Spectacular 'jets' seen by the UA2 detector at CERN's proton-antiproton collider in 1982, with The long road to the top sprays ofhigh energy particles at wide angles to the direction of the colliding beams (the by Christine Sutton Unes are proportional to the énergies of the emerging particles). The advent of such detectors surrounding the collision point and intercepting ail emerging particles provided a major impetus to jet studies.

Mann dubbed them "quarks", the name by which we know them today. Earlier this year (June, page 1), To make the known hadrons the initial évidence for the sixth quarks had to have some bizarre ('top') quark from the CDF properties. The hadrons fall naturally experiment at Fermilab's into two groups - the baryons (such Tevatron proton-antiproton as the proton and the neutron) with collider underlined the strength half-integer values of intrinsic spin, of the Standard Model. The top and the mesons (such as the pion quark mass is exactly in line and the kaon) with integer spins. To with prédictions from Standard make a baryon requires three quarks, Model data, including the mass each with a spin of 1/2, while to of précision data from LEP, make a meson requires a quark CERN's électron-positron coupled with an antiquark. Then for collider. In this specially-com- the baryons to have the correct missioned article, Christine electric charge, the quarks must Sutton, Oxford physicist and carry fractions of 1/3 and 2/3 the well-known science miter, usual unit of charge, the charge of reviews the story behind the top the électron. quark. For some time it was far from clear whether the quark model was basi- cally a mathematical device or whether it did indeed reflect a new The top quark occupies a spécial level of reality. As Zweig has since place among the quarks. Not only is said, "The reaction of the theoretical it the heaviest by far, it is also the physics community to the ace model only quark that particle physicists was generally not benign ... The idea have actively hunted. The hunt has that hadrons ... were made of el- been long, but now at last the quarry ementary particles with fractional is in sight and the net has tightened quantum numbers did seem a bit almost to a close. rich." For experimenters, however, It is 30 years since the idea of the hunt for quarks had begun. With quarks first arose. By the early a charge of 1/3, for example, a quark 1960s, experiments had revealed a should produce only 1/9 the ioniza- host of short-lived particles created in tion due to a standard particle, such nuclear collisions, both in cosmic as a pion. So single quarks passing rays and at accelerators. Many of through a detector should leave faint thèse particles, such as the pion and tracks with only 1/9 the density of the the kaon, as well as the more familiar usual high-energy tracks. But the type of experiment. While we may proton and neutron, interacted search for quark tracks was destined not be able to track quarks directly, it through a strong force, and became to fail. We now know that quarks are turns out that we can in effect spy on known collectively as hadrons, from always bound together as groups of them hiding with the proton and the the Greek for "strong". In 1963 three (baryons) or in quark-antiquark neutron. This type of experiment Murray Gell-Mann and George Zweig pairs (mesons), except perhaps at goes back to the days of Ernest came independently to the conclu­ collision énergies far higher than we Ftutherford, whose colleagues Hans sion that they could explain the can currently achieve. Geiger and Ernest Marsden discov- multitude of hadrons in terms of only The first évidence that quarks really ered that alpha-particles directed at a three basic constituents. Zweig called do exist within the proton and the thin gold foil could be scattered thèse constituents "aces", while Gell- neutron came instead from a différent through large angles, sometimes

CERN Courier, November 1994 1 The long road to the top

Rutherford revisited - end Station A at the Stanford Linear Accelerator Center (SLAC) first saw évidence for hard grains deep inside the proton.

