Jet Finders
Pythia
comes with three built-in
jet finders, ClusterJet
for e^+e^- events
and SlowJet
and CellJet
for 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
main71.cc
and main72.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.