As the name “Compact Muon Solenoid” suggests, detecting muons is one of CMS’s most important tasks. Muons are charged particles that are just like electrons and positrons, but are 200 times heavier. We expect them to be produced in the decay of a number of potential new particles; for instance, one of the clearest "signatures" of the Higgs Boson is its decay into four muons.

Because muons can penetrate several metres of iron without interacting, unlike most particles they are not stopped by any of CMS's calorimeters. Therefore, chambers to detect muons are placed at the very edge of the experiment where they are the only particles likely to register a signal.

A particle is measured by fitting a curve to hits among the four muon stations, which sit outside the magnet coil and are interleaved with iron "return yoke" plates (shown in red below, for the barrel region). By tracking its position through the multiple layers of each station, combined with tracker measurements the detectors precisely trace a particle’s path. This gives a measurement of its momentum because we know that particles travelling with more momentum bend less in a magnetic field. As a consequence, the CMS magnet is very powerful so we can bend even the paths of very high-energy muons and calculate their momenta.

In total there are 1400 muon chambers: 250 drift tubes (DTs) and 540 cathode strip chambers (CSCs) track the particles’ positions and provide a trigger, while 610 resistive plate chambers (RPCs) form a redundant trigger system, which quickly decides to keep the acquired muon data or not. Because of the many layers of detector and different specialities of each type, the system is naturally robust and able to filter out background noise.

DTs and RPCs are arranged in concentric cylinders around the beam line (“the barrel region”) whilst CSCs and RPCs, make up the “endcaps” disks that cover the ends of the barrel.

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;
  • formed the first and last slices of CMS to be lowered into the cavern with endcaps “YE+3” in November 2006 and “YE-1” in January 2008;
  • is aligned with the central tracker to within one sixth of a millimetre in order for the detectors to work together in reconstructing tracks;
  • is made of components built in 15 countries.

Making Tough Chambers

Many of the CSCs were built at two US universities and a national laboratory before being shipped all the way across the Atlantic to CERN, so the precision chambers needed to be robust. A test involving a road trip from Florida to California showed they could take some wear and tear, but the ultimate proof of their resilience came inadvertently during a beam test.

When physicists overseeing the beam test decided to reposition a chamber, one side of its support structure suddenly fell by more than 30 cm. Such an unprecedented test of robustness had the physicists worry about the fate of that unlucky chamber. Instead, they found it was still "taking data" in spite of its hard drop.