PYTHIA6 with Radiative Corrections


Based on PYTHIA 6.4.28 with radiative corrections and output modifications to support eic-smear.

Currently hosted on GitLab


Overview of PYTHIA

Pythia processes important in e-p
Subprocess # Description
V N → V N 91 elastic VMD
V N → X N 92 single-diffractive VMD
V N → V X 93 single-diffractive VMD
V N → X X 94 double-diffractive VMD
V N → X 95 soft non-diffractive VMD low-pT
QCD 2→2    
  96 semihard QCD 2→2
RESOLVED (hard VMD and anomalous)    
qq → qq 11 QCD 2 → 2(q)
q qbar → q qbar 12  
q qbar → gg 13  
gq → gq 28  
qg → qg 28 QCD 2 → 2(g)
gg → q qbar 53  
gg → gg 68  
γ*q → q 99 LO DIS
γ*T q → qg 131 (transverse) QCDC
γ*L q → qg 132 (longitudinal) QCDC
γ*T g → q qbar 135 (transverse) PGF
γ*L g → q qbar 136 (longitudinal) PGF

VMD Vector Meson Dominance, describing the elastic diffractive production of Vector Mesons.
QCDC: QCD-Compton, radiation of a gluon from incoming or outgoing quark lines.
PGF: Photon Gluon Fusion.

Running pythiaeRHIC

In the standard setup in the singularity cvmfs environment or at BNL, the code, executable, steering files, plus shared libraries for PYTHIA and RADGEN, can be found in


The executable pythiaeRHIC (in the future most likely renamed to pythia-eic) is the program for running PYTHIA. It is based on PYTHIA 6.4.28 and was modified to include radiative corrections using RADGEN.

To run, the executable reads options from standard input and is controlled via steering cards and input redirection, with optional output redirection to a log file. The name of the output file is specified in the steering card.

pythiaeRHIC < STEER_FILE > out.log

A practical example, assuming $EICDIRECTORY was set and the package installed as above, is:


The output ASCII format is described below.

Using Radiative Corrections
Steering files

A variety of steering files can be found in the STEER-* directories. Which ones appropriately reflect the physics you wish to implement is beyond the scope of this document. Please contact the EIC UG for advice.

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
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
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.


ROOT supports an interface to PYTHIA via the class TPythia6. This allows PYTHIA to be configured, run and analysed from within a ROOT session, which can be a more convenient than using PYTHIA as a separate programme. The EIC installations of ROOT support TPythia6, but be aware that the version of PYTHIA interfaced with ROOT is the “vanilla” distribution, and hence lacks the EIC additions. If you require these for your analysis you should only use the standalone pythiaeRHIC program. This means you miss:


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 cmake and make.

Basic installation

(Adapt directories etc as appropriate.)

git clone
mkdir build; cd build
make -j 8 install

This will produce warnings of the form

Warning: $ should be the last specifier in format at (1)

which should be okay (it is a g77 extension allowed by gfortran).

Executables require LHAPDF5 installed and links executables against it.

CHANGES with respect to the script

Radiative Corrections

The code implemented in PYTHIA to calculate radiative corrections is called RADGEN. The writeup on it can be found here. The following steps have been done to implement it in PYTHIA:

Additional Info on radiative corrections

Webpages with codes:

Other references: 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 here.