A beam crossing in a particle collider occurs when two packets of particle

Elementary particle

In particle physics, an elementary particle or fundamental particle is a particle not known to have substructure; that is, it is not known to be made up of smaller particles. If an elementary particle truly has no substructure, then it is one of the basic building blocks of the universe from which...

s, going in opposite directions, reach the same point in space. Most of the particles in each packet cross each other, but a few may collide, producing other particles that may be observed in a particle detector

Particle detector

In experimental and applied particle physics, nuclear physics, and nuclear engineering, a particle detector, also known as a radiation detector, is a device used to detect, track, and/or identify high-energy particles, such as those produced by nuclear decay, cosmic radiation, or reactions in a...

. In a linear collider there is only one location where beam crossings occur, while in a modern accelerator ring there are a few locations (LHC

Large Hadron Collider

The Large Hadron Collider is the world's largest and highest-energy particle accelerator. It is expected to address some of the most fundamental questions of physics, advancing the understanding of the deepest laws of nature....

, for example, has four); it is at these points that detectors are placed.