The Discovery of the Top Quark Finding the sixth quark involved the world’s most energetic collisions and a cast of thousands by Tony M. Liss and Paul L. Tipton VIOLENT COLLISION between a proton and an antiproton (center) creates a top quark (red) and an antitop (blue). These decay to other particles, typically producing a number of jets and possibly an electron or positron. n March 1995 scientists gathered quarks are the building blocks of mat- must exist since 1977, when its partner, at a hastily called meeting at Fer- ter. The lightest quarks, designated “up” the bottom, was discovered. But the top I milab—the Fermi National Accel- and “down,” make up the familiar pro- proved exasperatingly hard to find. Al- erator Laboratory in Batavia, Ill., near tons and neutrons. Along with the elec- though a fundamental particle with no Chicago—to witness a historic event. In trons, these make up the entire periodic discernible structure, the top quark MICHAEL GOODMAN back-to-back seminars, physicists from table. Heavier quarks (such as the charm, turns out to have a mass of 175 billion rival experiments within the lab an- strange, top and bottom quarks) and electron volts (GeV)—as much as an nounced the discovery of a new particle, leptons, though abundant in the early atom of gold and far greater than most the top quark. A decades-long search moments after the big bang, are now theorists had anticipated. The proton, for one of the last missing pieces in the commonly produced only in accelera- made of two ups and one down, has a Standard Model of particle physics had tors. The Standard Model describes the mass of just under 1 GeV. (The electron come to an end. interactions among these building blocks. volt is a unit of energy, related to mass The top quark is the sixth, and quite It requires that leptons and quarks each via E = mc 2.) possibly the last, quark. Along with come in pairs, often called generations. Creating a top quark thus required leptons—the electron and its relatives— Physicists had known that the top concentrating immense amounts of en- 54 Scientific American September 1997 Copyright 1997 Scientific American, Inc. The Discovery of the Top Quark ergy into a minute region of space. Phys- GeV. Meanwhile the collider at Fermilab beam energies, its collisions would be icists do this by accelerating two parti- was just coming into its own with our unlikely to create top quarks heavier cles and having them smash into each young CDF (Collider Detector at Fer- than 77 GeV. The competition was now other. Out of a few trillion collisions at milab). A brief flurry of intense compe- between CDF and a new experiment least a handful, experimenters hoped, tition between us and a group at CERN across the accelerator ring at Fermilab, would cause a top quark to be created brought the decade to a close without a called Dø (pronounced “dee zero,” af- out of energy from the impact. What we top but with the knowledge that its mass ter its location on the ring). did not know was how much energy it could be no lower than 77 GeV. In the early 1980s Leon M. Leder- would take. Although many properties By this time CERN had reached its man, then director of Fermilab, decided of the top, such as its charge and spin limit. With its comparatively lower that CDF needed some local competi- (intrinsic angular momentum), were predicted by the Standard Model, the mass was unconstrained. Although particles can be created from CONSTITUENTS OF MATTER nothing but energy, certain features, CHARGE u c such as electrical charge, cannot—these UP CHARM t TOP are “conserved.” A top quark cannot MASS +2/ be born all by itself. The easiest way to (GeV) 0.3 1.5 175 3 make a top is along with an antitop— d s b identical in mass but with opposite signs d DOWN STRANGE BOTTOM QUARKS MASS 0.5 4.5 1 for other properties, so that conserved (GeV) 0.3 – /3 quantities cancel out. In 1985, when the Fermilab collider - e- µ- τ- was first activated, the search for the ELECTRON MUON TAU MASS top had already been going on for eight 0.0005 0.106 1.7 –1 years. Early forays at the Stanford Lin- (GeV) ν ν ν ear Accelerator Center in Palo Alto, e ELECTRON µ MUON τ TAU Calif., and at DESY in Hamburg, Ger- NEUTRINO NEUTRINO NEUTRINO LEPTONS MASS 0? 0? 