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The Compact Muon Solenoid (CMS) experiment is one of two large general-purpose particle physics detectors being (as of 2006) built on the proton-proton Large Hadron Collider (LHC) at CERN in Switzerland. Approximately 2300 people from 159 scientific institutes form the collaboration building it. It will be located in an underground chamber at Cessy in France, just across the border from Geneva. The completed detector will be cylindrical, 21 metres long and 16 metres diameter and weigh approximately 12500 tonnes.

Goals
The main goals of the experiment are:
 * To explore physics at the TeV scale
 * To discover of the Higgs boson
 * To look for evidence of supersymmetry
 * To be able to study aspects of heavy ion collisions

Highlights
The name highlights features of the detector:
 * Its relatively small size
 * The powerful solenoid
 * Its optimization for tracking muons

Detector overview
CMS is designed as a general-purpose detector, capable of studying many aspects of proton collisions at 14 TeV, the center-of-mass energy of the LHC particle accelerator. It contains subsystems which are designed to measure the energy and momentum of photons, electrons, muons, and other products of the collisions. The innermost layer is a silicon-based tracker. Surrounding it is a scintillating crystal electromagnetic calorimeter, which is itself surrounded with a sampling calorimeter for hadrons. The tracker and the calorimetry are compact enough to fit inside the CMS solenoid which generates a powerful magnetic field of 4 T. Outside the magnet are the large muon detectors, which are inside the return yoke of the magnet.

The collision region - at the centre
This is where the protons smash into other. The focusing magnets in the LHC force the proton beams, travelling in opposite directions, to cross at the the centre of the CMS detector.

The beams are arranged into "bunches" of protons. Each bunch contains approximately 100 billion protons. The particles are so tiny that the chance of any two colliding is very small. When the bunches cross, there will be only about 20 collisions among 200 billion particles.

When two protons collide at such high energy, they are ripped apart and (needs revision by someone who understands this in more depth than I do) practically any particle you can think of may be created. Most of these processes are already well understood - only around 100 in every 1 billion collisions will produce "interesting" physics.

Consequently, the bunches are spaced closely in the beam, so that there are 40 million bunch crossings per second - one every 25ns.

Layer 1 – The Tracker
Finely segmented silicon sensors (strips and pixels) enable charged particles to be tracked and their momenta to be measured. They also reveal the positions at which long-lived unstable particles decay.

Layer 2 – The Electromagnetic Calorimeter
Nearly 80 000 crystals of scintillating lead tungstate (PbWO4) are used to measure precisely the energies of electrons and photons. A ‘preshower’ detector, based on silicon sensors, helps particle identification in the endcaps.

Layer 3 – The Hadron Calorimeter
Layers of dense material (brass or steel) interleaved with plastic scintillators or quartz fibres allow the determination of the energy of hadrons, that is, particles such as protons, neutrons, pions and kaons.

Layer 4 – The Magnet
Like most particle physics detectors, CMS has a large solenoid. This allows the charge/mass ratio of particles to be determined from the curved track that they follow in the magnetic field. It is 13 metres long and 6 metres in diameter, and its refrigerated superconducting niobium-titanium coils will produce a 4-tesla magnetic field.

The current in the magnet is (?anybody?), and the total energy stored is 2.7GJ, equivalent to half-a-tonne of TNT. There are dump circuits to safely dissipate this energy should the magnet quench.

Layer 5 – The Muon Detectors and Return Yoke
To identify muons (essentially heavy electrons) and measure their momenta, CMS uses three types of detector: drift tubes, cathode strip chambers and resistive plate chambers.

Pattern Recognition
New particles discovered in CMS will be typically unstable and rapidly transform into a cascade of lighter, more stable and better understood particles. Particles travelling through CMS leave behind characteristic patterns, or ‘signatures’, in the different layers, allowing them to be identified. The presence (or not) of any new particles can then be inferred.

Trigger System
To have a good chance of producing a rare particle, such as a Higgs boson, the particle bunches in the LHC collide up to 40 million times a second. Particle signatures are analyzed by fast electronics to save (or ‘trigger on’) only those events (around 100 per second) most likely to show new physics, such as the Higgs particle decaying to four muons in the figure below. This reduces the data rate to a manageable level. These events are stored for subsequent detailed analysis.

Data Analysis
Physicists from around the world use cutting-edge computing techniques (such as the Grid) to sift through millions of events from CMS to produce data that could indicate the presence of new particles or phenomena.