DISCOVERY of the TOP QUARK by BILL CARITHERS & PAUL GRANNIS Two leaders of the Fermilab experiments that isolated the top quark tell the adventure of its discovery. Fermilab Media Services Visual 4 FALL 1995 ANKIND has sought the elementary building blocks of matter ever since the days of the Greek philosophers. Over Mtime, the quest has been successively refined from the original notion of indivisible “atoms” as the fundamental elements to the present idea that objects called quarks lie at the heart of all matter. So the recent news from Fermilab that the sixth—and possibly the last—of these quarks has finally been found may signal the end of one of our longest searches. Fermilab Media Services Visual BEAM LINE 5 But the properties of this funda- combinations of three quarks or a CDF Collaboration mental constituent of matter are quark-antiquark pair. Quarks also bizarre and raise new questions. In seemed to form a counterpart to the Argonne National Laboratory particular, the mass of the top quark other class of elementary particles, Istituto Nazionale di Fisica Nucleare, is about forty times that of any the leptons, which then included the University of Bologna µ Brandeis University other—a fact which suggests that per- electron (e) and muon ( ) (both with University of California at Los Angeles haps it plays a fundamental role in unit charge) and their companion ν ν University of Chicago the question of how the mass of any chargeless neutrinos, e and µ. The Duke University object arises. leptons do not feel the strong inter- Fermi National Accelerator Laboratory action, but they do participate in the Laboratori Nazionali di Frascati, Istituto N 1964 Murray Gell-Mann and electromagnetic interactions and the Nazionale di Fisica Nucleare Harvard University George Zweig proposed the quark weak interaction responsible for ra- Hiroshima University Ihypothesis to account for the ex- dioactive decays. They have the University of Illinois plosion of subatomic particles dis- same spin as the quarks and also Institute of Particle Physics, McGill covered in accelerator and cosmic- have no discernible size or internal University and University of Toronto ray experiments during the 1950s and structure. The Johns Hopkins University early 1960s. Over a hundred new par- But most physicists were initially National Laboratory for High Energy ticles, most of them strongly inter- reluctant to believe that quarks were Physics (KEK) Lawrence Berkeley Laboratory acting and very short-lived, had been anything more than convenient ab- Massachusetts Institute of Technology observed. These particles, called stractions aiding particle classifica- University of Michigan hadrons, are not elementary; they tion. The fractional electric charges Michigan State University possess a definite size and internal seemed bizarre, and experiments re- University of New Mexico structure, and most can be trans- peatedly failed to turn up any indi- Osaka City University formed from one state into another. vidual free quarks. And—as became Università di Padova, Instituto Nazionale di Fisica Nucleare The quark hypothesis suggested that apparent from studies of fundamen- University of Pennsylvania different combinations of three tal theories of quarks and leptons— University of Pittsburgh quarks—the up (u), down (d), and major conceptual problems arise if Istituto Nazionale di Fisica Nucleare, strange (s) quarks—and their an- the numbers of quarks and leptons University and Scuola Normale tiparticles could account for all of the are not the same. Superiore of Pisa hadrons then known. Each quark has Two major developments estab- Purdue University University of Rochester an intrinsic spin of 1/2 unit and is lished the reality of quarks during Rockefeller University presumed to be elementary, like the the 1970s. Fixed-target experiments Rutgers University electron. So far, quarks appear to directing high energy leptons at pro- Academia Sinica have no size or internal structure and tons and neutrons showed that these Texas A&M University thus represent the smallest known hadrons contain point-like internal Texas Tech University constituents of matter. To explain constituents whose charges and University of Tsukuba the observed spectrum of hadrons, spins are just what the quark mod- Tufts University University of Wisconsin quarks had to have electric charges el had predicted. And in 1974 ex- Yale University that are fractions of the electron periments at Brookhaven National charge. The u quark has charge 2/3 Laboratory in New York and Stan- while the d and s quarks have charges ford Linear Accelerator Center −1/3 (in units where the electron (SLAC) in California discovered a charge is −1). striking new hadron at the then very The observed hadron spectrum large mass of 3.1 GeV—over three agreed remarkably well with the times that of the proton. This hadron expected states formed from (called the J/ψ after its separate 6 FALL 1995 Electric First Second Third