In particle-physics experiments, the discovery of increasingly more massive particles has brought deep understanding of the basic constituents of matter and the fundamental forces between them. Such discoveries are made possible by accelerator beams of ever more energetic particles.
Traditionally, such beams were directed at stationary targets. In that configuration, however, the total center-of-mass energy Ecm of a collision between a beam particle of energy Eb and a target particle grows only like √Eb. But Ecm is what limits the masses of particles a collision can produce.
One can get much more bang (Ecm) for the buck (Eb) by making counterpropagating particle beams collide head-on. In that case, Ecm grows much faster; for beams of equal mass and energy, it’s simply 2Eb. For almost half a century now, such colliding-beam accelerators—colliders—have been hurling beams...
