Top quark condensate
In particle physics, the top quark condensate theory (or top condensation) is an alternative to the Standard Model fundamental Higgs field, where the Higgs boson is a composite field, composed of the top quark and its antiquark. These are bound together by a new force called topcolor, analogous to the binding of Cooper pairs in a BCS superconductor, or mesons in the strong interactions. The idea of binding of top quarks is motivated because it is comparatively heavy, with a measured mass is approximately 173 GeV (comparable to the electroweak scale), and so its Yukawa coupling is of order unity, suggesting the possibility of strong coupling dynamics. at higher energy scales. This model attempts to explain how the electroweak scale may match the top quark mass.
Top quark condensation is based upon the "infrared fixed point" for the top quark Higgs-Yukawa coupling, proposed in 1981 by Hill, based upon an earlier proposal of Pendleton and Ross.  The infrared fixed point surprisingly predicted that the top quark would be heavy, contrary to the prevailing view of the early 1980's. Indeed, the top quark was discovered in 1995 at the large mass of 173 GeV. The infrared-fixed point implies that it is strongly coupled to the Higgs boson at very high energies, corresponding to the Landau pole of the Higgs-Yukawa coupling. At this high scale the boundstate Higgs forms, and the coupling relaxes in the infrared to its measured value of order unity by the renormalization group.
The idea in its present form was described by Yoichiro Nambu and subsequently by Miransky, Tanabashi, and Yamawaki   and Bardeen, Hill and Lindner, who connected the theory to the renormalization group and improved its predictions. The simplest top condensation models predicted that the Higgs boson mass would be larger than the observed 173 GeV top quark mass, and have now been ruled out by the LHC discovery of the Higgs boson at a mass scale of 125 GeV. However, extended versions introducing more particles can be made consistent with the observed top quark mass. The general idea of a composite Higgs boson, connected in a fundamental way to the top quark, remains compelling, though the full details are not yet understood.
A composite Higgs boson arises naturally in Topcolor models, that are extensions of the standard model in analogy to quantum chromodynamics. To be natural, without excessive fine-tuning (i.e. to stabilize the Higgs mass from large radiative corrections), the theory requires new physics at a relatively low energy scale. Placing new physics at 10 TeV, for instance, the model predicts the top quark to be significantly heavier than observed (at about 600 GeV vs. 171 GeV). "Top Seesaw" models, also based upon Topcolor, circumvent this difficulty.
- Bose–Einstein condensation
- Fermion condensate
- Hierarchy problem
- Technicolor (physics)
- Infrared fixed point
- Hill, C.T. (1981). "Quark and Lepton masses from Renormalization group fixed points". Phys. Rev. D24: 691. Bibcode:1981PhRvD..24..691H. doi:10.1103/PhysRevD.24.691.
- Pendleton, B.; Ross, G.G. (1981). "Mass and Mixing Angle Predictions from Infrared Fixed points". Phys. Lett. B98: 291.
- "Dynamical electroweak symmetry breaking with large anomalous dimension and t quark condensate," Vladimir A. Miransky, Masaharu Tanabashi, and Koichi Yamawaki, Published in Phys. Lett. B 221:177, 1989.
- "Is the t Quark Responsible for the Mass of W and Z Bosons?" Vladimir A. Miransky, Masaharu Tanabashi, and Koichi Yamawaki, http://adsabs.harvard.edu/abs/1989MPLA....4.1043M
- William A. Bardeen; Christopher T. Hill & Manfred Lindner (1990). "Minimal dynamical symmetry breaking of the standard model". Physical Review. D41 (5): 1647–1660. Bibcode:1990PhRvD..41.1647B. doi:10.1103/PhysRevD.41.1647.