Three Jet Production in Deep-Inelastic Scattering at HERA
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
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.