even knocked backwards. Rutherford realized that the positively-charged alphas were being deflected by the positive charge within the gold atoms. To explain the large angles, the positive charge within the atom had to be concentrated in a small région at the centre - Rutherford had found the nucleus. In 1969, experiments at the Stanford Linear Accelerator Center (SLAC) began to see a similar phenomenon when high-energy électrons from the 3-km linac struck protons in a hydrogen target. This time it appeared that the électrons were scattering from tiny concentra­ tions of charge within the protons. To show that thèse objects had the same fractional charges as the quarks required a comparison with similar experiments with uncharged Back in 1973, three types of quark - partner the strange quark would projectiles - neutrinos. As Richard up, down and strange - were suffi- complète the picture. Feynman said at the time, "If you cient to build the wide variety of Later more compelling arguments never did believe that "nonsense" particles that had already been for a fourth quark came from consid- that quarks have non-integral discovered. But since 1964 there had ering the lack of expérimental évi­ charges, we have a chance now, in been hints that a fourth quark should dence for interactions called comparing neutrino to électron exist. There were known to be four "strangeness-changing neutral- scattering, to finally discover for the particles that did not consist of currents". Neutral currents are first time whether the idea ... is quarks. Thèse were the weakly- interactions through the weak force in physically sensible ..." interacting "leptons": the électron, the which no electric charge changes At CERN, the team studying the electron-neutrino, the muon and the hands - they occur through the interactions of neutrinos in the huge muon-neutrino. Arguments based on exchange of the now famous Z bubble chamber Gargamelle found symmetry suggested that as there particle. At first sight there seems no the vital évidence. In a conférence in were four leptons, which seemed to reason why a strange quark (charge Hawaii in 1973, Don Perkins reported be fundamental, structure less -1/3) should not change to a down to Feynman and others, "The évi­ particles, then why should there not quark (-1/3) in this way, with no dence is rather compelling that be four quarks, as they likewise change in charge but a change in électrons and neutrinos are seeing appear structureless and fundamen­ "strangeness", a quantum number the same substructure inside the tal? possessed only by the strange quark. nucléon, with absolute rates standing There was more to such arguments In 1970, Sheldon Glashow, John in exactly the ratio predicted by the than simple aesthetics. The weak lliopoulos and Luciano Maiani found quark charge assignments." Later, force, responsible for the decays of that a fourth quark with a charge of the CDHS, CHARM and BEBC many particles, seemed to connect +2/3 would provide an explanation as experiments at CERN followed in the pairs of quarks and leptons. It could to why such interactions were not footsteps of Gargamelle, using the for example change a muon into a seen. The fourth quark could change high-energy neutrino beams at the muon-neutrino, in muon decay; or a to an up quark, also charge +2/3, SPS to probe in détail the complex down quark into an up quark, in through a second type of neutral world within the proton. neutron decay. A fourth quark to interaction. The existence of this

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CERN Courier, November 1994 3 The long road to the top

Installation of the CDF experiment at Fermilab's Tevatron proton-antiproton collider, showing how the detector modules fit around each other like a giant high-tech jigsaw. The CDF detector has provided first évidence for the top quark.

leptons to six, with the quarks lagging behind again with only four members to the family. Now, however, the quark hunters were on guard. When Léon Lederman's team at Fermilab found évidence for a new particle rather like the J/psi but some three times heavier still, the idea that this should be a fifth quark bound with its antiquark did not seem at ail absurd. By 1977, the bottom quark, with charge -1/3, had become an estab- lished member of the quark family, and the hunt was on for its partner, "top", with charge +2/3 to complète the picture. The quest for the top quark proved longer and more arduous than anyone could probably have ex- pected. It seemed always to be hiding just round the corner, to be caught by the next machine that would reach a little higher in energy. In 1977 no one would have predicted that it would eventually weigh in with a mass as great as that of a gold nucleus. The very nature of quarks makes them difficult to snare. Like intrinsi- cally shy créatures, they remain second pathway for a neutral current that became called the J/psi, discov- hidden within baryons or mesons. would lead to a quantum mechanical ered by Sam Ting's team at The experiments with électrons and cancellation between the two, so Brookhaven and Burt Richter's team neutrinos that probe within protons neither would be seen! The fourth at SLAC. see the average effects of a mixture quark would have a unique quantum The new particle was at first a of three "valence" quarks entangled number distinguishing it from the puzzle, but évidence gradually in a "sea" of ephemeral quark- other quarks. Echoing earlier work by mounted to show that it must be a antiquark pairs and gluons (which Glashow and James Bjorken this meson built from the fourth quark hold the mixture together). They can quantum number was called "charm". locked in a long but ultimately deadly tell us how the constituents move It is probably fair to say that in the embrace with its antiquark. A lepton- around inside the proton, but not how early 1970s, there was such a variety quark symmetry was therefore to identify a spécifie quark. When we of competing theoretical ideas that no assured - but not for long. Even as try to knock quarks out of particles, one tried seriously to search for the the consensus on the nature of the J/ ail that we succeed in doing is to fourth quark. Instead, it gave itself up psi was forming, Marty Perl and his create more quark-antiquark pairs, in a rather unexpected way. In the colleagues at SLAC began to dis- and hence more particles. In some autumn of 1974 two experiments cover évidence for another new circumstances we can use this effect found évidence for a new surprisingly particle, this time a third charged to "see" a quark, through the tight long-lived state, which at 3.1 GeV lepton - the tau. Together with its cluster or "jet" of particles the quark was more than three times heavier likely (but still unproven!) neutrino spawns. But again it is difficult to than the proton. This was the particle partner, the tau brought the tally of

