Knocking on Heaven's Door (19 page)

In truth, physicists have learned a lot about particles from both types of colliders—those colliding protons and those colliding electrons. Colliders with an electron beam don’t operate at the lofty energies that the highest-energy proton accelerators have attained. But the experiments at colliders with electron beams have achieved measurements more precise than proton collider people could even dream about. In particular, in the 1990s, experiments performed at SLAC and also the Large Electron-Positron collider (LEP) (the blandness of the names never ceases to amuse me) at CERN achieved spectacular precision in verifying the predictions of the Standard Model of particle physics.

These
precision electroweak measurement
experiments exploited the many different processes that can be predicted with knowledge of the electroweak interactions. For example, they measured the weak force carriers’ masses, the rates of decay into different types of particles, and asymmetries in the forward and backward parts of the detectors that tell even more about the nature of the weak interactions.

Precision electroweak measurements explicitly apply the effective theory idea. Once physicists perform enough experiments to pin down the few parameters of the Standard Model such as the interaction strengths of each of the forces, everything else can be predicted. Physicists check for consistency of all the measurements and look for deviations that would tell us whether something is missing. All told so far, measurements indicate that the Standard Model works extraordinarily well—so well that we still don’t have the clues we need to know what lies beyond except that whatever it is, its effects at LEP energies must be small.

That tells us that getting more information about heavier particles and higher-energy interactions requires directly investigating processes at energies that are considerably higher than those that were achieved at LEP and SLAC. Electron collisions simply won’t achieve the energies we think we’ll need to pin down the question of what gives particles mass and why they are the masses they are—at least not in the near future. That will require proton collisions.

That’s why physicists decided to accelerate protons rather than electrons inside the tunnel that had been built in the 1980s to house LEP. CERN ultimately shut down LEP operations to make way for preparations for its new colossal enterprise, the LHC. Because protons don’t radiate nearly as much energy away, the LHC far more efficiently boosts them to higher energies. Its collisions are messier than those involving electrons, and experimental challenges abound. But with protons in the beam, we have a chance to attain energies high enough to directly tell us the answers we’ve been seeking for several decades.

PARTICLES OR ANTIPARTICLES?

But we still have one more question to answer before we can decide what to collide. After all, collisions involve two beams. We’ve decided that high energies mandate that one beam consist of protons. But will the other beam be made of particles—that is, protons—or their anti-particles—namely, antiprotons? Protons and antiprotons have the same mass and therefore radiate at the same rate. Other criteria must be used to decide between them.

Clearly protons are more plentiful. We don’t see too many antiprotons lying around since they would annihilate with the abundant protons in our surroundings, turning into energy or other, more elementary particles. So why would anyone even consider making beams of antiparticles? What is to be gained?

The answer could be quite a bit. First of all, acceleration is simpler since the same magnetic field can be used to direct protons and antiprotons in opposite directions. But the most important reason has to do with the particles that could be produced.

Particles and antiparticles have equal masses but opposite charges. This means that the incoming particle and antiparticle together carry exactly the same charge as pure energy carries—namely, nothing. According to
E = mc
²
,
this means that a particle and its antiparticle can turn into energy, which can in turn create any other particle and antiparticle together, so long as they are not too heavy and have a strong enough interaction with the initial particle-antiparticle pair.

These particles that are created could in principle be new and exotic particles whose charges are different from those of particles in the Standard Model. A colliding particle and antiparticle have no net charge, and neither does an exotic particle plus its antiparticle. So even though the exotic particle’s charges can be different from those in the Standard Model, a particle and antiparticle together have zero charge and can in principle be produced.

Let’s apply this reasoning to electrons. Were we to collide together two particles with equal charges such as two electrons, we could make only objects that carry the same charge as whatever went in. It could produce either a single object with net charge two or two different objects like electrons that each carry a charge of one. That’s rather restrictive.

Colliding two particles with the same charge is very limiting. On the other hand, colliding together particles and antiparticles opens many new doors that wouldn’t be possible were we to collide only particles. Because of the greater number of possible new final states, electron-positron collisions have much more potential than electron-electron collisions. For example, collisions involving electrons and their antiparticles—namely, positrons—have produced uncharged particles like the
Z gauge boson
(that’s how LEP worked) as well as any particle-antiparticle pair light enough to be produced. Although we pay a steep price when we use antiparticles in the collisions—since they are so difficult to store—we win big when the new exotic particles we hope to discover have different charges than the particles we collide.

Most recently, the highest-energy colliders used one beam of protons and one beam of antiprotons. That of course required a way to make and store antiprotons. Efficiently stored antiprotons were one of CERN’s major accomplishments. Earlier on, before CERN constructed the electron-positron collider, LEP, the lab produced high-energy proton and antiproton beams.

