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.


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 and 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.