Editing Top quark (section)

== History ==
In 1973, [[Makoto Kobayashi (physicist)|Makoto Kobayashi]] and [[Toshihide Maskawa]] predicted the existence of a third generation of quarks to explain observed [[CP violation]]s in [[kaon]] [[particle decay|decay]]. The names top and [[bottom quark|bottom]] were introduced by [[Haim Harari]] in 1975,<ref name=Harari/><ref name=Staley/> to match the names of the first generation of quarks ([[up quark|up]] and [[down quark|down]]) reflecting the fact that the two were the "up" and "down" component of a [[weak isospin]] [[Doublet (physics)|doublet]].<ref name=Perkins/><ref name=Close/>

The proposal of Kobayashi and Maskawa heavily relied on the [[GIM mechanism]] put forward by [[Sheldon Glashow]], [[John Iliopoulos]] and [[Luciano Maiani]],<ref name=Glashow/> which predicted the existence of the then still unobserved [[charm quark]]. (Direct evidence for the existence of quarks, including the other [[Generation (physics)|second generation]] quark, the [[strange quark]], was obtained in 1968; strange particles were discovered back in 1947.) When in [[November Revolution (physics)|November 1974]] teams at [[Brookhaven National Laboratory]] (BNL) and the [[Stanford Linear Accelerator Center]] (SLAC) simultaneously announced the discovery of the [[J/ψ meson]], it was soon after identified as a bound state of the missing charm quark with its antiquark. This discovery allowed the GIM mechanism to become part of the Standard Model.<ref name=Pickering/> With the acceptance of the GIM mechanism, Kobayashi and Maskawa's prediction also gained in credibility. Their case was further strengthened by the discovery of the [[tau (particle)|tau]] by [[Martin Lewis Perl]]'s team at SLAC between 1974 and 1978.<ref name=Perl/> The tau announced a third generation of [[leptons]], breaking the new [[symmetry (physics)|symmetry]] between leptons and quarks introduced by the GIM mechanism. Restoration of the symmetry implied the existence of a fifth and sixth quark.

It was in fact not long until a fifth quark, the bottom, was discovered by the [[E288 experiment]] team, led by [[Leon Lederman]] at [[Fermilab]] in 1977.<ref name=Fermilab/><ref name=Lederman/><ref name=Herb/> This strongly suggested that there must also be a sixth quark, the top, to complete the pair. It was known that this quark would be heavier than the bottom, requiring more energy to create in particle collisions, but the general expectation was that the sixth quark would soon be found. However, it took another 18&nbsp;years before the existence of the top was confirmed.<ref name=LissTipton1997/>

Early searches for the top quark at [[SLAC]] and [[DESY]] (in [[Hamburg]]) came up empty-handed. When, in the early 1980s, the [[Super Proton Synchrotron]] (SPS) at [[CERN]] discovered the [[W&nbsp;boson]] and the [[Z&nbsp;boson]], it was again felt that the discovery of the top was imminent. As the SPS gained competition from the [[Tevatron]] at Fermilab there was still no sign of the missing particle, and it was announced by the group at CERN that the top mass must be at least {{val|41|u=GeV/c2}}. After a race between CERN and Fermilab to discover the top, the accelerator at CERN reached its limits without creating a single top, pushing the lower bound on its mass up to {{val|77|u=GeV/c2}}.<ref name=LissTipton1997/>

The Tevatron was (until the start of [[Large Hadron Collider|LHC]] operation at [[CERN]] in 2009) the only hadron collider powerful enough to produce top quarks. In order to be able to confirm a future discovery, a second detector, the [[DZero experiment|DØ detector]], was added to the complex (in addition to the [[Collider Detector at Fermilab]] (CDF) already present). In October&nbsp;1992, the two groups found their first hint of the top, with a single creation event that appeared to contain the top. In the following years, more evidence was collected and on 22&nbsp;April 1994, the CDF group submitted their article presenting tentative evidence for the existence of a top quark with a mass of about {{val|175|u=GeV/c2}}. In the meantime, DØ had found no more evidence than the suggestive event in 1992. A year later, on 2&nbsp;March 1995, after having gathered more evidence and reanalyzed the DØ data (which had been searched for a much lighter top), the two groups jointly reported the discovery of the top at a mass of {{val|176|18|u=GeV/c2}}.<ref name=CDF-1995/><ref name=D0-1995/><ref name=LissTipton1997/>

In the years leading up to the top-quark discovery, it was realized that certain precision measurements of the electroweak vector boson masses and couplings are very sensitive to the value of the top-quark mass. These effects become much larger for higher values of the top mass and therefore could indirectly see the top quark even if it could not be directly detected in any experiment at the time. The largest effect from the top-quark mass was on the [[S and T parameters|T&nbsp;parameter]], and by 1994 the precision of these indirect measurements had led to a prediction of the top-quark mass to be between {{val|145|u=GeV/c2}} and {{val|185|u=GeV/c2}}.<ref name=LissTipton1997/> It is the development of techniques that ultimately allowed such precision calculations that led to [[Gerardus 't Hooft]] and [[Martinus Veltman]] winning the [[Nobel Prize]] in physics in 1999.<ref name=Nobel-1/><ref name=Nobel-2/>