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Compact Muon Solenoid

The Compact Muon Solenoid (CMS) experiment is one of two large general-purpose particle physics detectors being (as of 2007) built on the proton-proton Large Hadron Collider (LHC) at CERN in Switzerland. Approximately 2600 people from 180 scientific institutes form the collaboration are 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 12,500 tonnes.


The main goals of the experiment are:

* to explore physics at the TeV scale

* to discover the Higgs boson

* to look for evidence of physics beyond the standard model, such as supersymmetry, or extra dimensions

* to study aspects of heavy ion collisions


The main highlight features of the CMS (Compact Muon Solenoid) detector:

* Its relatively small size (compact)

* Its optimization for tracking muons

* The powerful solenoid magnet

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 set up of the CMS. In the middle, under the so-called barrel there is a man for the scale. (HCAL=hadron calorimeter, ECAL=electromagnetic calorimeter)

The layers of CMS – from the center outwards

A slice of the CMS detector. Flash animation can be reached here

The collision region - at the center

This is where the protons smash into each other. The focusing magnets in the LHC force the proton beams, traveling in opposite directions, to cross at the center 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 the exchange of mass and energy means that particles which do not usually occur in the world around us can 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 25 ns.

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.

This part of the detector is the world's largest silicon detector. It has 205 m2 of silicon sensors (approximately the area of a tennis court) comprising 9.3 million microstrips and 66 million pixels.[1]

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 fibers allow the determination of the energy of hadrons, that is, particles such as protons, neutrons, pions and kaons. The brass used in the endcaps of the HCAL used to be Russian artillery shells. [1]

Layer 4 – The magnet

Like most particle physics detectors, CMS has a large solenoid magnet. 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 meters long and 6 meters in diameter, and its refrigerated superconducting niobium-titanium coils was originally intended to produce a 4-tesla magnetic field. It was recently announced that the magnet will run at 3.8T instead of the full design strength in order to maximize longevity.

The inductance of the magnet is 14 henries and the nominal current is 19,500 amperes, giving a total stored energy of 2.66 GJ, equivalent to about half-a-tonne of TNT. There are dump circuits to safely dissipate this energy should the magnet quench. The circuit resistance (essentially just the cables from the power converter to the cryostat) have a resistance of 0.1 milliohms which leads to a circuit time constant of nearly 39 hours. This is the longest time constant of any circuit at CERN.

Layer 5 – The muon detectors and return yoke

To identify muons and measure their momenta, CMS uses three types of detector: drift tubes (DT), cathode strip chambers (CSC) and resistive plate chambers (RPC). The DT's are used for precise trajectory measurements in the central barrel region, while the CSC's are used in the end caps. The RPC's provide a fast signal when a muon passes through the muon detector, and are installed in both the barrel and the end caps.

Collecting and collating the data

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. 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 billions of events from CMS to produce data that could indicate the presence of new particles or phenomena.


The insertion of the vacuum-tank, June 2002


Another experiment called ATLAS, installed at another point on the LHC ring, is meant to do similar physics; the ATLAS and CMS collaborations may compete to make major discoveries.

The Tevatron is a proton - antiproton collider at Fermilab, with a center-of-mass energy of about 2 TeV. It has been operating since 1987. There are two experiments on the Tevatron ring called CDF and D0.


  1. ^ CMS installs the world's largest silicon detector, CERN Courier, Feb 15, 2008


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