Measuring the number of hadronic jets.

A quantitative description of the qualitative feature of multihadron final states known as the “number of jets” is given by a sequence of infrared finite shape observables (jet discriminators) that: take continuous values between 0 and 1; are stable—unlike clustering algorithms—against small variations of the input (data errors, Sudakov effects etc.); have a form of multiparticle correlators that is natural in the context of quantum field theory and hence are better suited for a systematic study of theoretical uncertainties (logarithmic and power corrections).