Editing ATLAS experiment (section)

==ATLAS detector==
The ATLAS detector is 46&nbsp;metres long, 25&nbsp;metres in diameter, and weighs about 7,000&nbsp;tonnes; it contains some 3,000&nbsp;km of cable.<ref name=fact_sheets/><ref name=the_bible/><ref name="TPoveralldetector"/>

At 27&nbsp;km in [[circumference]], the [[Large Hadron Collider]] (LHC) at [[CERN]] [[collider|collides]] two beams of protons together, with each proton carrying up to 6.8&nbsp;[[electron volt#TeV|TeV]] of energy – enough to produce particles with masses significantly greater than any particles currently known, if these particles exist. When the proton [[particle beam|beams]] produced by the Large Hadron Collider interact in the center of the detector, a variety of different particles with a broad range of energies are produced.

===General-purpose requirements===
The ATLAS detector is designed to be general-purpose. Rather than focusing on a particular physical process, ATLAS is designed to measure the broadest possible range of signals. This is intended to ensure that whatever form any new physical processes or particles might take, ATLAS will be able to detect them and measure their properties. ATLAS is designed to detect these particles, namely their masses, [[momentum]], [[energy|energies]], lifetime, charges, and [[nuclear spin]]s.

Experiments at earlier colliders, such as the [[Tevatron]] and [[Large Electron–Positron Collider]], were also designed for general-purpose detection. However, the beam energy and extremely high rate of collisions require ATLAS to be significantly larger and more complex than previous experiments, presenting unique challenges of the Large Hadron Collider.

===Layered design===
In order to identify all particles produced at the [[interaction point]] where the particle beams collide, the detector is designed in layers  made up of detectors of different types, each of which is designed to observe specific types of particles.  The different traces that particles leave in each layer of the detector allow for effective [[particle identification]] and accurate measurements of energy and momentum. (The role of each layer in the detector is discussed [[#Detector systems|below]].) As the energy of the particles produced by the accelerator increases, the detectors attached to it must grow to effectively measure and stop higher-energy particles. As of 2022, the ATLAS detector is the largest ever built at a particle collider.<ref name="CERNpr">{{cite press release| publisher=CERN| date=2006-11-20| title= World's largest superconducting magnet switches on| url=http://press.cern/press-releases/2006/11/worlds-largest-superconducting-magnet-switches| access-date=2016-11-23}}</ref>

===Detector systems===
[[File:ATLAS Drawing with Labels.svg|thumb|upright=1.5|Computer generated cut-away view of the ATLAS detector showing its various components. <br>
[[#Muon Spectrometer|Muon Spectrometer]]: <br>
&nbsp;&nbsp; (1) Forward regions (End-caps) <br>
&nbsp;&nbsp; (1) Barrel region <br>
[[#Magnet System|Magnet System]]: <br>
&nbsp;&nbsp; (2) Toroid Magnets <br>
&nbsp;&nbsp; (3) Solenoid Magnet <br>
[[#Inner Detector|Inner Detector]]: <br>
&nbsp;&nbsp; (4) Transition Radiation Tracker <br>
&nbsp;&nbsp; (5) Semi-Conductor Tracker <br>
&nbsp;&nbsp; (6) Pixel Detector <br>
[[#Calorimeters|Calorimeters]]: <br>
&nbsp;&nbsp; (7) Liquid Argon Calorimeter <br>
&nbsp;&nbsp; (8) Tile Calorimeter <br>
]]

The ATLAS detector<ref name=fact_sheets/><ref name=the_bible/><ref name="TPoveralldetector"/> consists of a series of ever-larger concentric cylinders around the [[interaction point]] where the proton beams from the LHC collide. Maintaining detector performance in the high radiation areas immediately surrounding the proton beams is a significant engineering challenge.  The detector can be divided into four major systems: 
# Inner Detector; 
# Calorimeters; 
# [[Muon]] Spectrometer; 
# Magnet system.  
Each of these is in turn made of multiple layers.  The detectors are complementary: the Inner Detector tracks particles precisely, the calorimeters measure the energy of easily stopped particles, and the muon system makes additional measurements of highly penetrating muons.  The two magnet systems bend [[electric charge|charged]] particles in the Inner Detector and the Muon Spectrometer, allowing their [[electric charge]]s and [[momentum|momenta]] to be measured.
The only established stable particles that cannot be detected directly are [[neutrino]]s; their presence is inferred by measuring a momentum imbalance among detected particles.  For this to work, the detector must be "[[Hermetic detector|hermetic]]", meaning it must detect all non-neutrinos produced, with no blind spots.