The most important discoveries from the collision of protons and antiprotons at CERN were the electroweak gauge bosons that communicate the electroweak force for which Carlo Rubbia and Simon van der Meer received the Nobel Prize in 1984. As with the other forces, the weak force is communicated by particles. In this case they are known as the
weak gauge bosons
—the positively and negatively charged
W
and neutral
Z vector bosons
—and these three particles are responsible for the weak nuclear force. I still think of the
W
s and the
Z
as the “bloody vector bosons” due to a drunken exclamation of a British physicist who lumbered into the dormitories where visiting physicists and summer students—including me—resided at the time. He was concerned about America’s dominance and was looking forward to Europe’s first major discovery. When the
W
s and the
Z
vector bosons were discovered at CERN in the 1980s, the Standard Model of particle physics, for which the weak force was an essential component, was experimentally verified.

Critical to the success of these experiments was the method that Van der Meer developed to store antiprotons, which is clearly a difficult task since antiprotons want nothing better than to find protons with which to annihilate. In Van der Meer’s process, known as
stochastic cooling,
the electric signals of a bunch of particles drove a device that “kicked” any particle with particularly high momentum, eventually cooling the entire bunch so that they didn’t move as rapidly and therefore didn’t immediately escape or hit the container so that even antiprotons could be stored.

The idea of a proton-antiproton collider wasn’t restricted to Europe. The highest-energy collider of this type was the
Tevatron,
built in Batavia, Illinois. The Tevatron reached an energy of 2 TeV (an energy equivalent to about 2,000 times the proton’s rest energy).
³³
Protons and antiprotons collided together to make other particles that we could study in detail. The most important Tevatron discovery was the
top
quark, the heaviest and the last Standard Model particle to be found.

However, the LHC is different from either CERN’s first collider or the Tevatron. (See Figure 22 for a summary of the collider types.) Rather than protons and antiprotons, the LHC collides together two proton beams. The reason the LHC chooses two proton beams over a beam of protons and another of antiprotons is subtle but worth understanding. The most opportunistic collisions are those where the net charge of the incoming particles adds up to zero. That’s the type of collision we already discussed. You can produce anything plus its antiparticle (assuming you have enough energy) when your net charge is zero. If two electrons come in, the net charge of whatever is produced would have to be minus two, which rules out a lot of possibilities. You might think colliding together two protons is an equally bad idea. After all, the net charge of two protons is two, which doesn’t seem to be a big improvement.

If protons were fundamental particles, this would be absolutely right. However, as we explored in Chapter 5, protons are made up of subunits.

A COMPARISON OF DIFFERENT COLLIDERS

[
FIGURE 22
]
A comparison of different colliders showing their energies, what collides, and the accelerator shape.

Protons contain quarks that are bound together through gluons. Even so, if the three
valence
quarks—two up quarks and a down—that carry its charge were all there were inside a proton, that still wouldn’t be very good: the charges of two valence quarks never add to zero either.

However, most of the mass of the proton isn’t coming from the mass of the quarks it contains. Its mass is primarily due to the energy involved in binding the proton together. A proton traveling at high momentum contains a lot of energy. With all this energy, protons contain a sea of quarks and antiquarks and gluons in addition to the three valence quarks responsible for the protons’ charge. That is, if you were to poke a high-energy proton, you would find not only the three valence quarks, but also a sea of quarks and antiquarks and gluons whose charge adds up to zero.

Therefore, when we consider proton collisions, we have to be a little more careful in our logic than we were with electrons. The interesting events are the result of subunits colliding. The collisions involve the charges of the subunits and not the protons. Even though the sea quarks and gluons don’t contribute to the net proton charge, they do contribute to its composition. When protons collide together, it could be that one of the three valence quarks in the proton hits another valence quark and the net charge in the collision doesn’t add to zero. When the net charge of the event doesn’t vanish, interesting events involving the correct sum of charges might occasionally occur, but the collision won’t have the broad capacities that net-charge-zero collisions do.

But a lot of interesting collisions will happen because of the virtual sea, which allows a quark to meet an antiquark or a gluon to hit a gluon, yielding collisions that carry no net charge. When protons bang together, a quark inside one proton might hit an antiquark inside the other, even if that is not what happens most of the time. All of the possible processes that can happen, including those from the collision of the sea particles, play a role when we ask what happens at the LHC. These sea collisions in fact become more and more likely as the protons are accelerated to higher energy.

The total proton charge doesn’t determine the particles that get made, since the rest of the proton just goes forward, avoiding the collision. The pieces of the protons that don’t collide carry away the rest of the net proton charges, which just disappear down the beam pipe. This was the subtle answer to the question the Paduan mayor asked, which was where the proton charges go during an LHC collision. It has to do with the composite nature of the proton and the high energy that guarantees that only the smallest elements we know of—quarks and gluons—directly collide.

Because only pieces of the proton collide and those pieces can be virtual particles that collide with net zero charge, the choice of proton-proton versus proton-antiproton collider is not so obvious. Whereas in the past, it was worth the sacrifice at lower-energy colliders to make antiprotons in order to guarantee interesting events, at LHC energies that’s not such an obvious choice. At the high energies the LHC will achieve, a significant fraction of the energy of the proton is carried by sea quarks, antiquarks, and gluons.

LHC physicists and engineers made the design choice to collide together two proton beams, rather than a proton and an antiproton beam.
³⁴
This makes generating high luminosity—that is, a higher number of events—a far more accessible goal. It’s considerably easier to make proton beams than antiproton beams.