Difference between revisions of "PEPSI"

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====By now it is better to use DJANGOH for polarised studies as it is the more complete generator====
This page is now maintained at [https://eic.github.io/software/pepsi.html https://eic.github.io/software/pepsi.html].
PEPSI (Polarised Electron Proton Scattering Interactions) [[#Ref1|[1]]] is a Monte Carlo generator for polarised deep inelastic scattering (pDIS). It is based on the [http://www.isv.uu.se/thep/lepto/ LEPTO] 6.5 Monte Carlo for unpolarised DIS.
== PEPSI References ==
<span id="Ref1">(1)</span> ''"PEPSI - a Monte Carlo generator for polarized leptoproduction"'', L. Mankiewicz, A. Schäfer and M. Veltri, Comp. Phys. Comm. '''71''', 305-318 (1992) [https://wiki.bnl.gov/eic/index.php/File:PEPSI.paper.pdf PEPSI.paper.pdf].
==Parton distribution functions==
The distribution function to use in polarised leptoproduction is set via the variable <tt>LST(15)</tt> in the LEPTO <tt>COMMON</tt> block <tt>/LEPTOU/</tt>. [[PEPSI-pdf | Tables 1 and 2]] list internal allowed values of <tt>LST(15)</tt> for polarised and unpolarised distributions respectively.
Pepsi is linked with the pdflib such all PDFs included in there can be used by setting <tt>LST(15)</tt> to the respective PDF-ID
==== PEPSI 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" | DIRECT
!γ∗q → q
!γ∗ q → qg
!γ∗ g → q qbar
QCDC: QCD-Compton, radiation of a gluon from incoming or outgoing quark lines<br>
PGF: Photon Gluon Fusion<br>
== Running PEPSI ==
the code can be found on the afs directory for EIC at BNL
this code is based on PEPSI/LEPTO and was modified to include radiative corrections using [[#Documentation on Radiative Corrections: | RadGen]].<br>
The main program is in the same directory and called pepsiMaineRHIC_noradcorr.f / pepsiMaineRHIC_radcorr.f, several other routines are needed, which are in the same directory.<br>
The executable is in the same directory and called pepsieRHICnoRAD / pepsieRHICwithRAD<br>
There are several steer files (named: input.data. XXXXX.eic) provided in this directory to run PEPSI and get reasonable output.
=== How to Run the Code ===
PEPSI has to be run twice if polarized asymmetries should be generated, once for parallel, lepton and proton beam spin direction, and once antiparallel.<br>
Charge Current events can only be generated in the unpolarized mode.<br>
The LST(8) can only be used different from 0 or 1 in the unpolarised mode.
==== without radiative corrections ====
  pepsieRHICwithRAD < input.data_noradcor.eic.pol.anti > XXX.log 
input.data_noradcor.eic.pol.anti is one of the steer file examples in the directory to run PEPSI with settings tuned for Hermes, and/or H1 and ZEUS for the antiparallel polarized cross-section<br>
input.data_noradcor.eic.pol.par: for the parallel polarized cross-section<br>
input.data_noradcor.eic.unpol: for the unpolarised cross-section
==== 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>
    pepsieRHICwithRAD < input.data_make-radcor.eic.unpol
* or you can use one of the files already generated<br>
the files are in the directory
and called xytab1unp.04.050.dat or xytab1ant.04.050.dat or xytab1par.04.050.dat
* to run the code than with radiative corrections simply change the steer file to either
input.data_radcor.eic.unpol or input.data_radcor.eic.pol.par or .... and
  type pepsieRHICwithRAD < input.data_radcor.eic.unpol > XXX.log
=== Output file structure ===
the output file is in a text format which has the following structure.<br>
* 1st line:  PEPSI EVENT FILE
* 2nd line: "============================================"
* 3rd line: Information on event wise variables stored in the file
{| class="wikitable" style="text-align:left" border="1" cellpadding="2" cellspacing="0"
| I: || 0 (line index)
| ievent: || eventnumber running from 1 to XXX
| genevent: || trials to generate this event
| process: || pepsi subprocess (LST(23)), for details see table above
| subprocess: || pythia subprocess (LST(24)), for details see table above
| nucleon: || hadron beam type (LST(22))             
| struckparton: || parton hit in the target (LST(25))
| partontrack: || # or parton track (LST(26))
| 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
| mcfixedweight || weight calculated from generation limits
| weight || total weight including everything
| dxsec || cross section included in the weight
| mcextraweight || Pepsi total cross section in pb from numerical integration  parl(23)
| dilut, F1, F2, A1, A2, R, Depol, d, eta, eps, chi || true variables needed to calculate g1
| gendilut, genF1, genF2, genA1, genA2, genR, genDepol, gend, geneta, geneps, genchi  || variables needed to calculate g1
| SigRadCor: || information used and needed in the radiative correction code
| EBrems: || energy of the radiative photon in the nuclear rest frame
| nrTracks: || number of tracks in this event, includes also virtual particles
* 4th line: "============================================"
* 5th line: Information on track wise variables stored in the file
{| class="wikitable" style="text-align:left" border="1" cellpadding="2" cellspacing="0"
| 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
* 6th line: "============================================"
* 7th line: event information for first event
* 8th line: "============================================"
* 9th to X-1 line: trackwise info of 1st event
* Xth line "=============== Event finished ==============="
'''the information from line 7 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 (still under construction)=====
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
[[Category:Event Generators]]

Latest revision as of 13:42, 9 June 2020

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