The Compact Muon Solenoid

What does CMS do?

The Compact Muon Solenoid is a general purpose particle detector installed at point 5 of the Large Hadron Collider at CERN.

CMS is analogous to a very fast digital camera that captures information from proton-proton collisions at the LHC.

How is CMS constructed?

CMS is a collection of subdetectors that are successively layered around the interaction point. The layers are ordered so that longer-lived and more energetic particles reach the outer subdetectors.

The Solenoid

The magnetic field bends the path of moving charged particles.

Measuring these paths gives information about the momentum of particles, which can help identify them.

The solenoid yields a 4 Tesla magnetic field that is 100,000 times stronger than the Earth’s.

The Tracker . . .

Is the innermost layer of CMS.

Is a silicon device with both pixel and strip configuration.

Maximizes resolution.

Is primarily where the momentum of particles is measured.

The ECAL

The electromagnetic calorimeter finds the energy of electrons, positrons, and photons.

Primarily uses lead tungstate crystals with special avalanche photodiodes to capture the light.

ECAL endcap also uses lead and silicon strips.

ECAL

Crystals each weigh 1.5 kg but with a volume roughly equal to that of a small coffee cup.

ECAL contains nearly 80,000 such crystals, each of which took two days to grow.

The HCAL

Like the ECAL, it is situated inside the magnet.

Measures the energy of “hadrons,” particles made of quarks and gluons (e.g., protons, neutrons, pions and kaons).

HCAL also provides indirect measurement of neutrinos.

HCAL . . .

Used over a million World War II brass shell casements from the Russian Navy in making some of its detector components.

Is made up of 36 wedges, each of which weighs as much as 6 African elephants.

Contains over 400 “optical decoder” units, all of which were made by American high school students through the QuarkNet program.

The Muon System

In total there are 1400 muon chambers:

• 250 drift tubes

• 540 cathode strip chambers

• 610 resistive plate chambers

The Muon System . . .

Contains 2 million cathode strip chamber wires. Though each is as thin as a human hair, not a single one has broken.

Is aligned with the central tracker to within one sixth of a millimeter in order for the detectors to work together in reconstructing tracks.

Is made of components built in 15 countries.