In particle physics, the top quark condensate theory (or top condensation) is an alternative to the Standard Model fundamental Higgs field, replaced by a composite field of the top quark and its antiquark. These are bound together by a new force, analogous to the binding of Cooper pairs in a BCS superconductor, or mesons in the strong interactions. The top quark can "condense" 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, yielding the possibility of strong coupling dynamics.
The top and antitop quarks form a bound state that is a composite Higgs boson field. This model predicts how the electroweak scale may match the top quark mass. The idea was first described by Yoichiro Nambu and subsequently by Vladimir Miransky, Masaharu Tanabashi, and Koichi Yamawaki (Is the t Quark Responsible for the Mass of W and Z Bosons?) and developed into a predictive framework, based upon the renormalization group, by William A. Bardeen, Christopher T. Hill, and Manfred Lindner in the article Minimal Dynamical Symmetry Breaking of the Standard Model. Top quark condensation is essentially based upon the "quasi-infrared fixed point" for the top quark Higgs-Yukawa coupling, proposed in 1981 by Hill in the paper Quark and Lepton Masses from Renormalization Group Fixed Points. The simplest top condensation models predicted that the Higgs boson mass would be larger than the 175 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.
More complex schemes may still be viable. Top condensation 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. These theories will ultimately be tested at the LHC in its Run-II commencing in 2015.