The installation of all the above detector systems was finished in August 2008. The detectors collected millions of cosmic rays during the magnet repairs which took place between fall 2008 and fall 2009, prior to the first proton collisions. The detector operated with close to 100% efficiency and provided performance characteristics very close to its design values.<ref>{{cite journal|title= Performance of the ATLAS Detector using First Collision Data|journal=JHEP|volume=1009|year=2010|pages=056|doi= 10.1007/JHEP09(2010)056|arxiv = 1005.5254 |bibcode = 2010JHEP...09..056A|last1= Aad|first1= G.|author2=  (ATLAS Collaboration)|issue= 9|s2cid=118543167|display-authors=etal}}</ref>

===Inner Detector===
[[File:ATLAS TRT.jpg|thumb|The TRT (Transition Radiation Tracker) central section, the outermost part of the Inner Detector, assembled above ground and taking data from [[cosmic ray]]s<ref>{{cite journal|title=Readiness of the ATLAS detector: Performance with the first beam and cosmic data|author=F. Pastore|journal=Nuclear Instruments and Methods in Physics Research Section A|year=2010|volume=617|issue=1/3|doi=10.1016/j.nima.2009.08.068|pages=48–51|bibcode = 2010NIMPA.617...48P |url=https://cds.cern.ch/record/1177420}}</ref> in September 2005.]]

The Inner Detector<ref name=fact_sheets/><ref name=the_bible/><ref name="TPoveralldetector"/><ref>{{cite journal | title=Alignment of the ATLAS inner detector tracking system | author=Regina Moles-Valls | journal=Nuclear Instruments and Methods in Physics Research Section A | year=2010 | volume=617 | issue=1–3 | pages=568–570 | doi=10.1016/j.nima.2009.09.101|bibcode = 2010NIMPA.617..568M | arxiv=0910.5156 }}</ref> begins a few centimetres from the proton beam axis, extends to a radius of 1.2&nbsp;metres, and is 6.2&nbsp;metres in length along the beam pipe.  Its basic function is to track charged particles by detecting their interaction with material at discrete points, revealing detailed information about the types of particles and their momentum.<ref name="TPinnerdetector">{{cite book| year=1994| title= ATLAS Technical Proposal| chapter=Inner detector| publisher=CERN| chapter-url=http://atlas.web.cern.ch/Atlas/TP/NEW/HTML/tp9new/node10.html#SECTION00433000000000000000}}</ref>  
The Inner Detector has three parts, which are explained below.

The [[magnetic field]] surrounding the entire inner detector causes charged particles to curve; the direction of the curve reveals a particle's charge and the degree of curvature reveals its momentum.  The starting points of the tracks yield useful information for [[particle identification|identifying particles]]; for example, if a group of tracks seem to originate from a point other than the original proton–proton collision, this may be a sign that the particles came from the decay of a hadron with a [[bottom quark]] (see [[b-tagging]]).

====Pixel Detector====
The Pixel Detector,<ref>{{cite journal|title=The ATLAS pixel detector|author=Hugging, F.|journal=IEEE Transactions on Nuclear Science|year=2006|volume=53|issue=6|doi=10.1109/TNS.2006.871506|pages=1732–1736|arxiv = physics/0412138 |bibcode = 2006ITNS...53.1732H |s2cid=47545925}}</ref> the innermost part of the detector, contains four concentric layers and three disks on each end-cap, with a total of 1,744&nbsp;''modules'', each measuring 2&nbsp;centimetres by 6&nbsp;centimetres.  The detecting material is 250&nbsp;μm thick [[silicon]].  Each module contains 16 readout [[computer chip|chips]] and other electronic components. The smallest unit that can be read out is a pixel (50 by 400&nbsp;micrometres); there are roughly 47,000&nbsp;pixels per module.

