Three Jet Production in Deep-Inelastic Scattering at HERA

Jets, collimated sprays of particles emerging from the interaction of particle beams have long been interpreted as the echo of the underlying interaction at a microscopic level. Quarks and gluons make up the proton. In electron-proton interactions at HERA, the electron strikes a quark and expels it from the proton. Shouldn't then every electron-proton collision end up in a scattered electron and a jet emerging from the struck quark?

Not quite. The quark travels in the strong colour field inside the proton. These fields may become so strong locally that a gluon is radiated off the accelerated quark much alike the electromagnetic field surrounding an accelerated electric charge. The gluon may thus convert into a quark anti-quark pair, which may radiate itself and so on. Some of these quarks and gluons may be so energetic that additional jets are expelled from the proton. Single jets are abundant at HERA. Interactions producing two jets have also been studied in detail. These so called hard processes in which all of the participating partons acquire sufficient energy to leave the interaction region are theoretically calculable in perturbative Quantum Chromodynamics, pQCD. The theory has been tested both in electron-positron, electron-proton and proton-proton collisions.

The H1 experiment has now selected a number of events that show a distinct separation of the expelled particles into three jets as recorded in the experimental apparatus. These events are attributed to processes in which at least three quarks and gluons are involved. Since three jets are produced the strong interaction must have "worked" on these events several times. The rate of these events, considerably smaller than those in which a simple quark jet is expelled from the proton, is hence suppressed by several orders. However, because the involved process of interaction the sensitivity for tests of the theory is particularly large.

Kinematic Dependence of the Production Rate

The measurements can hence be compared to the expectation from pQCD in a very sensitive manner. Such calculations have just become available in the next reliable degree of approximation, i.e. expansion in the coupling strength (in next-to-leading order calculation). The adjacent figure shows the rate of such events as a function of the distance from the struck quark. (The scale Q2 is a convenient quantity that can be extracted from the experimental data. Large Q2 imply short distances).

The rate of these events is compared to the calculations in the adjacent figure (top). It is astounding that the rate of such events is very well predicted from the theoretical calculation over orders of magnitude in the distance scale. Pictorially speaking this implies that there is a good understanding of how a quark "dresses up" in a strongly interacting cloud of gluons and quarks inside the proton.

The experimental uncertainties for this distinct final state signature is small, they become smaller the smaller the distance is probed.


Angular Distribution

How do we know that QCD is at work? If the three jets just formed in a random fashion in the proton they would populate the available phase space volume in a uniform manner. The graph on the left shows the angular distribution measured between jets and production plane. This distribution is well described by the theory of strong interactions (QCD) while the phase space model does not describe the experimental observation.

Last modified Sep 19, 2001