After an event has been generated, one may wish to list it, or process it further in various ways. The first section below describes some simple study routines of this kind, while the subsequent ones describe more sophisticated analysis routines.
To describe the complicated geometries encountered in multihadronic events, a number of event measures have been introduced. These measures are intended to provide a global view of the properties of a given event, wherein the full information content of the event is condensed into one or a few numbers. A steady stream of such measures are proposed for different purposes. Many are rather specialized or never catch on, but a few become standards, and are useful to have easy access to. PYTHIA therefore contains a number of routines that can be called for any event, and that will directly access the event record to extract the required information.
In the presentation below, measures have been grouped in three kinds. The first contains simple event shape quantities, such as sphericity and thrust. The second is jet finding algorithms. The third is a mixed bag of particle multiplicities and compositions, factorial moments and energy-energy correlations, put together in a small statistics package.
None of the measures presented here are Lorentz invariant. The analysis will be performed in whatever frame the event happens to be given in. It is therefore up to you to decide whether the frame in which events were generated is the right one, or whether events beforehand should be boosted, e.g. to the c.m. frame. You can also decide which particles you want to have affected by the analysis.