The minute pixel size is designed for extremely precise tracking very close to the interaction point.  In total, the Pixel Detector has over 92 million readout channels, which is about 50% of the total readout channels of the whole detector. Having such a large count created a considerable design and engineering challenge. Another challenge was the [[radiation]] to which the Pixel Detector is exposed because of its proximity to the interaction point, requiring that all components be [[radiation hardened]] in order to continue operating after significant exposures.

====Semi-Conductor Tracker====
The Semi-Conductor Tracker (SCT) is the middle component of the inner detector.  It is similar in concept and function to the Pixel Detector but with long, narrow strips rather than small pixels, making coverage of a larger area practical.  Each strip measures 80 micrometres by 12 centimetres.  The SCT is the most critical part of the inner detector for basic tracking in the plane perpendicular to the beam, since it measures particles over a much larger area than the Pixel Detector, with more sampled points and roughly equal (albeit one-dimensional) accuracy.  It is composed of four double layers of silicon strips, and has 6.3&nbsp;million readout channels and a total area of 61&nbsp;square meters.

====Transition Radiation Tracker====
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&nbsp;centimetres long.  The uncertainty of track position measurements (position resolution) is about 200&nbsp;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 [[ion]]ized when a charged particle passes through.  The straws are held at about −1,500&nbsp;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 [[index of refraction|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 [[special relativity|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: [[electron]]s and their antiparticles, [[positron]]s. The TRT has about 298,000 straws in total.

===Calorimeters===
[[File:ATLAS HCal.jpg|thumb|September 2005: The main barrel section of the ATLAS [[hadronic]] calorimeter, waiting to be moved inside the toroid magnets.]]
[[File:CERN-Rama-33.jpg|thumb|One of the sections of the extensions of the hadronic [[Calorimeter (particle physics)|calorimeter]], waiting to be inserted in late February 2006.]]
[[File:ATLAS Tile Calorimeter.png|thumb|The extended barrel section of the hadronic calorimeter.]]

The [[calorimeter (particle physics)|calorimeters]]<ref name=fact_sheets/><ref name=the_bible/><ref name="TPoveralldetector"/> 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.<ref name="TPcalorimetry">{{cite book| year=1994| title= ATLAS Technical Proposal| chapter=Calorimetry| publisher=CERN| chapter-url=http://atlas.web.cern.ch/Atlas/TP/NEW/HTML/tp9new/node9.html#SECTION00432000000000000000}}</ref>  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.

====Electromagnetic calorimeter====
The electromagnetic (EM) calorimeter absorbs energy from particles that interact [[Electromagnetism|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&nbsp;[[radian]]s. 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.

====Hadron calorimeter====
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&nbsp;radians only).<ref name="PhysicsatLHC"/>  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&nbsp;metres in diameter and covers 12&nbsp;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.

===Muon Spectrometer===
The [[Muon]] [[Spectrometer]]<ref name=fact_sheets/><ref name=the_bible/><ref name="TPoveralldetector"/> is an extremely large tracking system, consisting of three parts: 
# A magnetic field provided by three toroidal magnets;  
# A set of 1200 chambers measuring with high spatial precision the tracks of the outgoing muons; 
# A set of triggering chambers with accurate time-resolution. 
The extent of this sub-detector starts at a radius of 4.25&nbsp;m close to the calorimeters out to the full radius of the detector (11&nbsp;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&nbsp;square meters.

