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{{Short description|General-purposes experiment at the Large Hadron Collider}}{{coord|46|18|34|N|6|4|37|E|type:landmark|display=title}}{{LHC}} The Compact Muon Solenoid (CMS) experiment is one of two large general-purpose particle physics detectors built on the Large Hadron Collider (LHC) at CERN in Switzerland and France. The goal of the CMS experiment is to investigate a wide range of physics, including the search for the Higgs boson, extra dimensions, and particles that could make up dark matter.CMS is 21 metres long, 15 m in diameter, and weighs about 14,000 tonnes.WEB,weblink Archived copy, 2014-10-18, dead,weblink" title="web.archive.org/web/20141018141526weblink">weblink 2014-10-18, Over 4,000 people, representing 206 scientific institutes and 47 countries, form the CMS collaboration who built and now operate the detector.WEB,weblink CMS Collaboration - CMS Experiment, cms.cern, 28 January 2020, It is located in a cavern at Cessy in France, just across the border from Geneva. In July 2012, along with ATLAS, CMS tentatively discovered the Higgs boson.NEWS , NEWS , WEB , By March 2013 its existence was confirmed.WEB ,

Background

Recent collider experiments such as the now-dismantled Large Electron-Positron Collider and the newly renovated Large Hadron Collider (LHC) at CERN, as well as the ({{As of|October 2011|lc=on}}) recently closed Tevatron at Fermilab have provided remarkable insights into, and precision tests of, the Standard Model of Particle Physics. A principal achievement of these experiments (specifically of the LHC) is the discovery of a particle consistent with the Standard Model Higgs boson, the particle resulting from the Higgs mechanism, which provides an explanation for the masses of elementary particles.WEB, The Higgs Boson,weblink CERN: Accelerating Science, CERN, 11 June 2015, However, there are still many questions that future collider experiments hope to answer. These include uncertainties in the mathematical behaviour of the Standard Model at high energies, tests of proposed theories of dark matter (including supersymmetry), and the reasons for the imbalance of matter and antimatter observed in the Universe.

Physics goals

(File:CERN LHC Compact Muon Solenoid - panorama.jpg|thumb|Panorama of CMS detector, 100m below the ground.)The main goals of the experiment are:

• to explore physics at the TeV scale
• to further study the properties of the Higgs boson, already discovered by CMS and ATLAS
• to look for evidence of physics beyond the standard model, such as supersymmetry, or extra dimensions
• to study aspects of heavy ion collisions.

The ATLAS experiment, at the other side of the LHC ring is designed with similar goals in mind, and the two experiments are designed to complement each other both to extend reach and to provide corroboration of findings. CMS and ATLAS uses different technical solutions and design of its detector magnet system to achieve the goals.

Detector summary

CMS is designed as a general-purpose detector, capable of studying many aspects of proton collisions at 0.9–13.6 TeV, the center-of-mass energy of the LHC particle accelerator.The CMS detector is built around a huge solenoid magnet. This takes the form of a cylindrical coil of superconducting cable that generates a magnetic field of 4 tesla, about 100 000 times that of the Earth. The magnetic field is confined by a steel 'yoke' that forms the bulk of the detector's weight of 12 500 t. An unusual feature of the CMS detector is that instead of being built in-situ underground, like the other giant detectors of the LHC experiments, it was constructed on the surface, before being lowered underground in 15 sections and reassembled.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 3.8 T. Outside the magnet are the large muon detectors, which are inside the return yoke of the magnet.(File:CMS 160312 06.png|thumb|none|400px|A cutaway diagram of the CMS detector)

CMS by layers

For full technical details about the CMS detector, please see the Technical Design Report.BOOK,weblink 9789290832683, CMS Physics: Technical Design Report Volume 1: Detector Performance and Software, Technical design report. CMS, 2006, Acosta, Darin,

The interaction point

This is the point in the centre of the detector at which proton-proton collisions occur between the two counter-rotating beams of the LHC. At each end of the detector magnets focus the beams into the interaction point. At collision each beam has a radius of 17 Î¼m and the crossing angle between the beams is 285 Î¼rad.At full design luminosity each of the two LHC beams will contain 2,808 bunches of {{val|1.15|e=11}} protons. The interval between crossings is 25 ns, although the number of collisions per second is only 31.6 million due to gaps in the beam as injector magnets are activated and deactivated.At full luminosity each collision will produce an average of 20 proton-proton interactions. The collisions occur at a centre of mass energy of 8 TeV. But, it is worth noting that for studies of physics at the electroweak scale, the scattering events are initiated by a single quark or gluon from each proton, and so the actual energy involved in each collision will be lower as the total centre of mass energy is shared by these quarks and gluons (determined by the parton distribution functions).The first test which ran in September 2008 was expected to operate at a lower collision energy of 10 TeV but this was prevented by the 19 September 2008 shutdown. When at this target level, the LHC will have a significantly reduced luminosity, due to both fewer proton bunches in each beam and fewer protons per bunch. The reduced bunch frequency does allow the crossing angle to be reduced to zero however, as bunches are far enough spaced to prevent secondary collisions in the experimental beampipe.

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 Î¼m, 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.WEB,weblink Tracker detector - CMS Experiment, cms.web.cern.ch, 20 December 2017, 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. BOOK, 10.1109/NSSMIC.2016.8069719, 978-1-5090-1642-6, The phase-1 upgrade of the CMS pixel detector, 2016 IEEE Nuclear Science Symposium, Medical Imaging Conference and Room-Temperature Semiconductor Detector Workshop (NSS/MIC/RTSD), 2016, Weber, Hannsjorg, 1–4, 1475062, 22786095,weblink The next four layers (up to 55 cm radius) consist of {{nowrap|10 cm × 180 μm}} silicon strips, followed by the remaining six layers of {{nowrap|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.CMS installs the world's largest silicon detector, CERN Courier, Feb 15, 2008

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 Ã— 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

The Hadron Calorimeter (HCAL) measures the energy of hadrons, particles made of quarks and gluons (for example protons, neutrons, pions and kaons). Additionally it provides indirect measurement of the presence of non-interacting, uncharged particles such as neutrinos.The HCAL consists of layers of dense material (brass or steel) interleaved with tiles of plastic scintillators, read out via wavelength-shifting fibres by hybrid photodiodes. This combination was determined to allow the maximum amount of absorbing material inside of the magnet coil.The high pseudorapidity region scriptstyle (3.0 ;