4 CERN Courier, November 1994 The long road to the top

détermine the exact nature of the mass of the top quark as they failed sions a quark within the proton may original quark (or gluon for that to reveal their prey. From PETRA annihilate with an antiquark within the matter) that produces a given jet. In and PEP, through TRISTAN, and antiproton to produce a top-antitop the 1970s initial évidence for such now to LEP, successive teams of pair. The CDF collaboration have jets at CERN's Intersecting Storage physicists have been disappointed to found 12 possible such events, which Rings hinted that there was some- discover that the top quark must lie together yield a central mass for the thing within protons that could be beyond their machine's reach. With top quark of 174 ±17 GeV - neatly knocked sideways in the head-on no sign of toponium at LEP's highest within the range predicted by LEP collisions - something which we can collision energy of 100 GeV, the top and the Standard Model. If thèse now identify as quarks. quark must weigh in at more than events eventually do prove to be the Later, clear jet signais were seen in half this, or 50 GeV. first examples of the top quark, the électron-positron annihilations and in However, LEP had a far more consistency of ail thèse numbers will experiments using lepton beams. A subtle way of indicating what the top be a great triumph, for the Standard new phase of jet studies began in quark's mass might be. With some 8 Model, and our understanding of the 1982 with the advent of big experi­ million Z particles collected up to the fundamental nature of matter. ments at CERN's proton-antiproton end of 1993 (and many more coming With the top quark mass accurately collider with calorimeters wrapped this year, see page 6), the four known, rather than just limited, the round the beam collision point to pick experiments at LEP measure the next step will be to probe the higgs up as much as possible of the parameters of the Z particle with symmetry breaking mechanism at the emerging energy, an important unprecedented accuracy - the mass heart of the electroweak theory. feature of ail today's major colliding- of the Z, for example, is fixed at beam experiments. 91.19 GeV to an accuracy of a few The charmed and bottom quarks MeV. Thèse and other LEP results were found through the discovery of can be compared with the prédictions the mesons that contain the quark of the Standard Model and constrain together with its antiquark. In princi­ as yet unknown quantities, such as pe, a similar meson formed from top the mass of the top quark. Other and anti-top should also exist, and input cornes from neutrino interac­ many hunts for the top quark have tions; from Stanford's linear collider, revolved around searches for such a SLC, using polarized beams; and particle. Electron-positron collisions from measuring the mass of the W provide a particularly clean way of particle, the electrically-charged producing mesons of this kind. In companion of the Z, in proton- thèse interactions the électron and its antiproton collision experiments - antiparticle (the positron) simply UA2 at CERN and CDF and D0 at annihilate into energy - a photon - Fermilab. (LEP will only be able to which can then rematerialize as any measure the W when its energy is quark or lepton together with its upgraded - January, page 6). antiparticle, providing there is enough For ail thèse précision results to be energy to create the mass of the new consistent within the Standard Model, pair. This is the way the J/psi ap- the top quark must exist, with a peared at the SPEAR collider at mass, to within about 10%, of 174 SLAC, when the machine's colliding GeV. électrons and positron beams had Now, however, it may be that the just the right energy to produce a top quark has at last appeared charm- anticharm pair. directly, in the CDF detector at Searches at électron-positron Fermilab. Fermilab's proton-proton colliders for a similar state of collisions at 1.8 TeV have just "toponium" gradually pushed up the enough energy that on rare occa­

CERN Courier, November 1994 5