Jet Finders
Pythia comes with three built-in
jet finders, ClusterJet for e^+e^- events
and SlowJet and CellJetfor hadron collider ones.
Especially the latter is not so well matched to the standards of its field,
however. (But it is closely related to the anti-kT algorithm,
so is also not completely disconnected [Cac08].)
SlowJet can do jet finding according to the current-day
kT, Cambridge/Aachen and anti-kT algorithms.
It can be run in two modes. The original one is a native implementation
which, as the name indicates, is rather slow. However, with the release
of the fjcore code from FastJet
[Cac06, Cac12], the default mode has become to use the
fjcore methods. This is transparent to the user.
FastJet
SlowJet does not exhaust all the posssibilities of the
fjcore code, so users are welcome to extend on the existing
functionality by a direct usage of the fjcore methods.
Missing from fjcore is a number of aspects, such as
jet areas functionality, background estimation, access to other algorithms
via plugins, interface to CGAL and tools such as filters and taggers.
Therefore, for more sophisticated jet studies the complete
FastJet package needs to be linked. This is foreseen in the
configure file in the examples subdirectory, and the
main212.cc and main213.cc programs contain
examples how it can be used with Pythia events. (Even if
these examples do not go beyond the functionality that
SlowJet can offer.)
The latter program makes use of the
include/Pythia8Plugins/FastJet3.h
header file, contributed by Gavin Salam. This allows simple input
of a Pythia particle into a FastJet one,
either retaining only the four-momentum or the full particle information.
Thereby more sophisticated selectors become possible at the
FastJet level. This code could be duplicated, with trivial
modifications, to augment the fjcore package functionality
in an identical manner, should the need arise.