PEPSI

Note: By now it is better to use DJANGOH for polarized studies as it is the more complete generator

• PEPSI
• Parton distribution functions
  • PEPSI processes important in ep
• Running PEPSI
  • Without radiative corrections
  • With radiative corrections
  • Output file structure
  • How to analyze events
• Installation
  • Changes made for more recent fortran versions
• Known Issues
• Table 1: Polarized parton distributions
• Table 2: Unpolarized parton distributions

PEPSI (Polarized Electron Proton Scattering Interactions) is a Monte Carlo generator for polarized deep inelastic scattering (pDIS). It is based on the LEPTO 6.5 Monte Carlo for unpolarized DIS.

See L. Mankiewicz, A. Schäfer and M. Veltri, Comp. Phys. Comm. 71, 305-318 (1992), PEPSI.paper.pdf

Parton distribution functions

The distribution function to use in polarized leptoproduction is set via the variable LST(15) in the LEPTO COMMON block /LEPTOU/. Table 1 and Table 2 list internal allowed values of LST(15) for polarized and unpolarized distributions respectively.

Pepsi is linked with the pdflib such that all PDFs included in there can be used by setting LST(15) to the respective PDF-ID

PEPSI processes important in ep

QCDC: QCD-Compton, radiation of a gluon from incoming or outgoing quark lines
PGF: Photon Gluon Fusion

Running PEPSI

In the standard setup in the singularity cvmfs environment or at BNL, the code can be found in

$EICDIRECTORY/PACKAGES/pepsi

The executables are in the same directory and called pepsieRHICnoRAD and pepsieRHICwithRAD, respectively. There are several steer files (named input.data.XXXXX.eic) provided in this directory to run PEPSI and get reasonable output.

PEPSI has to be run twice if polarized asymmetries should be generated, once for parallel, lepton and proton beam spin direction, and once antiparallel. Charge Current events can only be generated in the unpolarized mode. The LST(8) can only be different from 0 or 1 in the unpolarized mode.

Note that the executables expect the pdf/ directory in the directory of execution. Easiest way to achieve this is a softlink (adapt to your location)

ln -s $EICDIRECTORY/PACKAGES/PEPSI/pdf

Without radiative corrections

Choose or create a steering file. Some that are provided in $EICDIRECTORY/PACKAGES/pepsi/STEER-FILES:

• input.data_noradcor.eic.pol.anti is an example to run PEPSI with settings tuned for Hermes, and/or H1 and ZEUS for the antiparallel polarized cross-section
• input.data_noradcor.eic.pol.par: for the parallel polarized cross-section
• input.data_noradcor.eic.unpol: for the unpolarized cross-section

You can then run:

./pepsieRHICnoRAD < STEER-FILES/input.EW_noradcor.eic.posi.test > XXX.log

where the output redirect to XXX.log is optional.

With radiative corrections

Note: DO NOT USE radiative corrections. Currently this is no longer supported. See Known Issues

Output file structure

The output file is in a text format which has the following structure:

• 1st line: PEPSI EVENT FILE
• 2nd line: ============================================
• 3rd line: Information on event wise variables stored in the file:

• 4th line: ============================================
• 5th line: Information on track-wise variables stored in the file:

• 6th line: ============================================
• 7th line: event information for first event
• 8th line: ============================================
• 9th to X-1 line: track-wise info of 1st event
• Xth line ============================================

For each subsequent event, lines 7 through X repeat analogously .

How to analyze events

The recommended way is to create and use a ROOT tree with the BuildTree function and other tools provided by eic-smear. Some guidelines regarding Monte Carlo normalization can be found here.

Installation

It is recommended to take advantage of the pre-installed versions on the lab farms or the available stand-alone singularity or escalate containers.

However, the package can also be built using “make”. The Makefile should be customized to your environment before using, specifically you need to adapt the “install” target and point to a compatible CERNLIB installation.

Ignore warnings of the form:

 Warning: $ should be the last specifier in format

Which should be okay (it is a g77 extension allowed by gfortran). As well as:

Warning: Deleted feature: PAUSE statement at (1)

This feature is deleted in F95; here, it should eventually be replaced by write() + read().

Changes made for more recent fortran versions

Multiple warnings like this:

     >   './pdf/cteq5hj.tbl',                                           
          1
Warning: Initialization string starting at (1) was truncated to fit the variable (16/17)

indicate a too short variable length. Changed to

      Character Flnm(Isetmax)*100

in line 6 (of setctq5.F).

Known Issues

Note: DO NOT USE radiative corrections. Currently does not work.

If radiative corrections are restored, this would be the procedure to use them:

• 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

   pepsieRHICwithRAD < input.data_make-radcor.eic.unpol
  

  or you can use one of the files already generated in the directory $EICDIRECTORY/PACKAGES/pepsi/radgen 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 run

   pepsieRHICwithRAD < input.data_radcor.eic.unpol > XXX.log
  

However, as noted above, they cannot currently be used.

./pepsieRHICwithRAD < STEER-FILES/input.data_radcor.eic.pol.anti

At line 514 of file pepsiMaineRHIC_radcorr.v2.F (unit = 29, file =
'pepsi.ep.4x100.1Mevents.pol-anti.RadCor=1.txt')
Fortran runtime error: Expected REAL for item 42 in formatted
transfer, got INTEGER

Details about radiative corrections can be found here.

Table 1: Polarized parton distributions

Table 2: Unpolarized parton distributions