===Magnet System===
[[File:Installing the ATLAS Calorimeter - edit1.jpg|thumb|The eight toroid magnets of the ATLAS detector]] 
[[File:ATLAS Above.jpg|thumb|The ends of four of the eight ATLAS toroid magnets, looking down from about 90&nbsp;metres above, in September 2005]]

The ATLAS detector uses two large superconducting magnet systems to bend the trajectory of charged particles, so that their momenta can be measured.<ref name=fact_sheets/><ref name=the_bible/><ref name="TPoveralldetector"/>  This bending is due to the [[Lorentz force]], whose modulus is proportional to the [[electric charge]] <math>q</math> of the particle, to its speed <math>v</math> and to the intensity <math>B</math> of the magnetic field:
:<math>F = q \, v \, B.</math>
Since all particles produced in the LHC's [[proton]] collisions are traveling at very close to the speed of light in vacuum <math>(v \simeq c)</math>, the [[Lorentz force]] is about the same for all the particles with same [[electric charge]] <math>q</math>:
:<math>F \simeq q \, c \, B.</math>
The radius of curvature <math>r</math> due to the [[Lorentz force]] is equal to
:<math>r = \frac{p}{q \, B}.</math>
where <math> p = \gamma \, m \, v </math> is the [[mass in special relativity|relativistic]] [[momentum]] of the particle. As a result, high-momentum particles curve very little (large <math>r</math>), while low-momentum particles curve significantly (small <math>r</math>). The amount of [[curvature]] can be quantified and the particle [[momentum]] can be determined from this value.

====Solenoid Magnet====
The inner [[solenoid]] produces a two [[Tesla (unit)|tesla]] magnetic field surrounding the Inner Detector.<ref name="TPmagnet">{{cite book| year=1994| title= ATLAS Technical Proposal| chapter=Magnet system| publisher=CERN| chapter-url= http://atlas.web.cern.ch/Atlas/TP/NEW/HTML/tp9new/node8.html#SECTION00431000000000000000}}</ref>  This high magnetic field allows even very energetic particles to curve enough for their momentum to be determined, and its nearly uniform direction and strength allow measurements to be made very precisely.  Particles with momenta below roughly 400 [[MeV]] will be curved so strongly that they will loop repeatedly in the field and most likely not be measured; however, this energy is very small compared to the several [[TeV]] of energy released in each proton collision.

====Toroid Magnets====
The outer [[toroid]]al magnetic field is produced by eight very large air-core [[superconducting]] barrel loops and two smaller end-caps air toroidal magnets, for a total of 24 barrel loops all situated outside the calorimeters and within the muon system.<ref name="TPmagnet"/>  This magnetic field extends in an area 26&nbsp;metres long and 20&nbsp;metres in diameter, and it stores 1.6&nbsp;[[gigajoule]]s of energy.  Its magnetic field is not uniform, because a solenoid magnet of sufficient size would be prohibitively expensive to build. It varies between 2 and 8 Teslameters.

===Forward detectors===
{{further|ATLAS Forward Proton Project}}
The ATLAS detector is complemented by a set of four sub-detectors in the forward region to measure particles at very small angles.<ref>[http://atlas-project-lumi-fphys.web.cern.ch/ The ATLAS Forward Detector project]</ref> 
# LUCID (LUminosity Cherenkov Integrating Detector) <br> is the first of these detectors designed to measure luminosity, and located in the ATLAS cavern at 17 m from the interaction point between the two muon endcaps;  
# ZDC (Zero Degree Calorimeter) <br> is designed to measure neutral particles on-axis to the beam, and located at 140 m from the IP in the LHC tunnel where the two beams are split back into separate beam pipes; 
# AFP (Atlas Forward Proton) <br> is designed to tag diffractive events, and located at 204 m and 217 m; 
# ALFA (Absolute Luminosity For ATLAS) <br> is designed to measure elastic proton scattering located at 240 m just before the bending magnets of the LHC arc.

