Knocking on Heaven's Door - Lisa Randall [56]
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.
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