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<title>Jet Finders</title>
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<h2>Jet Finders</h2>
<code>Pythia</code> comes with three <a href="EventAnalysis.html" target="page">built-in
jet finders</a>, <code>ClusterJet</code> for <i>e^+e^-</i> events
and <code>SlowJet</code> and <code>CellJet</code>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-<i>kT</i> algorithm,
so is also not completely disconnected [<a href="Bibliography.html" target="page">Cac08</a>].)
<p/>
<code>SlowJet</code> can do jet finding according to the current-day
<i>kT</i>, Cambridge/Aachen and anti-<i>kT</i> 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 <code>fjcore</code> code from <code>FastJet</code>
[<a href="Bibliography.html" target="page">Cac06, Cac12</a>], the default mode has become to use the
<code>fjcore</code> methods. This is transparent to the user.
<h3>FastJet</h3>
<code>SlowJet</code> does not exhaust all the posssibilities of the
<code>fjcore</code> code, so users are welcome to extend on the existing
functionality by a direct usage of the <code>fjcore</code> methods.
<p/>
Missing from <code>fjcore</code> 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
<code>FastJet</code> package needs to be linked. This is foreseen in the
configure file in the <code>examples</code> subdirectory, and the
<code>main71.cc</code> and <code>main72.cc</code> programs contain
examples how it can be used with <code>Pythia</code> events. (Even if
these examples do not go beyond the functionality that
<code>SlowJet</code> can offer.)
<p/>
The latter program makes use of the <code>include/Pythia8/FastJet3.h</code>
header file, contributed by Gavin Salam. This allows simple input
of a <code>Pythia</code> particle into a <code>FastJet</code> one,
either retaining only the four-momentum or the full particle information.
Thereby more sophisticated selectors become possible at the
<code>FastJet</code> level. This code could be duplicated, with trivial
modifications, to augment the <code>fjcore</code> package functionality
in an identical manner, should the need arise.
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