A new era in physics will open up when the Large Hadron Collider (LHC) extends the reach of subatomic particle investigations to unprecedented energy scales. But even before researchers initiate the first high-energy collisions in the LHC’s giant storage ring, located under the French-Swiss border, they are already contemplating and working toward the next great particle accelerator. And the consensus choice of the particle physics community is a proposed facility called the International Linear Collider (ILC), a machine more than 30 kilometers long that would smash electrons and positrons together at velocities very close to the speed of light. (The positron is the antimatter counterpart of the electron, identical in mass but opposite in charge.)
Far more powerful than previous electron-positron colliders, the ILC would enable physicists to follow up any groundbreaking discoveries made by the LHC. The LHC is designed to investigate the collisions of protons, each of which is actually a bundle of three quarks bound together by gluons (the particles carrying the strong nuclear force). Because the quarks and gluons within a proton are constantly interacting, a proton-proton collision is an inherently messy affair. Researchers cannot be certain of the energy of each quark at the moment of the collision, and this uncertainty makes it difficult to determine the properties of novel particles produced by the impact. But the electron and positron are fundamental particles rather than composites, so physicists working with an electron-positron collider can know the energy of each collision to great accuracy. This capability would make the ILC an extremely useful tool for precisely measuring the masses and other characteristics of newly discovered particles.