Jet Phenomena

Abstract

While quarks do not appear as such, they may leave a clear kinematical signature. Spectacular trails of π mesons, such as those shown in Fig. l.a, have now become almost commonplace. An electron-positron annihilation here results in a two-jet pattern. Jets of π mesons appear instead of the quark-antiquark final state system, in terms of which many features of the reaction can be interpreted. The appearance of such jets is usually described in terms of the two-time scale, familiar to the parton model¹. A primeval process occurs on a very short time scale. It is described in terms of hadron constituents (partons) and results in a final set of partons moving away in a particular configuration. Partons then eventually turn into actual hadrons through a process which occurs on a longer time scale. This process is known to be “soft”. It involves only limited transverse momentum with respect to the parton direction (with an exponentially damped distribution associated with hadronic size). As a result, the final (π meson) pattern retains many kinematical features of the primeval parton one, provided that particle momenta are large enough, or that the reaction energy is sufficiently high, in practice of the order of 10 GeV or more at the parton level².