In particle physics

Particle physics

Particle physics is a branch of physics that studies the existence and interactions of particles that are the constituents of what is usually referred to as matter or radiation. In current understanding, particles are excitations of quantum fields and interact following their dynamics...

, 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 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...

s) at particle accelerator

Particle accelerator

A particle accelerator is a device that uses electromagnetic fields to propel charged particles to high speeds and to contain them in well-defined beams. An ordinary CRT television set is a simple form of accelerator. There are two basic types: electrostatic and oscillating field accelerators.In...

s 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

Hadron Elektron Ring Anlage

HERA was a particle accelerator at DESY in Hamburg. It began operating in 1992. At HERA, electrons or positrons were collided with protons at a center of mass energy of 318 GeV. It was the only lepton-proton collider in the world while operating...

, RHIC

Relativistic Heavy Ion Collider

The Relativistic Heavy Ion Collider is one of two existing heavy-ion colliders, and the only spin-polarized proton collider in the world. It is located at Brookhaven National Laboratory in Upton, New York and operated by an international team of researchers...

, Tevatron

Tevatron

The Tevatron is a circular particle accelerator in the United States, at the Fermi National Accelerator Laboratory , just east of Batavia, Illinois, and is the second highest energy particle collider in the world after the Large Hadron Collider...

 and 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....

can host several interaction regions and therefore several experiments taking advantage of the same beam.