Introduction to Found--The Top Quark

March 2, 1995, was a red-letter day for researchers at the Fermi National Accelerator Laboratory near Batavia, Ill., and for their colleagues around the world. At a dramatic press conference, two teams of Fermilab investigators jubilantly announced that there was no longer any serious doubt that they had discovered what may be nature's last missing building block of matter, a subatomic particle known as the top quark. The announcement, however, hardly came as a surprise. At an earlier press conference, in April 1994, one of the research teams had reported suggestive, though not convincing, evidence that it had found the top quark. What had changed in the intervening months was the degree of certainty regarding the existence of the elusive particle. By March 1995, three times as much data had been collected to strengthen the case, and a second team had compiled similar experimental results.

Fermilab particle physicists—investigators of matter at the smallest levels—emphasized that the finding, though significant, didn't point to anything new or unexpected. Researchers in the field had been convinced for years that the top quark had to exist—they simply hadn't been able to find it. Moreover, the discovery of the top quark did not signal the opening of an exciting new era of research, but rather the conclusion of what might be the last chapter in the age-old quest for the basic building blocks of matter.

The Top Quark Affirms the Standard Model

That chapter deals with the so-called Standard Model, a theoretical picture of the basic units of matter that had guided research in particle physics for more than 20 years. Failure to find the top quark would have cast doubt on the Standard Model and forced physicists to rethink their ideas of how the universe is constructed at the most fundamental levels. With the top quark in hand, physicists could begin turning their attention to other questions in their search for the ultimate nature of matter.

It is a search that goes back to the dawn of scientific thinking in ancient Greece. It has been above all a quest for simplicity, motivated by the belief that underlying the complexity and unruliness of the everyday world, there exists a much simpler order. In that realm, far too small for the human eye to see, all matter—whether in the stars above our heads or the dirt beneath our feet—would reveal itself to be made of just a few elementary building blocks.

One of the first thinkers to theorize about matter at the smallest scales was the Greek philosopher Democritus, who lived from about 460 to 370 B.C. Democritus said matter could be reduced to tiny particles that he called atoms, meaning “uncuttable.” He chose that name because he envisioned the particles as being the most basic bits into which anything could be divided. We still use the word atom for the smallest complete unit of an element, such as iron or gold. But in the past 100 years, scientists have learned that atoms, far from being uncuttable, can be split apart into a number of even smaller pieces.

Deeper Into the Atom

The discovery of the atom's true nature began in 1897 when the British physicist Joseph John Thompson discovered that atoms contain lightweight, negatively charged particles. Thompson theorized that these particles—electrons—were embedded in a positively charged sphere containing most of an atom's mass (quantity of matter).

In 1911, the work of another British physicist, Ernest Rutherford, led to a better view of the atom. Rutherford fired alpha particles (helium atoms stripped of their electrons) at thin sheets of gold foil and found that most of the particles passed through the foil while some were deflected. From this discovery, Rutherford concluded that the massive positive portion of an atom is concentrated in a very small central nucleus. The electrons, he said, circle the nucleus at enormous speed, leaving a great deal of empty space around the nucleus.

Physicists continued to develop their view of the atom. In 1913, the Danish theorist Niels Bohr theorized that an atom's electrons are confined to certain shells, or orbits, around the nucleus, an idea that was later confirmed and refined. The following year, Rutherford proposed that the proton, a massive, positively charged particle that had been discovered in 1902, must be a part of the nucleus. And in 1932, another British researcher, James Chadwick, found that the nucleus also contains other particles with a mass close to that of the proton but with no electric charge. These neutral particles were dubbed neutrons.

For a while the subatomic world looked simple enough, with just three basic parts—electrons, protons, and neutrons—required to make an atom. But as physicists soon learned, they had only begun to understand how matter is constructed. Further experiments with machines called particle accelerators, which slam beams of particles together or into stationary nuclei, showed that atoms were quite complex.

Particle Accelerators—atom Smashers

Particle accelerators, which can be thought of as giant microscopes able to peer deep into the atom, use electrical forces to boost protons or other charged particles to very high speeds. The faster the particles go, the more energy they possess. The earliest particle accelerators could fit on a tabletop, but they grew progressively larger as physicists sought to achieve higher and higher energies. Fermilab, the world's most powerful accelerator, which began operation in 1972, has an accelerating ring 6.4 kilometers (4 miles) around. And the Superconducting Super Collider (SSC), a huge accelerator that was being constructed in Texas until the United States Congress ended its funding in late 1993, would have been 87 kilometers (54 miles) in circumference.

