Discovery of Three-Jet Events and a Test of Quantum Chromodynamics at PETRA

Abstract

We report the analysis of the spatial energy distribution of data for e+e hadrons obtained with the MARK-J detector at PETRA. We define the quantity oblateness to describe the flat shape of the energy configuration and the three-jet structure which is unambiguously observed for the first time. Our data can be explained by quantum chromodynamic predictions for the production of quark-antiquark pairs accompanied by hard noncollinear gluons. In previous papers' we have reported the observation of a two-jet structure in the production of multihadronic states in e'e annihilation at PETRA, in terms of thrust and spherocity. At higher energies, the hadrons in each jet become increasingly collimated, and the identification of these hadrons with jets becomes increasingly unambiguous. Detailed analysis of the distribution of hadronic energy in three dimensions therefore can be done in order to see the effects of quantum chromodynamics (QCD), and, in particular, the emission of gluons. The detector we used, known as MARK-J, ' measures the energy distribution of both neutral and charged particles (except neutrinos). It covers a solid angle of cp=2~ and 8=9' to 171' (8 is the polar and y is the azimuthal angle). Lucite Cerenkov counters surround the intersection region, followed by two layers (A and B) of three radiation lengths each of lead-scintillator shower counters, with one fast photomultiplier tube at each end. The counters A and B enable us to locate shower maxima in various 9 and cp directions. They are followed by the sixteen C counters, consisting of twelve layers (twelve radiation lengths) of lead-scintillator sandwich also with one phototube at each end. Surrounding the electromagnetic shower counters are drift chambers which measure tracks from hadron showers and incident muon angle. The next layers are hadron calorimeters consisting of magnetized-iron- scintillator sandwiches. The last layer of calorimeter, composed of the D counters, is used for triggering on muons and for rejecting cosmic rays. The magnetic field in the iron is toroidal and its value is 1V kG. Finally, in the outermost layer there are drift chambers which are used to measure single- and double-muon exit angles and mom enta. In the small-angle region there are four layers of scintillation counters sandwiched between 10cm-thick iron plates to measure shower energy in the region 12 (8&30. The total energy of each interaction and the direction of a particle or group of particles is computed from the time and pulse-height information of the shower counters and calorimeter counters. The azimuthal position is determined by the finely segmented shower counters. This method enables us to determine the 6I and y angles to an accuracy of & 5' for e or y and &15' for hadrons. The jet analysis of the hadronic events was performed with use of the spatial distribution of the energy deposited in the detector. For each counter hit, a vector E' (the energy flow) is constructed, whose direction is given by the position of the signal in the counter, and magnitude by the corresponding deposited energy. To describe the energy distribution, three orthogonal axes are defined for each event as follows: (1) The thrust axis, e„ is defined as the direction along which the projected energy flow is