Knocking on Heaven's Door (18 page)

Dirac racked his brain before capitulating to the equation and admitting this mysterious particle had to exist. The American physicist Carl Anderson discovered the positron in 1932, verifying Dirac’s assertion that “The equation was smarter than I was.” Antiprotons, which are significantly heavier, were not discovered until more than twenty years later.

The discovery of antiprotons was important not only for establishing their existence, but also for demonstrating a matter-antimatter symmetry in the laws of physics essential to the workings of the universe. The world is, after all, made of matter, not antimatter. Most of the mass of ordinary matter is carried by protons and neutrons, not by their antiparticles. This asymmetry in matter and antimatter is critical to the world as we know it. Yet we don’t yet know how it arose.

DISCOVERY OF QUARKS

Between 1967 and 1973, Jerome Friedman, Henry Kendall, and Richard Taylor led a series of experiments that established the existence of quarks inside protons and neutrons. They did their work at a linear accelerator, which—unlike the circular cyclotrons and Bevatrons before it—accelerated electrons along a straight line. The accelerator center was named SLAC, the Stanford Linear Accelerator Center, located in Palo Alto. The electrons that SLAC accelerated radiated photons. These energetic—and hence short-wavelength—photons interacted with quarks inside the nuclei. Friedman, Kendall, and Taylor measured the change in interaction rate as the energy of the collision increased. Without structure, the rate would have gone down. With structure, the rate still decreased, but much more slowly. As with Rutherford’s discovery of the nucleus many years before, the projectile (the photon in this case) scattered differently than if the proton was a blob that lacked structure.

Nonetheless, even with experiments performed at the requisite energy, identifying quarks wasn’t entirely straightforward. Technology and theory both had to progress to the point that the experimental signatures could be anticipated and understood. Insightful experiments and theoretical analyses performed by the theoretical physicists James Bjorken and Richard Feynman showed that the rates agreed with the predictions based on structure inside the nucleus, thereby demonstrating that structure in protons and neutrons—namely, quarks—had been discovered. Friedman, Kendall, and Taylor were awarded the Nobel Prize in 1990 for their discovery.

No one could have hoped to use their eyes to directly observe a quark or its properties. The methods were necessarily indirect. Nonetheless, measurements confirmed quarks’ existence. Agreement between predictions and measured properties, as well as the explanatory nature of the quark hypothesis in the first place, established their existence.

Physicists and engineers have over time developed different and better types of accelerators that operated on increasingly larger scales, accelerating particles to ever higher energy. Bigger and better accelerators produced increasingly energetic particles that were used to probe structure at smaller and smaller distances. The discoveries they made established the Standard Model, as each of its elements was discovered.

FIXED-TARGET EXPERIMENTS VERSUS PARTICLE COLLIDERS

The type of experiment that discovered quarks, in which a beam of accelerated electrons was aimed at stationary matter, is known as a
fixed-target
experiment. It involves a single beam of electrons that is directed toward matter. The matter target is a sitting duck.

The current highest-energy accelerators are different. They involve collisions of two particle beams, both of which have been accelerated to high energy. (See Figure 21 for a comparison.) As one can imagine, those beams have to be highly focused into a small region to guarantee that any collisions can take place. This significantly reduces the number of collisions you can expect, since a beam is much more likely to interact with a chunk of matter than with another beam.

Fixed-Target Setup

Particle Collider

[
FIGURE 21
]
Some particle accelerators generate interactions between a beam of particles and a fixed target. Others collide together two particle beams.

However, beam-beam collisions have one big advantage. These collisions can achieve far higher energy. Einstein could have told you the reason that colliders are now favored over fixed-target experiments. It has to do with what is known as the
invariant mass
of the system. Although Einstein is famous for his theory of “relativity,” he thought a better name would have been “Invariantentheorie.” The real point of his quest was to find a way to avoid being misled by a particular frame of reference—to find the invariant quantities that characterize a system.

This idea is probably more familiar to you for spatial quantities such as length. Length of a stationary object doesn’t depend on how it is oriented in space. An object has a fixed size that has nothing to do with you or your observations, unlike its coordinates, which depend on an arbitrary set of axes and directions you impose.

Similarly, Einstein showed how to characterize events in a way that doesn’t depend on an observer’s orientation or motion. Invariant mass is a measure of total energy. It tells you how massive an object can be created with the energy in your system.

To determine the amount of invariant mass, one could ask this instead: if your system were sitting still—that is, if it had no overall velocity or momentum—how much energy would it contain? If a system has no momentum, Einstein’s equation
E = mc
²
applies. Therefore, knowing the energy for a system at rest is equivalent to knowing its invariant mass. When the system is not at rest, we need to use a more complicated version of his formula that depends on the value of momentum as well as energy.

