Dr. Leon Lederman, an experimental physicist and former director of the Fermi National Accelerator Laboratory, once offered the following portrait of the theoretical physicist as a young boy:
Mother: "Johnny, what are you doing?"Many people, scientists and otherwise, share Dr. Lederman's amusement at the seemingly arrogant confidence with which some theorists rely on pure reasoning in their construction of physical models. In the case of the top quark, however, it appears that the theorists may well have the last laugh. The top quark, for which experimenters have been searching for 17 years, is a subatomic particle whose existence is necessary to corroborate a theory that seeks to explain how matter and energy interact at the most basic, subatomic level. New evidence in support of this elusive particle's existence has recently been acquired from experiments conducted at Fermi National Accelerator Laboratory (commonly called Fermilab) in Batavia, Illinois. Several members of the Department of Physics at Duke participated in the experiments.
The top quark, sometimes called "truth," is one of six different types or "flavors" of quarks, which are subatomic particles that serve as the basic building blocks for hadrons, which encompass all particles that interact by means of the strong nuclear force. Aside from top, the remaining flavors of quarks are called up, down, charm, strange, and bottom. The top is the only remaining quark for which thoroughly conclusive evidence has yet to be produced.
Quarks have mass, electric charge, and spin (a kind of intrinsic angular momentum about an axis through the particle), but like the electron they do not appear to have any structure. It is for this reason that quarks (and leptons, a group of particles of which the electron is a member) are believed to be truly elementary: they cannot be resolved into something smaller.
If quarks and leptons prove to be truly elementary, then the discovery of the top quark (when and if its existence is finally confirmed) will represent the culmination of a 2400-year-old struggle to identify the fundamental building blocks of matter. The beginning of this effort is widely associated with the teachings of a Greek philosopher known as Democritus of Abdera in the fifth century B.C. He claimed that all matter is composed of tiny, indivisible particles he called atoms.
Democritus would probably be genuinely surprised at the abundance of truly elementary particles that have been identified by modern subatomic investigations. Current theory indicates the existence of twelve fundamental or elementary matter particles (six quarks and six leptons), not to mention their corresponding antiparticles. This may all seem relatively complicated, says Dr. Alfred Goshaw, professor of physics and director of the Duke high-energy research program, "but at the heart of it, the most remarkable thing is that nature is so simple." His illustration proceeds as follows: If the physical objects about us were to be reduced to their smallest, most fundamental components, there is no reason, a priori, why there could not be billions of different elementary particles.
This, however, is not the case. Most of the matter presently in the universe, explains Dr. Goshaw, is really composed of only three fundamental particles: the up quark, the down quark, and the electron. The up and down quarks combine to form protons and neutrons (collectively referred to as nucleons, or components of the nucleus), which along with electrons make up atoms, and hence all the matter of the discernible universe. "It is extraordinary," says Dr. Goshaw, "that these three particles and the interactions among them can give rise to an essentially infinite diversity."
If the bulk of the matter in the universe is composed of only three fundamental particles, then what is all the talk about other elementary particles, including the top quark? The answer lies in the fact that the universe was not always as it is today. A fraction of a second after the Big Bang, physicists believe that the extremely high temperatures of the early universe allowed for the creation of some particles that can no longer occur naturally. The top quark is a member of this category.
The high-energy conditions of the early universe are artificially recreated by particle accelerators, which produce beams of fast-moving, electrically charged atomic or subatomic particles. Under such conditions, particles that have not existed for billions of years may be created and studied.
The charm and bottom (or "beauty") quarks, for example, which are the last two quarks discovered, were created in the 1970's using particle accelerators. Partly because physicists believe that quarks come in pairs, the discovery of a fifth (bottom) quark in 1977 induced many theorists to affirm the existence of a companion quark, referred to as the top.
The theoretical significance of the top quark should not be underestimated. The prevailing theory of the particles and forces that determine the fundamental nature of matter and energy is called ("somewhat modestly," notes Dr. Goshaw) the Standard Model, which provides a detailed description of electromagnetism, and the strong and weak nuclear interactions. The Standard Model, explains Dr. Goshaw, "requires for its health the [existence of the] top quark, which fills out a pattern of fundamental quarks...and provides one of the last ingredient which allows you to make a self-consistent, predictive theory."
If the top quark is not found, writes Dick Teresi, "we'll see one of the greatest crises in the history of science...the Standard Model would crumble." Or still more forceful is a statement made by Dr. Lederman. If experiments prove that the top quark does not exist, he says, "Theorists will be jumping out of second story windows" (Lederman 1993).
The top quark is an extremely difficult particle to get a hold on. The primary obstacle involved in artificial top-quark production is its incredibly large mass. The greater the mass of a particle, the greater the energy required to produce it, in accordance with Einstein's famous equation illustrating the relationship between mass and energy: E = m*c–2, or energy equals mass times the square of the speed of light. This relationship motivates physicists to define the mass of a particle in terms of the energy needed to create it. The equation explains why the energy needed to create a particle is so high: the square of the speed of light is hardly a trivial value -- 9 * (10–16) * (m–2) / (s–2).
