Editing Top quark (section)

== Production ==
Because top quarks are very massive, large amounts of energy are needed to create one. The only way to achieve such high energies is through high-energy collisions. These occur naturally in the Earth's upper atmosphere as [[cosmic ray]]s collide with particles in the air, or can be created in a [[particle accelerator]]. In 2011, after the [[Tevatron]] ceased operations, the [[Large Hadron Collider]] at [[CERN]] became the only accelerator that generates a beam of sufficient energy to produce top quarks, with a [[center-of-mass frame|center-of-mass energy]] of 7&nbsp;TeV. There are multiple processes that can lead to the production of top quarks, but they can be conceptually divided in two categories: top-pair production, and single-top production.

=== Top-quark pairs ===
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| image1 = Ttbar production via gg fusion.png
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| caption1 = gluon–gluon fusion
| image2 = Ttbar production (t channel).png
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| caption2 = t-channel
| image3 = Ttbar production via qqbar annihilation.png
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| caption3 = quark–antiquark annihilation
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The most common is [[pair production|production of a top–antitop pair]] via [[strong interaction]]s. In a collision, a highly energetic [[gluon]] is created, which subsequently decays into a top and antitop. This process was responsible for the majority of the top events at Tevatron and was the process observed when the top was first discovered in 1995.<ref name=D0-2009/> It is also possible to produce pairs of top–antitop through the decay of an intermediate [[photon]] or [[Z-boson]]. However, these processes are predicted to be much rarer and have a virtually identical experimental signature in a [[hadron collider]] like Tevatron.

=== Single top quarks ===
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| image1 = Single-top production (s channel).png
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| caption1 = s-channel
| image2 = Single-top production (t-channel).png
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| caption2 = t-channel
| image3 = Single top production (tW channel).png
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| caption3 = tW channel
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The production of single top quarks via [[weak interaction]] is a distinctly different process. This can happen in several ways (called channels): Either an intermediate [[W-boson]] decays into a top and antibottom quarks ("s-channel") or a bottom quark (probably created in a pair through the decay of a gluon) transforms to a top quark by exchanging a W&nbsp;boson with an up or down quark ("t-channel"). A single top quark can also be produced in association with a W&nbsp;boson, requiring an initial-state bottom quark ("tW-channel"). The first evidence for these processes was published by the DØ collaboration in December&nbsp;2006,<ref name=Abazov2007b/> and in March&nbsp;2009 the CDF<ref name=Aaltonen2009/> and DØ<ref name=D0-2009/> collaborations released twin articles with the definitive observation of these processes. The main significance of measuring these production processes is that their frequency is directly proportional to the {{math|{{!}}''V''<sub>tb</sub>{{!}}<sup>2</sup>}}&nbsp;component of the [[CKM matrix]].