# What's jet multiplicity?

That's a basic thing, but, surprisingly, it is very difficult to find concise explanation of:

• What is the definition of jet multiplicity?
• Why is it interesting?
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well, it's the number of jets in whatever you're counting (probably in an event). it's interesting because in hadron collisions we have lots of jets and knowing the number of them is important. Maybe you're interested in how to deduce the number of jets in an event, starting from the hard process (CKKW and MLM matching might be keywords) –  luksen Sep 29 '11 at 11:01
To understand why jet multiplicity is interesting you have to understand what jets are in this context and how they form...are you good to go on that? From the experimenters point of view the jet multiplicity often chosen as part of event selection (as in we consider only events with two leptons, two jets and missing transverse momentum...). –  dmckee Sep 29 '11 at 14:15

Jet multiplicity , jets in general, came to the forefront with the parton model, advocated by Feynman way before QCD was established as the theory of strong interactions.

It was experimentally observed that colliding particles formed two jets, i.e. the particles in the debris of the interaction formed two jets. The parton model, modeled them as the parton that was incoming, and had a leading effect, most of the energy, and the parton at rest which had low energy . In the center of mass system they showed the two body reaction.

Back in 1977 John Ellis et al, analysing the phenomenology of QCD proposed the mercedes diagram, where one could see in the center of mass system three jets, the two interacting partons plus a gluon.

The model is that the hard scattering of the quarks may involve a hard gluon and each of these partons is dressed by soft gluons as it materializes. High pt 3jet events clinched the existence of gluons and made the simple parton model history.

So each high pt jet is a materialization of either a primary quark or a gluon, so the multiplicity of jets has to do with QCD predictions.

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