“The LHC is a like a collider of cherry pies,” says Berkeley’s Murayama. “Cherry pies are easy to throw, and they smash together rather easily, but they produce a huge splash, and all the goo comes out of the pie.” In the case of the LHC’s collisions, the “goo” consists of the protons’ components — quarks and gluons. The sheer messiness of proton-proton collisions makes it difficult to detect new particles, or to make accurate measurements of known particles.
Electrons and positrons, on the other hand, don’t have any components, so the collisions are cleaner. At the LHC, maybe one in a billion proton-proton collisions yields a Higgs boson. Physicists estimate that a Higgs should pop up roughly every hundred electron-positron collisions at the ILC. The challenge then will be aiming the electron beams accurately enough to ensure enough collisions occur.
“In circular colliders, you have collisions happening many times as the beam circulates inside the ring,” says Murayama. “But a linear collider gives you only one shot, and to get decent data, you have to squeeze the beam down to an incredibly small size so the probability of a collision between an electron and positron becomes high.”
The ILC’s collision point will be will be less than 10 nanometers — about a hundred atoms wide. “You have to operate these tiny objects coming at the speed of light and make sure they meet! I’m still surprised that people think they can do this,” says Murayama.
But the project’s supporters say the only thing the ILC needs now is funding, as well as a country willing to host the project — Japan is currently the front-runner. “The ILC is shovel-ready,” says consortium director Evans. Like the LHC, it will take more than a decade to build. Cost estimates range from $10 billion to $25 billion.
The Next Fix
The LHC is giving physicists their first look at an entirely new high-energy realm, but so far, aside from the Higgs boson, nothing new has turned up.
“For me, the next run of the LHC will be very important,” says Evans. “If SUSY is not found, then the justification for a linear collider will be even stronger.” It’s possible, he says, that the LHC may fail to find evidence for new physics — the energies required may simply lie beyond the reach of any machine we could conceivably build.
Even so, exacting measurements of the Higgs boson at the ILC might still allow physicists to tease out fine differences between the predictions of SUSY — or perhaps some other theory, should SUSY fizzle — and those of the Standard Model.
“We would be able to measure the properties of the Higgs boson with extreme precision,” says Evans, “and try to crack the Standard Model in that way.”