Discovery of the Top Quark

Ever since the existence of the bottom (or ) quark was inferred from the discovery of the Upsilon family of resonances at Fermilab in 1977, particle physicists have been on the lookout for its partner, called top (or ). The long search, which occupied experimenters at laboratories around the world, came to a successful conclusion in February 1995 with the announcement that the top quark had been observed in two experiments at the Tevatron proton-antiproton collider at Fermilab.

Top is the last of the fundamental constituents of subnuclear matter that theories of the strong, weak, and electromagnetic interactions and a wealth of experimental information had led particle physicists to expect. Theoretically, top's existence was required to make the electroweak theory internally consistent. Experimentally, the top's existence was signalled by the pattern of disintegrations of the quark and by the way quarks interacted with other particles via the exchange of bosons. A year before the formal discovery, a growing body of observations pointed to the existence of a top quark with a mass of about 180 GeV/c2. Finding top there emerged as a critical test of the understanding built up over two decades.

Creating top-antitop pairs in sufficient numbers to claim discovery demanded exceptional performance from the Tevatron, at 1.8 TeV the world's highest-energy collider. Observing traces of the disintegration of top into a quark and a boson required highly capable detectors and extraordinary attention to experimental detail. Both the and the are themselves unstable, with many complex decay modes.

Each of the Fermilab detectors is a complex apparatus operated by an international collaboration of about 450 physicists. The tracking devices, calorimeters, and surrounding iron for muon identification occupy a volume about three stories high and weigh about 5000 tons. The Collider Detector at Fermilab (CDF), a magnetic detector with solenoidal geometry, profited from its high-resolution silicon vertex detector to tag b   quarks with good efficiency. The DØ detector has no central magnetic field, emphasizing instead calorimetric measurement of the energies of produced particles.

Both groups found events in which one or both s decayed into an electron or muon, plus a neutrino. A significant number of these events showed evidence for one or two b quarks as well. Taken together, the populations and characteristics of different event classes provided irresistible evidence for a top quark with a mass in the anticipated region: 178 ± 8 ± 10 GeV/c2 for CDF(1) (the first uncertainty refers to that due to the statistical size of the data sample, while the second refers to that due to systematic uncertainties, which include imperfect knowledge of the detectors, theoretical modeling, and algorithms for estimating the mass); and 199 ± 20 ± 22 GeV/c2 for DØ.(2) Meanwhile, the top-antitop production rate was in line with theoretical predictions.

Top is a most remarkable particle, even for a quark. Although a single top quark weighs about as much as an atom of gold, we expect that it is structureless down to a scale of at least 10-18 m. Its expected lifetime of about 0.4x10-24 s makes it by far the most ephemeral of the quarks, and opens new possibilities for the study of quark dynamics.

Because of the great mass and short lifetime, it is popular to say that top quarks were produced in great numbers in the fiery cauldron of the Big Bang, that they disintegrated in the merest fraction of a second, and then vanished from the scene until physicists learned to create them in the Tevatron. To learn how it helped sow the seeds for the primordial universe that evolved into our world of great complexity and change would be reason enough to care about the top. But this cosmic role is not the whole story; it invests the top quark with a remoteness that veils its importance for the everyday world of today. The real wonder is that here and now, every minute of every day, the top quark affects the world around us.

For example, by virtue of the Heisenberg uncertainty principle, top quarks and antiquarks wink in and out of an ephemeral presence in our world. Though they appear virtually, fleetingly, on borrowed time, top quarks have real effects. Of immediate interest to particle physicists is the influence of top on the mass of the boson, which regulates the rate of radioactive decay and the rate of energy production in the Sun. Rapidly improving measurements of both the top mass and the mass set the stage for another incisive test of the electroweak theory.

The standard model suggests that the electroweak force separated into the separate electromagnetic and weak forces in the early universe, and that the Higgs boson bestowed a great mass on the and bosons, the carriers of the weak force, while allowing the photon, the carrier of the electromagnetic force, to remain massless. But what is the mass of the Higgs itself? Because the mass is fine-tuned by the influence of the Higgs boson, particle physicists hope that better estimates of the top mass and the mass will narrow the range of possible masses for the Higgs boson.

Like the end of many a scientific quest, the discovery of top marks a new opening. The first priority, with the doubled event samples that will be in hand by the end of 1995, is to refine the measurements of the top mass. It is already possible to begin asking how precisely the top conforms to prior expectations as to its production and decay rates. Also, because of top's great mass, its decay products may include unpredicted­or at least undiscovered­new particles. A very interesting development would be the observation of resonances in top-antitop production that would give new clues about the breaking of electroweak symmetry.

For the moment, the direct study of the top quark belongs to the Tevatron. By 1999, samples 20 times greater than the current samples should be in hand, thanks to the increased event rate made possible by Fermilab's main injector and upgrades to CDF and DØ. In addition, further improvements to the accelerator complex are under study. Furthermore, early in the next century the Large Hadron Collider at CERN will produce tops at more than 10,000 times the rate seen in the original discovery experiments. Also, electron linear colliders may add new opportunities for the study of top-quark properties and dynamics.

In the meantime, the network of understanding of elementary particle interactions known as the standard model links the properties of top quarks to a variety of phenomena to be explored in other experiments.

Chris Quigg
Fermi National Accelerator Laboratory
Telephone: +1 (630) 840-3578

1. F. Abe, et al.  (CDF collaboration), Phys. Rev. Lett.  74, 2626 (1995).
2. S. Abachi, et al.  (DØ collaboration), Phys. Rev. Lett.  74, 2632 (1995).
From APS News-Online/Physics News in 1995 (May 1996), Copyright 1996, The American Physical Society. The APS encourages the redistribution of the materials included in this newsletter provided that attribution to the source is noted, the materials are not truncated or changed. Physics News in 1995 in pdf format (702KB).