The energy imparted to a speeding particle in an accelerator is expressed in units called electronvolts. An electronvolt (eV) is a tiny energy unit, the energy gained by a single electron passing through a 1-volt battery. (A standard flashlight battery is 1.5 volts.) But particles can be accelerated to enormous energies. The first generation of particle accelerators, developed in the early 1930's, accelerated beams of particles to energies of about 1 million electronvolts (MeV), and by the early 1950's machines had been developed that produced energies in the billions of electronvolts (GeV, from giga, the prefix for billion).

The early particle accelerators were often called “atom smashers,” because they were used to break atomic nuclei apart for study. Over a period of 25 years, these machines unlocked most of the secrets of the nucleus. And with larger machines in the GeV range, physicists discovered that the proton and neutron were not simple objects but had complicated internal structures.

Imposing Order On A Profusion of Particles

The GeV accelerators of the 1950's and 1960's created a dizzying number of previously unknown nuclear particles rarely found in nature. They could do this because, as the famous German-American physicist Albert Einstein demonstrated in his equation E = mc², matter and energy are equivalent. Under the right conditions, matter can be converted into energy, and energy turned into matter. When two particles are accelerated in opposite directions to an energy of many GeV and then crashed together, the combined energy of the collision condenses into a swarm of particles that flash into momentary existence and then decay into other kinds of particles. The only visible remnant of a collision is an array of swirling lines captured with a camera or other recording device, tracing the myriad paths of the various particles.

By the 1960's, the number of particles created by the collisions was becoming bewildering. Surely, in their number and variety, the particles could not all be fundamental building blocks of matter. Physicists were desperate for a theory that would impose order on the apparent chaos of the subatomic world.

About Quarks

In 1964, two American physicists, Murray Gell-Mann and George Zweig, offered a theory that seemed to do just that. They proposed that protons, neutrons, and many of the new particles being created in particle accelerators were composed of combinations of truly fundamental particles that Gell-Mann called quarks.

Quarks, according to the theory, had one very unusual property: They were never seen alone, but only in combinations of two or three. Another peculiarity was that they carried electric charges that were a fraction—either -1/3 or +2/3—of the fundamental unit of electric charge carried by the proton. When quarks combined to make particles, these fractional charges added up to a single positive or negative charge or to no charge at all.

The theory predicted that only three kinds of quarks—called up, down, and strange—were required to make up all the known subatomic particles. Protons and neutrons were made of up and down quarks. A proton consisted of two up quarks and one down quark; a neutron contained two down quarks and one up quark. The strange quarks were found in several other kinds of heavy particles that are seldom seen in nature but which are often created during high-energy collisions in particle accelerators.

By 1978, high-energy research had created various new particles requiring the existence of a fourth and fifth quark, which were dubbed charmed and bottom. And the evolving quark theory suggested that quarks come in pairs, so physicists concluded that a sixth quark—the top quark—must also exist.

Fitting Quarks Into An Overall View of Matter

The quark theory fit into a larger scheme of matter, the Standard Model. According to the Standard Model, matter is made up of 12 fundamental particles divided equally into two major families, quarks and leptons. Leptons, which are thought to be fundamental particles with no smaller parts, include the electron, two heavier versions of the electron, and three uncharged particles called neutrinos. Neutrinos were long thought to be massless, but recent research has indicated that they may possess a tiny amount of mass.

Only the two lightest members of each family—the up and down quarks, and the electron and electron neutrino—play any major role in the universe today. All the heavier quarks and heavier charged leptons have only a momentary existence, because they are highly unstable. These particles can be created in particle accelerators, but they existed in nature in abundance only in the first few moments after the big bang, the colossal explosion of matter and energy that astronomers believe gave birth to the universe billions of years ago. When physicists push particle accelerators to higher and higher energies, they are in effect duplicating the conditions of the big bang.


Another kind of matter, known as antimatter, is also rarely seen in our world, though it plays a large role in the subatomic realm and is created in profusion in particle accelerators. Antimatter is like regular matter but opposite in electric charge and certain other properties. Thus, for example, the antimatter equivalent of the proton is the negatively charged antiproton, which is made of three antiquarks. It is a rule of nature that for every particle of matter that comes into existence, an antiparticle must also be created at the same time. It is fortunate for us that antimatter is not found in the everyday world, because when regular matter and antimatter come into contact, they annihilate each other in a burst of pure energy.

