CMS stands for Compact Muon Solenoid: compact because it is small for its enormous weight, muon for one of the particles it detects, and solenoid for the coil inside its huge superconducting magnet. It is a high-energy physics experiment in Cessy, France, part of the Large Hadron Collider (LHC) at CERN.
CMS is designed to see a wide range of particles and phenomena produced in high-energy collisions in the LHC. Like a cylindrical onion, different layers of detector stop and measure the different particles, and use this key data to build up a picture of events at the heart of the collision.
Scientists then use this data to search for new phenomena that will help to answer questions such as: What is the Universe really made of and what forces act within it? And what gives everything substance? CMS will also measure the properties of previously discovered particles with unprecedented precision, and be on the lookout for completely new, unpredicted phenomena.