Hermetic detector

In particle physics, a hermetic detector (also called a 4π detector) is a particle detector designed to register as many particles as possible produced by a high-energy collision in a particle accelerator. The name "hermetic" refers to the detector being conceptually "airtight," aiming to ensure that few particles from the collision escape undetected. The name "4π detector" comes from the fact that such detectors are designed to cover nearly all of the 4π steradians of solid angle around the interaction point.

The main goal of a hermetic design is to allow for a complete accounting of the energy and momentum from an interaction. This is critical for identifying the presence of particles like neutrinos, which do not interact with the detector directly. Their presence is instead inferred by measuring an imbalance in the total momentum, known as missing transverse energy. By maximizing its acceptance—the range of particle trajectories it can observe—a hermetic detector ensures such measurements are as accurate as possible.

The first such detector was the Mark I at the Stanford Linear Accelerator Center. The design proved essential for studying interactions involving large exchanges of energy and has been used for all subsequent general-purpose collider detectors, including the CDF and DØ detectors at the Tevatron, and the ATLAS and CMS detectors at the LHC.