Top Matters
Chris Quigg

Scientists gathered in Rio for the LAFEX International School in High-Energy Physics are paying close attention to an extraordinarily rare form of matter. They are hearing reports from modern explorers representing more than eight hundred physicists and students from Brazil, Canada, Colombia, France, Italy, India, Japan, Korea, Mexico, Russia, Taiwan, and the United States, who are engaged in the quest to find the top quark and measure its properties.

While debating the evidence for top, charting the next round of experimental studies, and theorizing about what it all means, the participants in LISHEP95 are living out a revolution in our perception of nature: our human dimensions are not favored over others for experiencing and comprehending the physical universe.

An astonishing aspect of nature’s richness is that whenever we explore an unfamiliar regime of distance, or time, or energy, we find new and interesting phenomena. Ours is not a universe of Russian dolls! When we study matter under unusual conditions, surprises are everywhere—from the superconductivity exhibited by many materials at very low temperatures to the new forms of matter created in particle accelerators at very high energies.

Poets speak of attaining some distance from the world—finding some strangeness—to express the truths that make a rock stony or a love sublime. Like the poet, the physicist seeks seek not merely to name the properties of nature, but to plumb their essence. To truly understand the world around us, to fully delight in its marvels, we must step outside everyday experience and look at the world through new eyes.

Physicists have known since the 1920s that to explain why a table is solid, or why a metal gleams, we must explore the atomic and molecular structure of matter. That realm is ruled not by the customs of everyday life, but by the laws of quantum mechanics. The hunt for the top quark reported in Rio makes us think anew about how the microworld influences our surroundings.

It is popular to say that top quarks were produced in great numbers in the fiery cauldron of the Big Bang some fifteen billion years ago, disintegrated in a fraction of a second, and vanished from the scene until my colleagues learned to create them in a giant accelerator near Chicago. That would be reason enough to be interested in top: to learn how it helped sow the seeds for the primordial universe that has evolved into the world of diversity and change we live in. But it is not the whole story, and it invests the top quark with a remoteness that hides its real importance to our lives.

The real wonder is that here and now, every minute of every day, the top quark affects the world around us. Through the uncertainty principle of quantum mechanics, top quarks and antiquarks wink in and out of an ephemeral presence in our world. Though they appear virtually, fleetingly, on borrowed time, top quarks have real effects.

A few numbers determine the dimensions and character of the everyday world, from the size of atoms to the energy output of the sun. Only a generation ago, these parameters of the quotidian—the mass of the proton, the mass of the electron, and the strengths of the fundamental interactions—seemed givens, beyond the reach of science. Today, we have begun to discern links among them. We see how each of them might be understood in principle, and even computed.

Of most pressing concern to particle physicists is top’s influence on the the weak force. The next crucial test will come from precise measurements of the masses of the top quark and the W-boson, the carrier of the weak force. If they stand in the predicted relation, we will have new confidence that we understand how the top quark influences our world. And our everyday world it is, because the weak force governs not only the rate of radioactive decay but also the intensity of the sun’s rays on Copacabana beach.

In fact, the mass of the top quark is encoded in the strengths of all the forces that rule the everyday world. We believe that the strong, weak, and electromagnetic forces all have equal strengths at some very high energy called the unification energy. The differences we observe in our low-energy world arise because the interactions evolved differently as the universe cooled. The way each one evolved depends on the character of the forces themselves and on the spectrum of particles that appear from very high energies down to the energy scale of common experience. Because the top quark stands apart as very much heavier than the other quarks, it has a special influence.

But that is not all. The proton’s mass is determined mostly by the energy stored up in the force that holds together the up and down quarks that make up the proton. The strength of the force between quarks is influenced by the top quark. The top quark is not a constituent of the proton, but if top weighed ten times more or less, the proton mass would shift up or down by about twenty percent. This world—our world—would have a very different character.

While particle physicists work far from the domain of everyday experience to establish the properties of the top quark, we can savor the realization that those properties are encoded in the form of every flower and grain of sand, in every human face. Top matters!

Prepared for the LAFEX International School in High-Energy Physics · Rio de Janeiro · 30 January 1995 · © Chris Quigg.