Knocking on Heaven's Door - Lisa Randall [67]
Even so, while the SpbarpS was operating, scientists and engineers were already planning a collider known as LEP, which would collide together electrons and their antiparticles known as positrons to study the weak interactions and the Standard Model in exquisite detail. This dream came to fruition in the 1990s, when through its very accurate measurements, LEP studied millions of weak gauge bosons that taught physicists a great deal about Standard Model physics interactions.
LEP was a circular collider with a 27 kilometer circumference. Electrons and positrons were repeatedly boosted in this ring as they orbited around. As we saw in Chapter 6, circular colliders can be inefficient when accelerating light particles such as electrons, since such particles radiate when accelerated on a circular path. The electron beams at the LEP energy of about 100 GeV lost about three percent of their energy each time they went around. This wasn’t too great a loss, but if anyone had wanted to accelerate electrons around this tunnel at any higher energy, the loss during each rotation would have been a deal breaker. Increasing the energy by a factor of 10 would have increased energy loss by a factor of 10,000, which would have made the accelerator far too inefficient to be acceptable.
For this reason, while LEP was being envisioned, people were already thinking about CERN’s next flagship project—which would presumably run at even higher energy. Because of the electron’s unacceptable energy losses, if CERN was to ever build a higher-energy machine, it would require proton beams, which are much heavier and therefore radiate much less. The physicists and engineers who developed LEP were aware of this more desirable possibility so they built the LEP tunnel sufficiently wide to accommodate a possible proton collider in the future, after the electron-positron machine would be dismantled.
Finally, some 25 years later, proton beams now race through the tunnel originally excavated for LEP. (See Figure 24.) The Large Hadron Collider is a couple of years behind schedule and about 20 percent over budget. That’s a pity, but perhaps not so unreasonable given that the LHC is the biggest, most international, most expensive, most energetic, most ambitious experiment ever built. As the screenwriter and director James L. Brooks jokingly said when hearing about the LHC’s setbacks and recovery, “I know people who take approximately the same amount of time to get their wallpaper just so. Understanding the universe just might have a better kick to it. Then again there’s some pretty great wallpaper out there.”
[ FIGURE 24 ] The setting for the Large Hadron Collider, with the underground tunnel illustrated in white, and Lake Geneva and mountains in the background. (Photo courtesy of CERN)
THE FELLOWSHIP OF THE RINGS
Protons are everywhere around and within us. However, they are generally bound into nuclei surrounded by electrons inside atoms. They aren’t isolated from those electrons and they aren’t collimated (aligned into columns) inside beams. The LHC first separates and accelerates protons and then steers them to their ultimate destiny. In doing so, they utilize the LHC’s many extremes.
The first step in preparing proton beams is to heat hydrogen atoms, which strips off their electrons and leaves the isolated protons that are their nuclei. Magnetic fields divert these protons so that they are channeled into beams. The LHC then accelerates the beams in several stages in distinct regions, with the protons traveling from one accelerator to another, each time increasing their energy before they are diverted from one