High-energy conditions may be generated by particle accelerators using a variety of methods. The fixed target type of accelerator, of which the early Stanford Linear Accelerator Center (SLAC) was an example, involves a beam hitting a target at rest. In the late 1960's experiments at SLAC showed that electrons fired at protons and neutrons are deflected at much wider angles than had been anticipated. The wide-angle scattering suggested that protons and neutrons are complex, or that they are composed of smaller particles. These investigations led to the discoveries of the up, down and strange quarks.
In spite of its many successes, however, the fixed target accelerator has one major drawback: energy wastage. This is explained by Dr. Seog Oh, associate professor of physics at Duke, in the following way: Imagine you have two balls, one stationary and the other set in motion towards the first. When the two balls collide, a large fraction of the initial energy of the system is diverted towards the resulting motion of the two balls. This motion-associated energy diminishes the available supply of energy from which new particles can be created. Furthermore, the proportion of energy wasted increases with the energy of the initial beam.
Most if not all of this energy wastage is avoided by using colliders, in which two beams of particles hit head on. Particle colliders, however, also have their drawback. As the well-known physicist Abraham Pais points out in his book Inward Bound, "This wonderful gain in energy is bought at the heavy price of low event rate. Fixed targets, usually solid foils, provide ever so many more chances for hits by a beam than will occur when two colliding beams, exceedingly dilute gases, encounter each other" (Pais 1986).
The "cost" of a low event rate notwithstanding, particle colliders employing electron-positron collisions succeeded in producing the charm and bottom quarks. (The positron is the antiparticle of the electron, or an electron with positive electric charge.) Collisions between electrons and positrons were, however, unable to produce top quarks.
Physicists reacted to this failure not by abandoning their belief in the top, but by increasing the value of its predicted mass. Dr. Goshaw provides some facts and numbers which help to explain this conclusion:
The highest-energy electron-positron collider (called LEP, located in Europe) has a center of mass energy on the order of 100 GeV (10–11 electron volts; an electron volt is the energy acquired by an electron that passes through a potential difference of one volt). In accordance with a conservation law, however, top quarks are always created in pairs, in top-antitop combinations. In order for an electron-positron collision to produce a pair of top quarks, therefore, the mass of the top must not exceed 50 GeV (recall that mass is measured in terms of energy); if the mass of the top does exceed this amount, then insufficient energy will be available for the production of a top-antitop pair.
As different experiments set up to discover the top have failed to do so, the predicted mass of the top quark has progressively increased. This in turn has made energy requirements for top quark production a limiting factor for accelerator laboratories across the world, forcing many would-be enthusiasts out of the race to complete what Dick Teresi describes as "the last great experiment of the 20th century." Fermilab's Tevatron, presently the most powerful accelerator in the world, is today the sole remaining facility with any hope of discovering the top quark.
In the Tevatron, which can attain total collision energies of 1.8 TeV (1.8 x 10–12 eV), beams of protons and antiprotons (often called p-bars) are circulated in opposite directions about a four-mile-long tunnel. The beams are focused and steered by over a thousand superconducting magnets. The tube inside of which the two beams circulate is a stainless steel oval pipe about two inches high and three inches wide. At two separate points in the accelerator the bunches of protons and p-bars, which are contracted by the magnets down to the radius of a human hair, are steered into a head-on collision. Fermilab's two detectors, called CDF and DZero, are positioned at these two collision sites. Here the composition of the beam tube changes from stainless steel to beryllium, a light metal that is relatively permeable to the particles created by the collision; this allows the particles to pass freely into the detectors (Teresi 42).
Detectors at Fermilab are in themselves incredibly complex, sophisticated pieces of machinery. A description of the CDF and DZero detectors in a Fermilab publication states that "Fermilab's detectors weigh 5,000 tons apiece; they are three stories high, crammed with the intricate circuitry for 100,000 electronic channels of information. Millions of lines of computer code control and monitor the detectors' operations. During collider operations at the Tevatron accelerator, control room crews operate the detector, taking eight hour shifts, 24 hours a day."
Such precision equipment is necessary to monitor the Tevatron's quarter-million collisions per second. Showers of particles are created from the proton-antiproton collisions, making the detection of any single particle an extremely difficult task.
The first job of the detector, therefore, is to weed out uninteresting or commonplace events from the mess of particles that are created from a collision. The detector next records as much information as possible about those particles associated with the remaining events. This involves a specific determination of each particle's path, energy, and charge. From this information it is possible to identify a particle's characteristic electronic signature, which proves that the particle was present at the scene of the collision.
Identification of the electronic signature of the top is made more difficult by the instability of the particle: a top quark, once created, immediately decays into secondary particles, which are the particles actually identified by the detector. In some cases, the secondary particles themselves decay, so that identification of the top often partially depends on the detection of what are really tertiary products of the collision. To make matters even more complex, the top may decay in several different ways, so that the detectors must be on the lookout for many different combinations of particles.
