129 lines
6.8 KiB
Markdown
129 lines
6.8 KiB
Markdown
---
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title: "ATLAS experiment"
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chunk: 4/5
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source: "https://en.wikipedia.org/wiki/ATLAS_experiment"
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category: "reference"
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tags: "science, encyclopedia"
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date_saved: "2026-05-05T13:02:57.471723+00:00"
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instance: "kb-cron"
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---
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==== Transition Radiation Tracker ====
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The Transition Radiation Tracker (TRT), the outermost component of the inner detector, is a combination of a straw tracker and a transition radiation detector. The detecting elements are drift tubes (straws), each four millimetres in diameter and up to 144 centimetres long. The uncertainty of track position measurements (position resolution) is about 200 micrometres. This is not as precise as those for the other two detectors, but it was necessary to reduce the cost of covering a larger volume and to have transition radiation detection capability. Each straw is filled with gas that becomes ionized when a charged particle passes through. The straws are held at about −1,500 V, driving the negative ions to a fine wire down the centre of each straw, producing a current pulse (signal) in the wire. The wires with signals create a pattern of 'hit' straws that allow the path of the particle to be determined. Between the straws, materials with widely varying indices of refraction cause ultra-relativistic charged particles to produce transition radiation and leave much stronger signals in some straws. Xenon and argon gas is used to increase the number of straws with strong signals. Since the amount of transition radiation is greatest for highly relativistic particles (those with a speed very near the speed of light), and because particles of a particular energy have a higher speed the lighter they are, particle paths with many very strong signals can be identified as belonging to the lightest charged particles: electrons and their antiparticles, positrons. The TRT has about 298,000 straws in total.
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=== Calorimeters ===
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The calorimeters are situated outside the solenoidal magnet that surrounds the Inner Detector. Their purpose is to measure the energy from particles by absorbing it. There are two basic calorimeter systems: an inner electromagnetic calorimeter and an outer hadronic calorimeter. Both are sampling calorimeters; that is, they absorb energy in high-density metal and periodically sample the shape of the resulting particle shower, inferring the energy of the original particle from this measurement.
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==== Electromagnetic calorimeter ====
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The electromagnetic (EM) calorimeter absorbs energy from particles that interact electromagnetically, which include charged particles and photons. It has high precision, both in the amount of energy absorbed and in the precise location of the energy deposited. The angle between the particle's trajectory and the detector's beam axis (or more precisely the pseudorapidity) and its angle within the perpendicular plane are both measured to within roughly 0.025 radians. The barrel EM calorimeter has accordion shaped electrodes and the energy-absorbing materials are lead and stainless steel, with liquid argon as the sampling material, and a cryostat is required around the EM calorimeter to keep it sufficiently cool.
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==== Hadron calorimeter ====
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The hadron calorimeter absorbs energy from particles that pass through the EM calorimeter, but do interact via the strong force; these particles are primarily hadrons. It is less precise, both in energy magnitude and in the localization (within about 0.1 radians only). The energy-absorbing material is steel, with scintillating tiles that sample the energy deposited. Many of the features of the calorimeter are chosen for their cost-effectiveness; the instrument is large and comprises a huge amount of construction material: the main part of the calorimeter – the tile calorimeter – is 8 metres in diameter and covers 12 metres along the beam axis. The far-forward sections of the hadronic calorimeter are contained within the forward EM calorimeter's cryostat, and use liquid argon as well, while copper and tungsten are used as absorbers.
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=== Muon Spectrometer ===
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The Muon Spectrometer is an extremely large tracking system, consisting of three parts:
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A magnetic field provided by three toroidal magnets;
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A set of 1200 chambers measuring with high spatial precision the tracks of the outgoing muons;
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A set of triggering chambers with accurate time-resolution.
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The extent of this sub-detector starts at a radius of 4.25 m close to the calorimeters out to the full radius of the detector (11 m). Its tremendous size is required to accurately measure the momentum of muons, which first go through all the other elements of the detector before reaching the muon spectrometer. It was designed to measure, standalone, the momentum of 100 GeV muons with 3% accuracy and of 1 TeV muons with 10% accuracy. It was vital to go to the lengths of putting together such a large piece of equipment because a number of interesting physical processes can only be observed if one or more muons are detected, and because the total energy of particles in an event could not be measured if the muons were ignored. It functions similarly to the Inner Detector, with muons curving so that their momentum can be measured, albeit with a different magnetic field configuration, lower spatial precision, and a much larger volume. It also serves the function of simply identifying muons – very few particles of other types are expected to pass through the calorimeters and subsequently leave signals in the Muon Spectrometer. It has roughly one million readout channels, and its layers of detectors have a total area of 12,000 square meters.
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=== Magnet System ===
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The ATLAS detector uses two large superconducting magnet systems to bend the trajectory of charged particles, so that their momenta can be measured. This bending is due to the Lorentz force, whose modulus is proportional to the electric charge
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q
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{\displaystyle q}
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of the particle, to its speed
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v
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{\displaystyle v}
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and to the intensity
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B
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{\displaystyle B}
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of the magnetic field:
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F
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=
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q
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v
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B
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.
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{\displaystyle F=q\,v\,B.}
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Since all particles produced in the LHC's proton collisions are traveling at very close to the speed of light in vacuum
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(
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v
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≃
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c
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)
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{\displaystyle (v\simeq c)}
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, the Lorentz force is about the same for all the particles with same electric charge
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q
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{\displaystyle q}
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:
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F
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≃
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q
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c
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B
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.
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{\displaystyle F\simeq q\,c\,B.}
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The radius of curvature
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r
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{\displaystyle r}
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due to the Lorentz force is equal to |