI-VII) , T. Sloan, (Lancaster U.) , M. Strikman, (Penn State U.) . Sep 1996. 40pp.
To be published in the proceedings of Workshop on Future Physics at HERA,
Hamburg, Germany, 25-26 Sep 1995.
In *Hamburg 1995/96, Future physics at HERA* 887-926.
e-Print: hep-ph/9610423
and the references 5 and 6 in this article
to find them check [http://www.desy.de/~heraws96/ [here]]

Latest revision as of 14:06, 9 June 2020

This page is now maintained at https://eic.github.io/software/pythia6.html.