Knocking on Heaven's Door - Lisa Randall [57]
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 = mc2, 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 Ws 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 Ws 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