ATLAS experiment: Difference between revisions

Revision as of 23:52, 25 July 2013

Revision as of 12:39, 19 September 2013

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ATLAS is designed to be a general-purpose detector. 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. 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. Experiments at earlier colliders, such as the [[Tevatron]] and [[Large Electron-Positron Collider]], were designed based on a similar philosophy. However, the unique challenges of the Large Hadron Collider – its unprecedented energy and extremely high rate of collisions – require ATLAS to be larger and more complex than any detector ever built.

At 27 kilometres in [[circumference]], the [[Large Hadron Collider]] (LHC) [[collider|collides]] two beams of protons together, each proton carrying presently about 4 [[electron volt#TeV|TeV]] of energy – enough energy to produce particles with masses up to roughly fiveten times more massive than any particles currently known – assuming of course that such particles exist. When upgraded in 2014, the LHC with an energy seven million times that of the first accelerator, will represent a "new generation" of particle accelerators.

Particles that are produced in accelerators must also be observed, and this is the task of particle detectors. While interesting phenomena may occur when protons collide it is not enough to just produce them. Particle detectors must be built to detect particles, their masses, [[momentum]], [[energy|energies]], lifetime, charges, and [[nuclear spin]]s. In order to identify all particles produced at the [[interaction point]] where the particle beams collide, particle detectors are usually designed in layers like an onion. The layers are 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 [[A Toroidal LHC ApparatuS#Components|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. ATLAS is the largest detector ever built at a particle collider .<ref name="CERNpr"/>

==Physics Program==

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ATLAS is intended to investigate many different types of physics that might become detectable in the energetic collisions of the LHC. Some of these are confirmations or improved measurements of the [[Standard Model]], while many others are possible clues for new physical theories.

One of the most important goals of ATLAS is to investigate a missing piece of the Standard Model, the [[Higgs boson]].<ref name="TPintro">{{cite book |year=1994| title= ATLAS Technical Proposal| chapter=Introduction and Overview| publisher=CERN| chapterurl=http://atlas.web.cern.ch/Atlas/TP/NEW/HTML/tp9new/node4.html#SECTION00400000000000000000}}</ref> The [[Higgs mechanism]], which includes the Higgs boson, is hypothesized to give mass to elementary particles, giving rise to the differences between the [[weak force]] and [[electromagnetism]] by giving the [[W and Z bosons]] mass while leaving the [[photon]] massless. On July 4, 2012, ATLAS (together with CMS – its sister experiment at the LHC) reported evidence for the existence of a particle consistent with the Higgs boson at the level of five sigma,<ref name="cern1207"/> with a mass around 125 GeV or 125133 times the proton mass. This new "Higgs-like" particle was detected by its possible decay into two [[photon]]s, and its decay to four [[lepton]]s. Many further studies are needed in order to confirm whether or not the new particle is indeed the sought after Standard Model Higgs boson.

The asymmetry between the behavior of matter and [[antimatter]], known as [[CP violation]], is also being investigated.<ref name="TPintro"/> Current CP violation experiments, such as [[BaBar]] and [[Belle experiment|Belle]], have not yet detected sufficient CP violation in the Standard Model to explain the lack of detectable antimatter in the universe. It is possible that new models of physics will introduce additional CP violation, shedding light on this problem. Evidence supporting these models might either be detected directly by the production of new particles, or indirectly by measurements of the properties of B-[[meson]]s. ([[LHCb]], an LHC experiment dedicated to B-mesons, is likely to be better suited to the latter).<ref name="PhysicsatLHC">{{cite journal |author= N. V. Krasnikov, V. A. Matveev |year= 1997 |month = September |title = Physics at LHC |journal= Physics of Particles and Nuclei| volume= 28 |issue= 5 | pages= 441–470 |arxiv = hep-ph/9703204 |doi = 10.1134/1.953049 |bibcode = 1997PPN....28..441K }}</ref>

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===Inner Detector===

[[Image:ATLAS TRT.jpg|thumb|The '''ATLAS 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, Accelerators, Spectrometers, Detectors and Associated Equipment|year=2010|volume=617|issue=1/3|doi=10.1016/j.nima.2009.08.068|pages=48|bibcode = 2010NIMPA.617...48P }}</ref> in September 2005 ]]

The Inner Detector<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, Accelerators, Spectrometers, Detectors and Associated Equipment|year=2010|volume=617|issue=1/3}}</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| chapterurl=http://atlas.web.cern.ch/Atlas/TP/NEW/HTML/tp9new/node10.html#SECTION00433000000000000000}}</ref> 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]]). The Inner Detector has three parts, which are explained below.

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|arxiv = physics/0412138 |bibcode = 2006ITNS...53.1732H }}</ref> the innermost part of the detector, contains three concentric layers and three disks on each end-cap, with a total of 1,744 ''modules'', each measuring two&nbsp;centimetres by six&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 80 million readout channels, which is about 50% of the total readout channels of the whole experiment. 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.

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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 million readout channels and a total area of 61&nbsp;square meters.

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&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 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 its antiparticles, [[positron]]s. The TRT has about 298,000 straws in total.

===Calorimeters===

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===Muon Spectrometer===

The [[Muon]] [[Spectrometer]] is an extremely large tracking system, consisting of 3 parts: (1) a magnetic field provided by three toroidal magnets, (2) a set of 1200 chambers measuring with high spatial precision the tracks of the outgoing muons, (3) a set of triggering chambers with accurate time-resolution. The extend of this sub-detector goes starts at a radius of 4.25&nbsp;m close to the calorimeters out to the full radius of the detector (11&nbsp;m).<ref name="TPoveralldetector"/> 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===