0 many, turned up nothing. Over the years (GeV) 0? the hunt moved on to different acceler- ators as they came into operation with ever more energetic particle beams. In TRANSMITTERS OF FORCE the early 1980s at CERN, the European laboratory for particle physics near Ge- VECTOR BOSONS PHOTON GLUON neva, beams of protons and antiprotons W + W – 0 γ g hitting one another at energies up to W Z g 315 GeV generated two new particles, MASS the W and the Z. (GeV) 80 80 91 0 0 Whereas quarks and leptons consti- tute matter, these particles transmit CHARGE +1 –1 0 0 0 force—in particular the weak force, re- FORCE WEAK WEAK WEAK ELECTRO- STRONG sponsible for some types of radioactive MAGNETIC decay. Their discovery provided further confirmation of the Standard Model, which had accurately predicted their masses. It was widely believed that the discovery of the top quark at CERN Characters of the Standard Model was imminent. Finding it would still be a difficult atter consists of two types of particles: quarks and leptons. These are feat. When protons and antiprotons hit Massociated into generations. Up and down quarks, for instance, occur one another at high energies, the actual along with electrons inside atoms; they are members of the first generation. collision is between their internal quarks Much heavier quarks such as the top and bottom are created only in acceler- and gluons. Each quark or gluon car- ators. For each quark or lepton, there is an antiquark or antilepton with oppo- ries just a modest fraction of the total site charge (not shown). energy of its host proton or antiproton, Force is transmitted by a different set of particles: the W, Z, photon and glu- yet the collision must be energetic ons. The W and Z “bosons” transmit the weak nuclear force, involved in ra- enough to generate top quarks. Such dioactive decays. For instance, an up quark may change into a down quark by collisions are rare, and the higher the emitting a W particle, which then decays into a quark or lepton pair. The pho- ton transmits the electromagnetic force, which at high energies is unified required energy—that is, the higher the with the weak force. The gluons transmit the strong force that binds up and top mass—the rarer they are. down quarks into protons and neutrons. An extra particle that is believed to By 1988 the top had not yet been ob- exist, the Higgs, has not yet been found. —T.M.L. and P.L.T. served at CERN; the experimenters con- cluded its mass must be greater than 41 MICHAEL GOODMAN The Discovery of the Top Quark Copyright 1997 Scientific American, Inc. Scientific American September 1997 55 a b W JET W JET BOTTOM/ ANTIBOTTOM JET Y OR Y T A OR T A POSITRON ABOR ABOR OR L T A OR L T A BOTTOM/ CELER ANTIBOTTOM C CELER C JET TIONAL A TIONAL A 3 METERS FERMI NA FERMI NA tion. So we acquired in-house rivals: be- the best theoretical calculations, we ex- that quarks always appear stuck togeth- ginning in 1992 the Dø collaboration pected that about one out of every 10 er with other quarks and antiquarks— began to take data. In addition to spur- billion collisions would produce a top in pairs called mesons or in triplets ring on our efforts, which it certainly quark. The rest, though interesting for a called baryons. (Protons and neutrons did, having two complementary experi- host of other projects, would be a com- are examples of baryons.) When a quark ments studying the same physics was plicated backdrop from which the top emerges from a collision, it gets “dressed healthy in another way. Despite the best would have to be extracted. up” by a cloud of other quarks and an- efforts of experimenters, spurious re- Over the course of a decade, both the tiquarks. What is observed is a jet, a di- sults can occur. Having a second exper- CDF and Dø collaborations construct- rected beam of particles that have rough- iment provides a cross-check. ed enormous, complicated instruments, ly the same direction of motion as the Both CDF and Dø are international with hundreds of thousands of chan- original quark. collaborations of more than 400 physi- nels of electronics, in order to isolate cists. There are also numerous engineers, the top’s “signature”—the trace it would A Barrage of Jets technicians and support personnel. The leave in the detectors. Whereas the CDF rival teams are independent of each oth- detector emphasizes the ability to track he W can decay into a quark and er and never collaborate on their analy- accurately the paths of individual parti- Tan antiquark from the same gener- ses. Each tries to beat the other to the cles in a magnetic field (in order to mea- ation, such as an up and an antidown. punch. But it is friendly competition, sure their momenta), the Dø device re- In this case, the quark and antiquark and we regularly share tables in the lies on an extremely precise segmented show up in a particle detector as two cafeteria and enjoy both serious scien- calorimeter, which measures the energy jets.