Bosons—messenger Particles

In addition to the fundamental building blocks of matter and their antimatter opposites, there are particles called bosons that act as “messengers,” transmitting the three basic forces that operate in the microworld. One of these messenger particles is the massless photon, or particle of light, which transmits electric and magnetic forces. Another is the gluon, also without mass, which conveys the strong force that binds quarks together in protons and neutrons. And finally, there are three heavy particles—the W+, W-, and Z°—that transmit a much weaker force that is responsible for some forms of radioactivity.

The force-carrying messenger particles are emitted by one particle and quickly absorbed by another. Thus, they have only a temporary, fleeting existence. But without them, our world would not be possible. Bosons are the “mortar” that holds together the “bricks,” the fundamental building blocks, to form atoms and molecules—and people.

The Standard Model, while bringing order to the subatomic world, is hardly the elegantly simple picture of reality that physicists had been hoping for—not with 12 kinds of bricks and several kinds of mortar. This complexity causes many scientists to believe that the Standard Model is not the ultimate picture of nature we have been seeking. Perhaps, they say, quarks and leptons are not fundamental particles after all but rather are constructed from an even simpler set of building blocks. Other scientists, however, think that quarks and leptons may be as basic as things are going to get. They contend that nature has no obligation to conform to our ideas of how it should be constructed and that its essence may be complexity, not simplicity.

The Elusive Top Quark

For years, the validity of the Standard Model was open to question because the reality of quarks could not be verified. Quarks produced in particle accelerators leave only indirect evidence of their vanishingly brief existence, so it took nearly a decade of experimentation before most physicists accepted quarks as real particles. But by the 1980's, the quark theory was firmly established, and researchers had found convincing evidence for the up, down, charmed, strange, and bottom quarks. The top quark, however, continued to elude them.

What made the top quark so hard to find was its enormous mass, now estimated at 176 GeV. (Because of the equivalence of mass and energy, the mass of a particle is usually expressed as the number of electronvolts needed to create it.) Since the top quark could be produced only in a pair with an antitop quark, more than 350 GeV of energy was required for its creation. Until 1985, no accelerator in the world, including the one at Fermilab, was powerful enough to produce such energies. In that year, Fermilab completed modifications to its accelerator ring that changed it from a single-beam accelerator to one that uses two beams of particles—one of protons, the other of antiprotons. The beams are accelerated in opposite directions to just a shade under the speed of light (about 300,000 kilometers [186,000 miles] a second) and then brought to a sharp focus. There, many of the protons and antiprotons collide and disappear in violent flashes of energy.

With 2 Trillion Electronvolts, the Search Is On

The alterations enabled Fermilab to achieve an energy of nearly 1 trillion electron volts (TeV, from tera, the prefix for trillion) per beam. When the beams collide, their total combined energy—close to 2 TeV—is available for particle creation. With the upgraded machine, named the Tevatron, the search for the top quark could begin.

The production of top quarks was expected to be a very rare event, happening in fewer than 1 collision in a billion. Identifying that 1-in-a-billion occurrence would be no easy task. The Fermilab researchers would have to examine the particle tracks left by countless individual collisions. The track left by a top quark itself would not be visible because the particle (and its antimatter counterpart) would survive for less than a trillionth of a quadrillionth of a second before decaying into other particles, far too short a time for it to emerge from the collision site. Moreover, the particles the top quark breaks up into are themselves highly unstable and short-lived. Only after two more particle decays would particles stable enough to be observed be created. By the end of this complicated chain of events, 20 or so particles would have been generated. Physicists would be looking for particular patterns, or signatures, of particle creation in which the production of a top quark was the most likely initial event.

Electronic Particle Detectors Get In On the Act

They would be aided in that task by an impressive array of electronic particle detectors. In modern accelerators, the point where the particle beams collide is surrounded by layers of detectors that produce an electronic signal whenever an electrically charged particle passes through them. These signals go to computers that decide instantaneously whether something noteworthy has happened, and, if so, record all the signals on magnetic tape. Other computers then use the recorded data to reconstruct the paths of the particles and create a detailed image of the debris emerging from the collision.

The Tevatron has two such detector systems. One of them, called the Collider Detector at Fermilab (CDF), was responsible for the data presented in April 1994. The other is known simply as “D-Zero,” a code name designating its location along the Tevatron ring. The detectors are huge and expensive devices. The CDF, for example, is the size of a three-story building and contains more than 100,000 individual particle detectors arranged in more than 100 layers.

