Discovery of the Top Quark

Andy Lankford

D0 DetectorD0 Detector
This spring, researchers in the UCI Department of Physics and Astronomy participated in one of the most exciting recent advances in particle physics, the discovery of the top quark. Two large experiments at the Fermi National Accelerator Laboratory (Fermilab) in Illinois, including the D0 Experiment in which UCI collaborates, shared in this discovery. Last year the other large experiment, called the Collider Detectory Facility (CDF) had published suggestive "evidence" for the existence of the top quark; however, limited numbers of detected examples of the characteristic decays of top quarks prevented them from claiming discovery. This year, with much improved operation of the Fermilab accelerator, which provides head-on collisions of protons with antiprotons at the highest laboratory energies in the world, both experiments accumulated enough statistics to independently, but simultaneously, announce the long-awaited discovery of the top quark. UCI researchers from Prof. Lankford's high energy physics group working on D0 are associate research physicist David Stoker, postgraduate researchers John Drinkard and Ray Hall, and graduate students Greg Griffin and Jamal Tarazi.

Why was the discovery of the top quark long awaited, and why is it exciting? Over the last two decades a "Standard Model" of elementary particle physics has been developed which properly describes all known phenomena. Indeed, the Standard Model is more than a model. It is based on elegant and simple mathematical theory and upon a (relatively) simple set of assumptions; moreover, it describes all of the known interactions of matter with the notable exception of gravity. One of the assumptions of this theory is that there exist three nearly identical generations, of four fundamental particles each. Here, fundamental means that these particles have no known substructure, and nearly identical means that they are the same except that the particle masses are different. The first generation of fundamental particles consists of the electron, the electron neutrino (discovered by UCI's Frederick Reines), and the up and down quarks. Up and down quarks are the building blocks of the protons and neutrons of ordinary matter and, together with the electron, comprise our worlds of nuclear and atomic physics. During the 1970's, discovery of the second generation was completed, and discovery of all but one of the four fundamental particles of the third generation occurred. Meanwhile, an experiment at the Stanford Linear Accelerator Center (SLAC) in which Dave Stoker and I participated, followed by even more precise measurements in Europe at CERN, Geneva, conclusively showed that there are no more than three such generations. By the late 1970's, the one missing particle in the Standard Model was the top quark, and until this year its discovery remained elusive.

The discovery of the top quark required two important ingredients: an accelerator with enough energy to produce top quarks and an apparatus sufficiently sensitive to detect them. The accelerator at Fermilab is the first with adequate collision energies, because top quarks have very large mass (175-200 GeV - giga or 10^9 electron volts), nearly two hundred times that of protons (1 GeV). The D0 and CDF experiments provided detectors sufficiently sophisticated to detect top quarks via characteristic decay patterns. Each experiment is composed of a large system of complex detectors surrounding the point of collision. Contrast the scale of the experimental apparatus, of approximately ten meters in radius, with the objects of its study, particles such as the top quark. which have radii less than 10^(-15) meters.

Even with these tools, the discovery of the top quark was a challenge. Top quarks are rarely produced, they decay instantaneously after production, and their decay patterns are complex and look much like other more mundane phenomena. The Standard Model, however, provided enough clues as to the characteristics of the decays to allow scientists to simultaneously search for a number of likely decay patterns. By piecing together the data gathered in each of these decay modes, enough self-consistent evidence was accumulated for discovery.

D0 and CDF will now turn their attention to studying the properties of the top quark in detail, attempting to establish whether or not all its properties are consistent with the Standard Model. Improvements to the D0 detector upon which the UCI group is working will increase the sensitivity of its studies of top. Furthermore, following the discovery of the top quark, some of the most basic questions of particle physics, namely, the origin of mass and why the identical generations have different masses, become ever more compelling. The measured mass of the top quark will shed light on these questions; however, answers will await future accelerators with even higher energies. The UCI group is also now becoming involved in development of the ATLAS experiment for the planned Large Hadron Collider (LHC) of seven times the Fermilab energy, at the European laboratory CERN, aimed at addressing these and other questions at the frontier of high energy physics.