Suppose we collide together two beams with the same energy and equal and opposite momentum. When they collide, the momenta add up to zero. That means that the total system is already at rest. Therefore, all the energy—the sum of the energy of the particles in the two individual beams—can be converted to mass.

A fixed-target experiment is very different. One beam has large momentum, but the target itself has none. Not all the energy is available to make new particles because the combined system of the target and the beam particle that hit it is still moving. Because of this motion, not all the energy from the collision can be transferred into making new particles, since some of the energy remains as kinetic energy associated with the motion. It turns out that the available energy scales only with the square root of the product of the energy of the beam and the target. That means, for example, that if we were to increase the energy of a proton beam by 100 and collide it with a proton at rest, the energy available to make new particles would increase by only a factor of 10.

This tells us there is a big difference between fixed-target and beam-beam collisions. The energy of a beam-beam collision is far greater—much bigger than twice as big as a beam-target collision, which is perhaps what you might assume. But that guess would be based on Newtonian thinking, which doesn’t apply for the relativistic particles in that beam that travel at nearly the speed of light. The difference in net energy of fixed-target compared with beam-beam collisions is much bigger than the simple guess because at near the speed of light, relativity comes into play. When we want to achieve high energies, we have no choice but to turn to particle colliders, which accelerate two beams of particles to high energy before colliding them together. Accelerating two beams together allows for much higher energy, and hence much richer collisions.

The LHC is an example of a collider. It bangs together two beams of particles that magnets deflect so that they will be aimed toward each other. The principal parameters that determine the capabilities of a collider such as the LHC are the type of particles that collide, their energy after acceleration, and the machine’s
luminosity
(the intensity of the combined beams and hence the number of events that occur).

TYPES OF COLLIDERS

Once we have decided that two beams colliding can provide higher energy (and hence explore shorter distances) than fixed-target experiments, the next question is what to collide. This leads to some interesting choices. In particular, we have to decide which particles to accelerate so that they participate in the collision.

It’s a good idea to use matter that’s readily available here on Earth. In principle, we could try to collide together unstable particles, such as particles called
muons
that rapidly decay into electrons, or heavy quarks such as
top quarks
that decay into other lighter matter.

In that case, we would first have to make these particles in a laboratory since they are not readily available. But even if we could make them and accelerate them before they decayed, we’d have to ensure that the radiation from the decay could be safely diverted. None of these problems are necessarily insurmountable, particularly in the case of muons, whose feasibility as particle beams is currently under investigation. But they certainly pose additional challenges that we don’t face with stable particles.

So let’s go with the more straightforward option: stable particles available here on Earth that don’t decay. This means light particles or at least bound stable configurations of light particles such as protons. We also would want the particles to be charged, so that we can readily accelerate them with an electric field. This leaves protons and electrons as options—particles that are conveniently situated in abundance.

Which should we choose? Both have their advantages and their downsides. Electrons have the advantage that they yield nice clean collisions. After all, electrons are fundamental particles. When you collide an electron into something, the electron doesn’t partition its energy into lots of substructure. So far as we know, the electron is all there is. Because the electron doesn’t divide, we can follow very precisely what happens when it collides with anything else.

That’s not true for protons. Recall that protons are composed of three quarks bound together by the strong nuclear force with gluons exchanged among the quarks that “glue” the whole thing together, as was discussed in Chapter 5. When a proton collides at high energy, the interaction you are interested in—that could produce some heavy particle—generally involves only one individual particle inside the proton, such as a single quark.

That quark certainly won’t carry all the energy of the proton. So even though the proton might be very energetic, the quark will generally have much less energy. It can still have quite a bit of energy, just not as much as if the proton could impart all its energy into that single quark.

On top of that, collisions involving protons are very messy. That’s because the other stuff in the proton still hangs around, even if it’s not involved in the super-high-energy collision we care about. All the remaining particles still interact through strong interactions (aptly named), which means there is a flurry of activity surrounding (and obscuring) the interaction you are interested in.

So why would anyone ever want to collide a proton in that case? The reason is that the proton is heavier than an electron. In fact, the proton mass is about 2,000 times greater than that of an electron. It turns out that’s a very good thing when we try to accelerate a proton to high energy. To get to these enormous energies, electric fields accelerate particles around a ring so that they can be accelerated more and more in each successive go-round. But accelerated particles radiate, and the lighter they are, the more they do so.

This means that even though we’d love to collide together super-high-energy electrons, this won’t happen any time soon. We can accelerate electrons to very high energies, but high-energy electrons radiate away a significant fraction of their energy when they are accelerated around a circle. (That’s why the Stanford Linear Accelerator Center [SLAC] in Palo Alto, California, which accelerated electrons, was a linear collider.) So in terms of pure energy and discovery potential, protons win out. Protons can be accelerated to sufficiently high energy that even their quark and gluon subcomponents can carry more energy than an accelerated electron.