Hail and Farewell, Grand Colliders

Physicists, to their credit, are notoriously unsentimental and future-oriented. "Today's sensation is tomorrow's calibration" describes their modus operandi.

Nevertheless! Today America's flagship collider, the Tevatron at Fermilab in Illinois, will cease operation, and Europe's Large Hadron Collider (LHC) at CERN, near Geneva, will lead the exploration of the deep microcosmos. That changing of the guard inspires reflection, and merits celebration too.

Some historical perspective will highlight the special role of colliders in fundamental physics. The goal of experimental work in fundamental physics, crudely speaking, is to find out what the smallest, most basic building-blocks of the material world are, and how they behave (which, if you think about it, effectively defines what they are). In the early days of science, optical microscopes revealed the existence of tiny creatures and the cellular structure of life in general. But light cannot resolve structures much smaller than its wavelength, and the wavelength of ordinary light is tens of thousands of times larger than the size of atoms.

X-rays, discovered much later, get closer to atomic scales. Rosiland Franklin's x-ray pictures of DNA crystals enabled Crick and Watson to decipher DNA's molecular structure. At this point, a simple question may suggest itself: Why was any deciphering necessary--can't you just look at the darned picture? The answer is profound, and central to our story. It's easy to take for granted a most fortunate and unusual feature of ordinary light, namely that lenses can bend it and (when suitably arranged) automatically form images of illuminated objects. Our very own eyes do that trick, which is what makes it so easy to overlook! But there are no good lenses for x-rays. Instead of images, when we scatter x-rays off matter we get patterns of greater or lesser brightness called diffraction patterns. Then we've got to use our brains to make models, using everything else we know about x-rays and matter, for what could have caused the observed diffraction pattern.

To probe structures much smaller than atoms we must get well beyond x-rays, using more extreme forms of illumination. Energetic particles are the tools for this job; and the smaller the structures we aspire to "see," the higher the energies we need. Physicists study what emerges when those projectiles impact on matter--or, in the jargon, scatter on targets. Then, like Crick and Watson, they make models of what could be responsible. (Actually, nowadays theorists usually provide myriad models in advance, and then experimentalists disprove all but one of them!) In the early days of nuclear physics, particles emitted in natural radioactivity (especially "alpha particles," later identified as helium nuclei) were the workhorse probes; later cosmic rays--high-energy particles raining down from space--despite their obvious inconvenience, and unreliability, took the lead. Those techniques led to some tantalizing discoveries, but their limitations were crippling.

Further progress required that experimentalists wean themselves from natural sources. They had to learn how to pump up the energy of particles, collect them, and guide them to targets. A long series of brilliant innovations led to the modern collider. One innovation in particular is so unlikely-sounding, yet so crucial, that it deserves special mention. According to the theory of relativity, particles moving close to the speed of light are flattened in the direction of motion, but retain their size in the transverse direction, so that they appear as narrow pancakes. For our purposes, that's great--it allows the probes to be sharply localized, so they can take high-resolution pictures. It's so advantageous that physicists double down on it. Rather than impacting energetic particles on a stationary target, at a modern collider highly energetic particles moving in one direction impact other highly energetic particles moving in the opposite direction. At the Tevatron protons collide with antiprotons; at the LHC it's protons on protons. To make such collisions happen, though, is no mean feat, because the targets are comparatively few, and each is very small indeed. It takes powerful, intricately patterned electric and magnetic control fields, and ultra-fast monitoring and feedback, to bring tight counter-circulating beams to the same place at the same time.

For this and many other reasons modern colliders are fantastic engineering projects. They employ instruments and ideas more complex and much more varied than are involved, for example, in space exploration. They are big, and expensive. The main Tevatron ring, where the beams circulate, is almost four miles around, and the various pieces of the project cost about $1 billion altogether. The LHC is about five times as big, and five times as expensive.

These great colliders are, I think, monuments to our dynamic scientific civilization. They are our Pyramids; but they are better motivated and much better engineered than the originals. There's been some progress in five thousand years!

A bittersweet corollary of dynamism, however, is that once-great things eventually become passé. With the coming of the LHC, which makes more and more energetic collisions, the Tevatron, its glory days gone, is ready for retirement.

What did the Tevatron teach us? I think most physicists would agree that its single most spectacular achievement was the discovery of the top quark, in 1995. The top quark is the next-to-last piece in the wildly successful Standard Model of fundamental physics. That set of ideas provides a reasonably compact census of the building blocks of matter, and precise, beautiful equations for their observed interactions; but it is less informative when it comes to their masses. The mere existence of the top quark was a firm prediction of the Standard Model since at least 1977, but theory gave no firm prediction for its mass. In fact the large value of that mass--about 185 times the mass of a proton, and more than 40 times the mass of the next-heaviest quark (bottom)--came as a shock to most. Together with the large mass comes an extraordinarily short lifetime, estimated at 5 ×10−25 seconds. Its large mass makes the top quark difficult to produce, and its short lifetime makes it challenging to detect, so the discovery was a tremendous technical achievement.

How can a single elementary particle be so heavy? We still don't know the answer to that question, or even whether it's a sensible question to ask. (A better question, I think, is why the other quarks are so light; but that's a story for another time.) In any case, the striking divergence among masses of otherwise very similar particles--i.e., different kinds of quarks--brings us face to face with our ignorance.

Pending deeper understanding, we can already draw important inferences from the large top-quark mass. Masses of quarks, within the Standard Model, reflect the strength of their interaction, or coupling, with the mass-giving Higgs field. The large top-quark mass implies quite a strong coupling. That coupling is in effect a powerful new force, that must be taken into account in constructing more encompassing models of physics. Its ultimate significance is presently unclear, but it makes the idea of supersymmetry--a most interesting and attractive hypothesis on other grounds--work more smoothly; so that is the direction it might be pointing us toward.

Several other pretty discoveries were made at the Tevatron, but I think its other most important result, besides the top quark discovery, was to confirm, in many demanding quantitative tests, the correctness of the core theories of the Standard Model. These triumphs of a beautiful, economical theory put many gratuitous speculations to rest, and demonstrated Nature's good taste. As a practical matter, this result provides a firm platform upon which we can stand, as we reach toward still more beautiful, unified, and encompassing understanding.

What's next?

The last, still missing piece of the Standard Model is the so-called Higgs particle. Just as it predicted the top quark, theory firmly predicts the existence of the Higgs particle, but not the value of its mass. The Tevatron was able to constrain that mass to a fairly narrow range (between about 122 and 160 proton masses), but ran out of time before reaching a conclusive result. The LHC will get to the finish line, very likely, within the next year or so. A Higgs particle in that mass range would be yet another favorable omen for supersymmetry. Unless Nature is a shameless tease, we'll see supersymmetry itself--that is, some of the new particles supersymmetry predicts--discovered at the LHC, though that might take longer. Should those profound discoveries occur, as I hope and expect, they will bring our understanding of Nature's foundational principles to a new, higher level. We will build upon the Tevatron's achievements even as we transcend them.

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Frank Wilczek

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