===Data systems===

====Data generation====
Earlier particle detector read-out and event detection systems were based on parallel shared [[Bus (computing)|buses]] such as [[VMEbus]] or [[FASTBUS]]. Since such a bus architecture cannot keep up with the data requirements of the LHC detectors, all the ATLAS data acquisition systems rely on high-speed point-to-point links and switching networks. Even with advanced [[electronics]] for data reading and storage, the ATLAS detector generates too much raw data to read out or store everything: about 25 [[megabyte|MB]] per raw event, multiplied by 40 million [[beam crossing]]s per second (40 [[Hertz#SI multiples|MHz]]) in the center of the detector. This produces a total of 1 [[Byte#Multiple-byte units|petabyte]] of raw data per second. By avoiding to write empty segments of each event (zero suppression), which do not contain physical information, the average size of an event is reduced to 1.6 [[megabyte|MB]], for a total of 64 [[terabyte]] of data per second.<ref name=fact_sheets/><ref name=the_bible/><ref name="TPoveralldetector"/>

====Trigger system====
The [[trigger (particle physics)|trigger]] system<ref name=fact_sheets/><ref name=the_bible/><ref name="TPoveralldetector"/><ref>{{cite journal|title=ATLAS Trigger and Data Acquisition: Capabilities and commissioning|author=D. A. Scannicchio|journal=Nuclear Instruments and Methods in Physics Research Section A|year=2010|volume=617|issue=1/3|doi=10.1016/j.nima.2009.06.114|pages=306–309|bibcode = 2010NIMPA.617..306S}}</ref> uses fast event reconstruction to identify, in real time, the most interesting [[event (particle physics)|events]] to retain for detailed analysis.  In the second data-taking period of the LHC, Run-2, there were two distinct trigger levels:<ref>{{cite journal|title=ATLAS Run-2 status and performance|author=ATLAS collaboration|journal=Nuclear and Particle Physics Proceedings|year=2016|volume=270|doi=10.1016/j.nuclphysbps.2016.02.002|pages=3–7|bibcode=2016NPPP..270....3P|url=https://cds.cern.ch/record/2048973}}</ref>

# The Level 1 trigger (L1), implemented in custom hardware at the detector site. The decision to save or reject an event data is made in less than 2.5 μs. It uses reduced granularity information from the calorimeters and the muon spectrometer, and reduces the rate of events in the read-out from 40&nbsp;[[Hertz#SI multiples|MHz]] to 100&nbsp;[[Hertz#SI multiples|kHz]]. The L1 rejection factor in therefore equal to 400.
# The High Level Trigger trigger (HLT), implemented in software, uses a computer battery consisting of approximately 40,000&nbsp;[[Central processing unit|CPUs]]. In order to decide which of the 100,000 events per second coming from L1 to save, specific analyses of each collision are carried out in 200 μs. The HLT uses limited regions of the detector, so-called Regions of Interest (RoI), to be reconstructed with the full detector granularity, including tracking, and allows matching of energy deposits to tracks.  The HLT rejection factor is 100: after this step, the rate of events is reduced from 100 to 1&nbsp;[[Hertz#SI multiples|kHz]]. The remaining data, corresponding to about 1,000 events per second, are stored for further analyses.<ref name="CERN">{{cite news |work=[[ATLAS collaboration]] Research News |title=Trigger and Data Acquisition System|url=https://atlas.cern/discover/detector/trigger-daq |date=October 2019}}</ref>

====Analysis process====
ATLAS permanently records more than 10 [[Byte#Multiple-byte units|petabyte]]s of data per year.<ref name=fact_sheets/>
Offline [[event reconstruction]] is performed on all permanently stored events, turning the pattern of signals from the detector into physics objects, such as [[Particle jet|jets]], [[photon]]s, and [[lepton]]s. [[Grid computing]] is being used extensively for event reconstruction, allowing the parallel use of university and laboratory computer networks throughout the world for the [[central processing unit|CPU]]-intensive task of reducing large quantities of raw data into a form suitable for physics analysis. 
The [[software]] for these tasks has been under development for many years, and refinements are ongoing, even after data collection has begun.
Individuals and groups within the collaboration are continuously writing their own [[computational physics|code]] to perform further analyses of these objects, searching the patterns of detected particles for particular physical models or hypothetical particles. This activity requires processing 25 [[Byte#Multiple-byte units|petabyte]]s of data every week.<ref name=fact_sheets/>