By the 1990's, scores of Fermilab investigators were occupied with studying the images being generated by the CDF. Calculations showed that during one eight-month period, from August 1992 to May 1993, the Tevatron had probably created nearly 100 top quarks. But finding the telltale signatures for that handful of events among the tens of millions of collisions that the CDF had recorded was a difficult task. Nobody could look at all of the events recorded by the computers—there were simply too many of them. The hunt for top-quark signatures thus began by ordering the computers to sort through the mass of data looking for events with certain desired characteristics. The researchers continued to narrow the sample of possible signatures until they arrived at a few “gold-plated” candidates for the top-quark.

The Top-quark Signature

Actually, the hardest part of the search was not identifying the best signatures but showing that those signatures were indeed caused by the top quark. Some sequences of particle decay initiated by other kinds of particles produce signatures very similar to those of the top quark. These “fake” patterns must be ruled out before an event can be accepted as a true top-quark signature.

Separating the fakes from the genuine article is based on probabilities. Physicists might determine, for example, that a certain signature should occur no more than once in 10 million collisions if it is caused by a non top-quark event. If the pattern shows up several times that often, it is more likely to be a top-quark signature than a fake.

Tentative Success, and Then Certainty

By October 1993, two Fermilab teams working with the CDF had found three different signatures that they felt were reasonably likely to have resulted from the top quark. However, there were too few examples of any one of the signatures for the results to be conclusive. Together, the two teams had found 12 possible top-quark events. Calculations showed that about four of those should be fakes, indicating that eight were true top-quark signatures. But because the probabilities of the fake-event rates were somewhat uncertain, the physicists could not say with complete confidence that the events were not all fake.

Under normal circumstances, the CDF researchers would have opted to wait for more convincing data before making an announcement. But in early 1994, it looked like it might take another year or two before a convincing case could be made for the top quark. In the meantime, news of the 12 top-quark candidates was spreading through computer networks to particle physicists around the world. The Fermilab experimenters therefore decided to combine the data compiled by the two CDF teams into a single report that was then subjected to an intensive review by all of those involved with the research. In the end, 399 members of the Fermilab group decided that the results were solid enough to risk putting their names on the report.

Despite the uncertainty, most physicists accepted the Fermilab findings as “probably real.” And further data from the Tevatron supported the earlier evidence. By end of 1994, the CDF researchers and a team using the D-Zero detector were finding additional signatures that were most likely due to the top quark, and the D-Zero investigators had submitted their own paper confirming the CDF results. In early 1995, still more signatures were found with both detectors. In March, Fermilab held another press conference, this time to announce that the top quark had definitely been discovered.

The End of the Frontier For Particle Physicists

The confirmation of the top quark was a triumph for particle physicists. But by filling in the final blank in the Standard Model's list of particles, they were left with little more to do. In a field that has been based on moving ever forward to new and deeper levels of understanding, coming to the end of the frontier was for particle physicists a frustrating situation.

The SSC, which would have accelerated two proton beams to energies of about 20 TeV, had been designed to break particle physics out of that impasse. For physicists who continued to dream of a few “ultimate” particles from which all others are made, the SSC provided the hope of finding evidence that both quarks and leptons are constructed of smaller, simpler parts. Aside from that quest, a primary mission of the SSC was to find one final piece of the Standard Model, a messenger particle called the Higgs boson that physicists think endows other particles with mass.

With the demise of the SSC, the search for the Higgs boson and for ultimate building blocks will most likely shift to Europe. The European Laboratory for Particle Physics (CERN) near Geneva, Switzerland, is planning to build a particle accelerator called the Large Hadron Colider (LHC), a proton-proton collider that initially will generate beam energies of about 5 TeV and may eventually reach 7 TeV. (Hadrons include any particles made of quarks.) For a while, it appeared that the LHC, too, might be axed, because several of the 19 CERN member nations were balking at paying for the expensive facility. But in January 1995, the project got the go-ahead.

For physicists who were gearing up to work at the SSC, the LHC was a decided comedown. It may not generate enough energy to produce significant numbers of Higgs particles, and in any event it will not be ready before the year 2003. Particle physicists say that, given such prospects, it is getting harder and harder to attract promising young people to the field. Other areas of physics, such as astrophysics and laser optics, appear to be “where the action is.” It is possible that after a highly productive 60-year reign on the forefront of physics, particle accelerators may finally be put to rest. Finding the top quark may prove to be one of their final triumphs.