The project starts with the creation of the VENUS model by Klaus WERNER,
during a postdoc stay at the Brookhaven National Laboratory between 1986
and 1989. This was one of the first codes able to simulate relativistic
proton-proton and heavy-ion collisions, the latter ones being realized
at the time at the SPS accelerator at CERN. VENUS was already using the
concept of parallel scatterings in the Gribov-Regge framework, but the
scatterings were considered to be completely "soft", sufficient for the
moderately high SPS energies.
In the early 2000s, the RHIC collider in Brookhaven started operation,
with Au+Au collisions at an CMS energy of 200 AGeV. This made it
necessary (in the late 90s) to reconsider the VENUS project in order to
implement "hard processes". It was decided to fuse the QGSJET code of
Sergej OSTAPCHENKO and the VENUS code of Klaus WERNER, with the help of
several PhD students (Hajo DRESCHER, Michael HLADIK, Tanguy PIEROG), to
have a consistent description of soft and hard scatterings, and
therefore an up-to-date model for RHIC simuations, named NEXUS.
In the NEXUS approach, it was clear from the beginning that a simple
approach with just elementary Pomerons (I-diagrams) will lead to
contradictions, and it was decided to explicitely implement as well
Y-diagrams, which amounts to splitting a parton ladder into two. But
then one also needs to allow for splitting the two legs again, and so
on. Unfortunately, such a cascade of splittings is impossible to do in a
framework with rigorous energy conservation. Two possible solution
emerged, and were further developed around 2006 as EPOS (keeping full
energy consevation, but treating Y diagrams in an effective way), and as
QGSJETII (giving up energy conservation, but summing up the splittings
to infinit order).
In EPOS, in addition to the multiple Pomeron exchanges, referred to as
primary scatterings happening at t=0, secondary scatterings were added,
treating the reinteractions of the particles produced initially. In
2007, the core-corona picture was introduced (Klaus WERNER), which
amounts to identify a core part, which then evolves macroscopically as a
fluid. Those particles which do not participate to the core are called
corona particles. In EPOS1, the core expansion was simply parameterized,
with a flow put in by hand. EPOS1 turned out to b quite successful to
descibe the first LHC results (with the code being contructed and tuned
before LHC). Around 2012, EPOS LHC was created (Tanguy PIEROG, K. Werner),
which is essentially the last EPOS1 version (1.99) with some finetuning
compared to LHC results from run 1.{" "}
In EPOS2 (I. KARPENKO, T. PIEROG, and K. WERNER), an efficient code for
solving the hydrodynamic equations in 3+1 dimensions was implemented,
inluding the conservation of baryon number, strangeness, and electric
charge, employing a realistic equation-of-state, compatible with lattice
gauge results, using a complete hadron resonance table, making our
calculations compatible with the results from statistical models.
Nonlinear effects (parton ladder fusions) were treated by simply adding
by hand a term x^epsilon to the Pomeron amplitudes, with x being an
energy fraction (epsilon method).
In EPOS3 (B. GUIOT, I. KARPENKO, T. PIEROG, and K. WERNER), a 3D+1 viscous
hydrodynamical evolution was implemented, starting from flux tube initial
conditions, being generated in the Gribov-Regge multiple scattering framework.
An individual scattering is referred to as Pomeron, identified with a
parton ladder, eventually showing up as flux tubes (or strings). Each
parton ladder is composed of a pQCD hard process, plus initial and final
state linear parton emission. Klaus WERNER introduced a new way to deal
with nonlinear effects, namely by associating a saturation scale Q_s to
each parton ladder (Q_s method). Tanguy PIEROG introduced a smart way to
combine the epsilon and the Q_s method, by first using epsilon to define
an effective Pomeron amplitude G_eff, and then making the link to the
QCD ladder via G_eff = k*G_QCD(Q_s), where the momentum dependence of k
should be chosen such that factorization is assured (which turned out to
be difficult). Finally, Benjamin GUIOT made major contributions
concerning the implementation of heavy flavor.