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CMS Particle Detector


The purpose of a particle detector is to trap, identify and count all the particles resulting from the break up of larger target particles resulting from controlled high energy collisions.

The CMS (Compact Muon Solenoid) detector is a massive device 21.6 metres long and 14.6 metres in diameter, weighing 14,500 tons. It is one of two general purpose particle detectors mounted on the circumference of CERN's Large Hadron Collider (LHC).

The other is the ATLAS (A Toroidal LHC ApparatuS) detector which is even bigger at 45 metres long and 25 metres diameter.

The image opposite shows the two halves of the "compact" CMS detector separated for maintenance purposes.

Note the size of the engineer standing on the floor below.


It is ironic that it takes equipment this big to detect particles which are so small that you can't even see them under a microscope.

Image of CERN LHC CMS Particle Detector

The diagram below is a transverse section through a sector of the main CMS detector structure. It shows the arrangement of the different special purpose detecting devices, housed concentrically in the walls of the main structure, and their responses to the various types particles resulting from the collisions.

The tracks of negatively charged particles such as electrons and the heavier muons are bent in one direction by the magnetic field of the CMS while the tracks of positively charged particles are bent in the opposite direction. Pions which may be positively or negatively charged are thus bent to one side or the other depending on their charge, while neutral particles such as photons and gluons are not affected by the magnetic field and follow straight paths through the detector. The curved trail of charged particle tracks in the magnetic field allows their charge and momentum to be measured. See an early example of charged particle tracks.

Section through CMS Particle Detector
  • The Accelerator (Not shown here) - The LHC accelerator's beam pipe brings its two counter rotating beams, each carrying bunches of 115 billion protons at a velocity close to the speed of light, to opposite ends of the detector directing them to the collision point at its centre. See a description of the LHC synchrotron accelerator.
  • When LHC's two proton beams collide they generate temperatures of more than a billion times those in the center of the sun, but in a very tiny space. These extreme temperatures approach those in the very early moments (one billionth of a second) of the Big Bang and assist in the creation of new particles.

  • The Collision Point - To increase the probability of collisions occurring, powerful magnets at opposite ends of the detector focus the two opposing beams down to an extremely small diameter of around 16 x 10-6 metres (16 microns - about a fifth the diameter of a human hair) at the collision point where the beams meet. The size of the collision point is still many times greater than the 2 x 10-15 metres diameter of the individual hydrogen protons in the bunches so that the collision area is mostly empty space. Bunches are designed to collide every 50 nanoseconds but even with 115 billion protons from each beam impinging on eachother the probability of a collision is still very small and only about 20 particle collisions are recorded from each pair of colliding bunches.
  • The Silicon Tracker - The first detector immediately around the collision point forms the inner tracker with 75 million electronic sensors which record the tracks of individual particles for subsequent analysis. It does not impede the motion of the particles. Like the Tevatron silicon strip detectors, they were constructed from long parallel strips of closely spaced diode materials etched into silicon wafers and connected to a central computer which recorded the location, magnitude and timing of the electrical impulses arising when a particle landed on the diode strip.
  • The Electromagnetic Calorimeter - Calorimeters measure particle energies with high accuracy and, unlike trackers, they are designed to stop the particles in their tracks. The Electromagnetic Calorimeter measures the energies of electrons and photons and is constructed from extremely dense but optically clear crystals of scintillating lead tungstate which stop high energy particles and produce light when hit. Silicon photo-diodes provide a particle count for subsequent analysis.
  • The Hadronic Calorimeter - The Hadronic Calorimeter finds the position, energy and arrival time of heavier particles such as quarks and gluons as well as composite particles such as protons and neutrons using layers of brass and steel absorber alternating with fluorescent plastic scintillator materials that produce a rapid light pulse when a particle passes through. Optic fibres feed the light pulses to photo-detectors which amplify the signal. The total light energy captured over a given region provides a measure of a particle's energy. The Hadronic Calorimeter provides wide angle coverage and records the energy and continuous position of individual hadrons.
  • The Super-Conducting Solenoid Magnet - The massive solenoid magnet provides the magnetic field which causes charged particles to curve and from the radius of the curve their charge and momentum can be calculated as well as their charge/mass (e/m) ratio. The e/m ratio is unique to individual particle types and this enables different particles to be identified.
  • The CMS solenoid generates a magnetic field of an incredible 4 Tesla, which is about 100,000 times higher than the Earth's magnetic field. The magnet with its return yoke weighs 13,000 tons and contains almost twice as much iron as the Eifel Tower. Superconductivity (zero resistance) of the solenoid magnetising coils is achieved by chilling them to almost the absolute temperature at -271.3 ºC using liquid helium. The nominal current energising the solenoid magnet is 18,500 amps.

  • The Muon Detectors - Muons are charged particles like electrons, but 200 times heavier and relatively easy to detect and measure. In general purpose detectors, the outermost layers are usually designed to capture all the remaining deeply penetrating muons and neutrinos which can penetrate several metres of iron and are not stopped by the calorimeters.
  • The CMS muon detector contains 1400 muon chambers in three types. 250 drift tubes (DTs) and 540 cathode strip chambers (CSCs) track the particles' positions. In order to reduce the massive data analysis tasks, these detectors are progammed to identify "interesting" events, heavy particles and unusual particle combinations and to provide a trigger to switch on more detailed data gathering only when such events occur. A further 610 resistive plate chambers (RPCs) form a fast acting redundant trigger system. Like the drift tracker these detectors depend on the ionisation of gas atoms by the charged particles giving rise to a current between pairs of electodes which can be measured. Though the physics of the operating principle is the similar, their configuration and constructions are quite different and unique to each task.

  • The Return Yoke - The return yoke is a huge steel structure which provides the return route for the magnetic flux and also holds the muon detectors in four concentric layers. The magnetic field associated with the return flux is in the opposite direction to the magnetic field of the main magnet and hence the tracks of the muons in the outer field are also bent in the opposite direction.






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