Editing ATLAS experiment (section)

===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.