Measurement of Dijet Electroproduction at Small Jet Separation

Much of the research performed at the electron-proton collider HERA is directed towards a better understanding of the strong force. While the much better known electrical force is responsible for the interaction of electrically charged objects like electrons and ions, the strong force determines the interaction of quark and gluons - the constituents of nuclear matter.

Quarks and gluons are usually confined into hadronic objects like protons or pions and are not directly observable as such. In the extremely violent electron-proton collisions at HERA, however, quarks and gluons are accelerated to the speed of light and become visible in the physicists' particle detectors as jets of highly energetic hadronic particles. The electron and proton collisions where (at least) two of these quark- and gluon-jets are detected
are particularly suited to study the strong interaction between quarks and gluons.

The topological properties of such events have extensively been investigated in the past  for jet configurations where the (angular) distance between the jets is large. One may wonder if this is the most likely topology. It is not !

Events with small jet distance are by far more abundant. Here, we select (predominantly) such events and measure various jet properties precisely. The sample of jet events is nearly three times as large as the typical sample with two jet at larger distance.

Why have such events not been studied earlier? It is because the most rigorous theoretical predictions - QCD perturbation theory in next-to-leading order - which correspond to the current best understanding of the strong interaction, are expected to be most reliable for the rare topologies with jets at large distances. It seems to make little sense to compare measurements with a theory in a range where it may not be valid.

We do it nevertheless!  To our surprise we find that the theory performs very well far beyond the range in which it was tested previously. The range of validity of the QCD calculations seems to be much larger than anticipated!

While this may increase the confidence in the correctness of Quantum Chromodynamics, the theory of the strong interaction, it also calls for more refined higher-order calculations, which will make possible even more complete tests of our understanding of the strong interaction.

The figure shows the distribution of jet distance - expressed in the variable y2 - in events with two jets. (Large jet distances correspond to large y2 values.) The solid line, which corresponds to the mentioned prediction, describes the data well up to y2 = 0.001. Thus the theory performs well in a range that is about three times larger than known previously !

Last Update on 26/10/2001