Charge Family Family Family DØ Collaboration +2/3 up (u) charm (c) top (t) QUARKS Universidad de los Andes -1/3 down (d) strange (s) bottom (b) University of Arizona -1 electron (e) muon (µ) tau (τ) Brookhaven National Laboratory LEPTONS ν ν ν 0 electron neutrino ( e) muon neutrino ( µ) tau neutrino ( τ) Brown University University of California, Davis names in the two experiments) was chart above). Ordinary matter is com- University of California, Irvine found to be a bound state of a new posed entirely of first-generation par- University of California, Riverside kind of quark, called charm or c, with ticles, namely the u and d quarks, LAFEX, Centro Brasileiro de Pesquisas its antiquark. The c quark has a plus the electron and its neutrino. Físicas Centro de Investigacion y de much greater mass than the first But the third-generation quark dou- Estudios Avanzados three, and its charge is 2/3. With two blet seemed to be missing its charge Columbia University quarks of each possible charge, a +2/3 member, whose existence was Delhi University symmetry could be established be- inferred from the existing pattern. In Fermi National Accelerator Laboratory tween the quarks and the leptons. advance of its sighting, physicists Florida State University Two pairs of each were then known: named it the top (t) quark. Thus be- University of Hawaii ν University of Illinois, Chicago (u,d) and (c,s) for quarks and (e, e) and gan a search that lasted almost µ ν Indiana University ( , µ) for leptons, satisfying theo- twenty years. Iowa State University retical constraints. Korea University But this symmetry was quickly SING THE RATIOS of the ob- Kyungsung University broken by unexpected discoveries. In served quark masses, some Lawrence Berkeley Laboratory 1976 experiments at SLAC turned up Uphysicists naively suggested University of Maryland a third charged lepton, the tau lep- that the t might be about three times University of Michigan Michigan State University ton or τ. A year later at Fermi as heavy as the b, and thus expect- Moscow State University National Accelerator Laboratory in ed that the top would appear as a – University of Nebraska Illinois a new hadron was discovered heavy new hadron containing a tt New York University called the upsilon or ϒ, at the huge pair, at a mass around 30 GeV. The Northeastern University mass of about 10 GeV; like the J/ψ, it electron-positron colliders then un- Northern Illinois University was soon found to be the bound state der construction (PEP at SLAC and Northwestern University University of Notre Dame of yet another new quark—the bot- PETRA at DESY) raced to capture the University of Panjab tom or b quark—and its antiparticle. prize, but they found no hint of the Institute for High Energy Physics, Experiments at DESY in Germany top quark. Protvino and Cornell in New York showed In the early 1980s a new class of Purdue University that the b quark has spin 1/2 and a accelerator came into operation at Rice University charge of −1/3, just like the d and s CERN in Switzerland, in which University of Rochester quarks. counter-rotating beams of protons Commissariat à l’Energie Atomique, Saclay With these discoveries, and and antiprotons collided with an en- Seoul National University through the development of the Stan- ergy of about 600 GeV. The protons State University of New York, dard Model, physicists now under- and antiprotons brought their con- Stony Brook stood that matter comes in two stituent quarks and antiquarks into Superconducting Super Collider parallel but distinct classes—quarks collision with typical energies of 50 Laboratory and leptons. They occur in “gener- to 100 GeV, so the top quark search Tata Institute of Fundamental Research University of Texas, Arlington ations” of two related pairs with dif- could be extended considerably. Be- Texas A&M University fering electric charge—(+2/3, −1/3) sides the important discovery of the for quarks and (−1, 0) for leptons (see W and Z bosons that act as carriers BEAM LINE 7 of the unified electroweak force, the rely on the production of separate t Jet Production CERN experiments demonstrated an- and t¯ quarks from annihilation of in- other aspect of quarks. Though coming quarks and antiquarks in the π+ quarks had continued to elude direct proton and antiproton, with subse- u – detection, they can be violently scat- quent decays into observable parti- d π– d tered in high energy collisions. The cles (see box on the right). The design u– Jet high energy quarks emerging from of DØ stressed recognition of the tell- + u K the collision region are subject to the tale leptons and jets over as large a s– strong interaction as they leave the solid angle as possible. Meanwhile p s K– – scene of the collision, creating ad- CDF had installed a new vertex de- u d p– d ditional quark-antiquark pairs from tector of silicon microstrips near the u u– g the available collision energy (us- beams intended to detect short-lived d u– 2 – ing E = mc ).