That such harrowing challenges have been willingly taken up by the high-energy physics community, that they have identified and struggled against every dimensional limitation imposed on human perception, more than vindicates one insider's description of two of the most salient characteristics of the particle physics community: "exceptional intellectual fearlessness, and a high incidence of the Shiva complex -- unhappiness with not having been born with three eyes and four arms" (Pais 1986).
Three eyes and four arms can wait. The CDF collaboration (Collider Detector at Fermilab), of which the Duke high-energy physics research team forms a part, may have just acquired the first direct experimental evidence for the existence of the top quark.
"Being quite conservative," remarks Dr. Goshaw, "in our analysis of the data and our interpretation of it, we believe that we have seen evidence for the production of the top quark in these collisions...We're confident on the level that there is only a few tenths of a percent probability that we are wrong."
The possibility of error stems from data contamination by non-top quark processes (called background) that mimic top-decay. Such background phenomena, however, are preemptively incorporated into the statistical expectations of top quark production. The number of plausible top-events recorded during the ten-month experiment (called a "run") in question significantly exceeds the predicted number of illegitimate events due to background interference.
According to a report issued by Fermilab: "In order to test whether the data requires the presence of a top quark, the experimenters calculated the probability that the entire observation is due to a statistical fluctuation of the background. This probability is one in four-hundred."
Most people, omni-skeptical philosophy students excepted of course, would find these statistics overwhelming. But with a staunch adherence to the rigor of scientific methodology, the CDF collaboration has cautiously refrained from proclaiming the "discovery" of the top quark.
Instead of "Discovery of the top quark" or "Search for the top quark," says Dr. Goshaw, "we have, somewhat ambiguously, entitled our papers something in between: `Evidence for the top quark.'" The CDF report sanctions the meticulousness of this approach by noting that, "Although [the statistical probability of error] appears very small, scientists generally require a smaller probability of a background fluctuation before a new phenomenon is firmly established."
Difficulties with issuing a definitive statement at present, says Dr. Goshaw, are largely associated with an inability to conduct cross-checks from the small handful of top-events that have been recorded.
"Small handful" doesn't quite tell the whole story: the actual numbers are staggering. During the ten-month (August 1992-May 1993) experimental run from which all data collected have already been analyzed, the total collisions which took place number on the order of 10–12. One million (10–6) of these were considered interesting enough to record on magnetic tape. And out of these 10–6 collisions, 12 top-events were singled out.
"It's like looking for a needle in a field, not a haystack," says Dr. Oh, with which statement he observes that even our most deeply-entrenched non-technical idioms break down at the frontiers of high-energy physics.
Whatever physical form the jumbled source may resemble, the good news is that new needles are still being painstakingly gathered. "We have taken another ten months of data," says Dr. Goshaw, "and we are continuing to take new data." This, he explains, makes the comparison of recorded top-events possible: cross checks will be conducted so that statistical fluctuations may be ruled out.
Assuming the results obtained from the ten-month data set were not a product of such statistical fluctuations, the measured mass of the top quark is 174 GeV, with an associated uncertainty of 17 GeV. This makes the top quark about as heavy as an entire gold atom! This also makes the top the heaviest of all the elementary particles, with a mass exceeding that of the next heaviest competitor, the bottom, by a factor of 35.
Another unusual feature of the top quark is concerned not with the nature of the particle itself, but with the mechanism by which "search" is translated into "discovery" for this particular investigation. Since experimental confirmation of the top's existence is predicated on the progressive accumulation of evidence over a long period, many scientists note that the top quark investigation lacks a well-defined "eureka moment." As Fermilab experimenter Claudio Campagnari observes, "Some physics discoveries stare you in the face -- you almost can't miss them. You see the signature event and you know you've got it. But top is probably not one of those." In the search for the top and similar experiments, it is extremely difficult to pin down the "discovery" to any specific point in time. And by the time the discovery is made, many view the finding as old news.
Lest these observations cast the current top quark experiments at Fermilab in a somewhat dreary light, it should be noted that the actual "discovery" of the top is only the necessary first step of a broader investigation. Once its existence has been unequivocally established, the top will be extensively studied, opening up many exciting areas of research. Susanne Hauger, a Duke graduate student in high-energy physics, emphasizes this point in saying, "A project like this never really ends. It just moves to different regions. There is still plenty of work to be done."
Perhaps the most significant, fascinating question to be addressed by future top quark investigations concerns the origin of mass, one of the unexplained missing pieces of the previously mentioned Standard Model. The enormous mass of the top, the most massive of all the elementary particles, leads many physicists to believe that the top may shed some light on this issue. Theorist Chris Hill of Fermilab claims that an understanding of the origin of mass would rank as "an achievement on a par with the greatest scientific strides in history, like Newton's establishing the universal law of gravitation or Einstein's connection of energy to mass and the speed of light."
Special thanks to Dr. Goshaw and Dr. Oh, and to graduate students Susanne Hauger and Malie Yin, for their generous contributions of time and information. Thanks also to the Fermilab Public Information Office.
At the time this article was written, Amit Agarwal was a Trinity college sophomore interested in physics and biology.
Winter 1994 contents
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