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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Compact Muon Solenoid | 2/5 | https://en.wikipedia.org/wiki/Compact_Muon_Solenoid | reference | science, encyclopedia | 2026-05-05T13:03:00.987311+00:00 | kb-cron |
=== Layer 1 – The tracker === Momentum of particles is crucial in helping us to build up a picture of events at the heart of the collision. One method to calculate the momentum of a particle is to track its path through a magnetic field; the more curved the path, the less momentum the particle had. The CMS tracker records the paths taken by charged particles by finding their positions at a number of key points. The tracker can reconstruct the paths of high-energy muons, electrons and hadrons (particles made up of quarks) as well as see tracks coming from the decay of very short-lived particles such as beauty or "b quarks" that will be used to study the differences between matter and antimatter. The tracker needs to record particle paths accurately yet be lightweight so as to disturb the particle as little as possible. It does this by taking position measurements so accurate that tracks can be reliably reconstructed using just a few measurement points. Each measurement is accurate to 10 μm, a fraction of the width of a human hair. It is also the inner most layer of the detector and so receives the highest volume of particles: the construction materials were therefore carefully chosen to resist radiation. The CMS tracker is made entirely of silicon: the pixels, at the very core of the detector and dealing with the highest intensity of particles, and the silicon microstrip detectors that surround it. As particles travel through the tracker the pixels and microstrips produce tiny electric signals that are amplified and detected. The tracker employs sensors covering an area the size of a tennis court, with 75 million separate electronic read-out channels: in the pixel detector there are some 6,000 connections per square centimetre. The CMS silicon tracker consists of 14 layers in the central region and 15 layers in the endcaps. The innermost four layers (up to 16 cm radius) consist of 100 × 150 μm pixels, 124 million in total. The pixel detector was upgraded as a part of the CMS phase-1 upgrade in 2017, which added an additional layer to both the barrel and endcap, and shifted the innermost layer 1.5 cm closer to the beamline. The next four layers (up to 55 cm radius) consist of 10 cm × 180 μm silicon strips, followed by the remaining six layers of 25 cm × 180 μm strips, out to a radius of 1.1 m. There are 9.6 million strip channels in total. During full luminosity collisions the occupancy of the pixel layers per event is expected to be 0.1%, and 1–2% in the strip layers. The expected HL-LHC upgrade will increase the number of interactions to the point where over-occupancy would significantly reduce track-finding effectiveness. An upgrade is planned to increase the performance and the radiation tolerance of the tracker. 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) in 9.3 million microstrip sensors comprising 76 million channels.
=== Layer 2 – The Electromagnetic Calorimeter === The Electromagnetic Calorimeter (ECAL) is designed to measure with high accuracy the energies of electrons and photons. The ECAL is constructed from crystals of lead tungstate, PbWO4. This is an extremely dense but optically clear material, ideal for stopping high energy particles. Lead tungstate crystal is made primarily of metal and is heavier than stainless steel, but with a touch of oxygen in this crystalline form it is highly transparent and scintillates when electrons and photons pass through it. This means it produces light in proportion to the particle's energy. These high-density crystals produce light in fast, short, well-defined photon bursts that allow for a precise, fast and fairly compact detector. It has a radiation length of χ0 = 0.89 cm, and has a rapid light yield, with 80% of light yield within one crossing time (25 ns). This is balanced however by a relatively low light yield of 30 photons per MeV of incident energy. The crystals used have a front size of 22 mm × 22 mm and a depth of 230 mm. They are set in a matrix of carbon fibre to keep them optically isolated, and backed by silicon avalanche photodiodes for readout. The ECAL, made up of a barrel section and two "endcaps", forms a layer between the tracker and the HCAL. The cylindrical "barrel" consists of 61,200 crystals formed into 36 "supermodules", each weighing around three tonnes and containing 1,700 crystals. The flat ECAL endcaps seal off the barrel at either end and are made up of almost 15,000 further crystals. For extra spatial precision, the ECAL also contains pre-shower detectors that sit in front of the endcaps. These allow CMS to distinguish between single high-energy photons (often signs of exciting physics) and the less interesting close pairs of low-energy photons. At the endcaps the ECAL inner surface is covered by the pre-shower subdetector, consisting of two layers of lead interleaved with two layers of silicon strip detectors. Its purpose is to aid in pion-photon discrimination.
=== Layer 3 – The Hadronic Calorimeter ===