The CMS Experiment
The acronym CMS stands for "Compact Muon Solenoid". Most detectors for particle physics are based around a magnet system, of one sort or another, to facilitate the measurement of the momenta of charged particles. CMS is no exception: we have decided to use a large superconducting Solenoid, with a length of around 12m and an inner diameter of about 6m.
The field strength is 4 Tesla - about 100000 times that of the earth's magnetic field. This is the largest magnet of its type ever constructed, and allows all of the tracking devices and calorimetry to be placed inside the coil of the solenoid - resulting in a Compact overall detector. Outside of the coil there is a steel "return yoke". This yoke is built in layers, interspersed with muon detectors; the configuration of the magnetic field means that the momentum of muons is measured both inside the coil (by the tracking devices) and outside of the coil (by the muon chambers). Muons are extremely important in particle physics as they signify that "something interesting has happened" so can be used to help find that elusive "needle in a haystack".
The complete CMS detector is about 21m long, with a diameter of about 16m and weighs in excess of 12500 tonnes. Even if the access shaft was large enough to accomodate the complete detector (as large as the underground cavern!) it was not possible to lower the detector down into the cavern. A modular design allowed CMS to be constructed, and tested, in pieces.
Our current best estimate of the mass of the Higgs (from theory and experiment) is relatively light - around 120 GeV. For Higgs masses less than about 150 GeV the most promising channel is its two-photon decay, even though the branching fraction is small. For this mass the decay width is of the order of tens of keV - the measured mass resolution is entirely dominated by the energy resolution of the electromagnetic calorimeter (ECAL) used to detect and measure the photons. This is the reason why CMS has placed such a large emphasis on the performance of its ECAL.
