Knocking on Heaven's Door - Lisa Randall [58]
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