September 16, 2011
When you smack two billiard balls together, they can go off in a multitude of different ways, depending on all of the myriad details of how fast they came in, how they were spinning, whether the pool table is warped, and even tiny imperfections in the balls themselves. A map of the results of many collisions would produce a scatter of different results all over the pool table. When microscopic particles interact, the sensitivity of the outcome to their incoming energies and momenta, internal properties, and the nature of the forces involved produces an analogous scatter of outcomes. This is why we call interactions among microscopic particles in which they exchange energy and momentum “scattering.”
Scattering is ubiquitous both in the laboratory, such as when high-energy sub-atomic particles are crashed into each other in particle physics experiments, and in nature, as when electrons in a distant galaxy cluster deflect each other and emit photons of X-ray light. In particle colliders such as BaBar or now the Large Hadron Collider at CERN, the outcomes of billions of scattering events are tracked by layers of detectors sensitive to the different kinds of particles that can result from the impacts. We can identify the outgoing particles and reconstruct their trajectories after a collision. In combination with what is known about the incoming momenta and energies, this tells us a great deal about the particles and their interactions.