PEPSI


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

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
Subprocess # Description
DIRECT    
γ*q → q 1 LO DIS
γ*q → qg 2 QCDC
γ*g → q qbar 3 PGF


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:

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:

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

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


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:

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

LST(15) Polarized parton distribution function
101 Schaefer, Phys. Lett. B 208,2 (1988) 175 for comparison with older PEPSI versions
102 free
103 free
104 Schaefer et al hep-ph/9505306 using Glueck et al. Z. Phys. C 53 (1992) 127
105 Bartelski et al hep-ph/9502271 Set II(p,n)
106 Bartelski et al hep-ph/9502271 Set II(P,n)
107 Gehrmann hep-ph/9512406 Gluon A (NLO) + DGLAP
108 Gehrmann hep-ph/9512406 Gluon A (NLO) + DGLAP
109 Gehrmann hep-ph/9512406 Gluon A (NLO) + DGLAP
110 Gehrmann et al hep-ph/9512406 Gluon A (LO)
111 Gehrmann et al hep-ph/9512406 Gluon B (LO)
112 Gehrmann et al hep-ph/9512406 Gluon C (LO)
113 Gehrmann et al hep-ph/9512406 Gluon A (LO) + (DGLAP)
114 Gehrmann et al hep-ph/9512406 Gluon B (LO) + (DGLAP)
115 Gehrmann et al hep-ph/9512406 Gluon C (LO) + (DGLAP)
116 M. Glueck, E. Reya, M. Stratmann and W. Vogelsang, DO-TH 95/13, RAL-TR-95-042 ‘standard’ scenario, next-to-leading order
117 M. Glueck, E. Reya, M. Stratmann and W. Vogelsang, DO-TH 95/13, RAL-TR-95-042 ‘valence’ scenario, next-to-leading order
118 M. Glueck, E. Reya, M. Stratmann and W. Vogelsang, DO-TH 95/13, RAL-TR-95-042 ‘standard’ scenario, leading order
119 M. Glueck, E. Reya, M. Stratmann and W. Vogelsang, DO-TH 95/13, RAL-TR-95-042 ‘valence’ scenario, leading order
120 Stanley J.Brodsky Nucl.Phys. B441(1995)
121 S.Keler & J.F.Owens Phys.Lett. B266(1991) & Phys.Rev. D19(1994)1199
124 D. de Florian et al., hep-ph/9711440 LO set 1
125 D. de Florian et al., hep-ph/9711440 LO set 2
126 D. de Florian et al., hep-ph/9711440 LO set 3
127 D. de Florian et al., hep-ph/9711440 NLO set 1
128 D. de Florian et al., hep-ph/9711440 NLO set 2
129 D. de Florian et al., hep-ph/9711440 NLO set 3
130 Fake sample: unpolarized Gehrmann et al hep-ph/9512406 with Delta u(x) = 0.5 * u(x) and Delta d(x) = 0.
131 Fake sample: unpolarized Gehrmann et al hep-ph/9512406 with Delta d(x) = 0.5 * d(x) and Delta u(x) = 0.
132 fit routine. (Is outside the official code.)
133 CTEQ4LQ . UNPOL: Low Q2 parametrization is the only one used here. POL: BOGUS, du=0.5* u(x) dd=-0.3*d(x) 0.0 all else
137 MRS low Q2
144 grsv2000 hep-ph/0011215 LO standard scenario
145 grsv2000 hep-ph/0011215 LO valence scenario
146 grsv2000 hep-ph/0011215 NLO standard scenario
147 grsv2000 hep-ph/0011215 NLO valence scenario


Table 2: Unpolarized parton distributions

LST(15) Unpolarized parton distribution function
150 cteq5l LO
151 cteq5m NLO MSBAR
152 cteq5m1 NLO MSBAR (update)
161 mrs99 cor01 central gluon, a_s
162 mrs99 cor02 higher gluon
163 mrs99 cor03 lower gluon
164 mrs99 cor04 lower a_s
165 mrs99 cor05 higher a_s
166 mrs99 cor06 quarks up
167 mrs99 cor07 quarks down
168 mrs99 cor08 strange up
169 mrs99 cor09 strange down
170 mrs99 cor10 charm up
171 mrs99 cor11 charm down
172 mrs99 cor12 larger d/u
173 cteq6l LO
174 cteq6d DIS NLO
175 cteq6m NLO MSBAR