Interaction point
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In particle physics, an interaction point (IP) is the place where particles collide. One differentiates between the nominal IP, which is the design position of the IP, and the real or physics IP, which is the position where the particles actually collide. The real IP is the primary vertex of the particle collision.
For fixed target experiments, the IP is the point where beam and target interact. For colliders (and collider experiments), it is the place where the beams interact. Experiments (particle detectors) at particle accelerators are built around the nominal IPs of the accelerators. Therefore the whole region around the IP (the experimental hall) is called an interaction region. Particle colliders such as LEP, HERA, RHIC, Tevatron and LHC can host several interaction regions and therefore several experiments taking advantage of the same beam.
