Composite Higgs models

In particle physics, composite Higgs models (CHM) are speculative extensions of the Standard Model (SM) where the Higgs boson is a bound state of new strong interactions. These scenarios are the leading alternative to supersymmetric models for physics beyond the SM presently tested at the Large Hadron Collider (LHC) in Geneva. In particle physics, composite Higgs models (CHM) are speculative extensions of the Standard Model (SM) where the Higgs boson is a bound state of new strong interactions. These scenarios are the leading alternative to supersymmetric models for physics beyond the SM presently tested at the Large Hadron Collider (LHC) in Geneva. According to CHM the recently discovered Higgs boson is not an elementary particle (or point-like) but has finite size, typically around 10−18 meters. This dimension is related to the Fermi scale (100 GeV) that determines the strength of the weak interactions such as in β-decay. Microscopically the composite Higgs will be made of smaller constituents in the same way as nuclei are made protons and neutrons. The main prediction of CHM are new particles with mass around a TeV that are excitations of the composite Higgs. This is analogous to the resonances in nuclear physics. The new particles could be produced and detected in collider experiments if the energy of the collision exceeds their mass and/or could produce deviations from the SM predictions in 'low energy observables' — results of experiments at lower energies. Within the most compelling scenarios each Standard Model particle has a partner with equal quantum numbers but heavier mass. For example, the photon, W and Z bosons have heavy replicas with mass determined by the compositeness scale, expected around 1 TeV. CHM are motivated by the so-called naturalness or hierarchy problem of the SM, the difficulty to explain the different energy scales that appear in the fundamental interactions of particle physics, in particular, the large difference between the mass of the Higgs boson and the Planck scale. CHM can solve the naturalness problem because the Higgs boson is not an elementary particle so that a new energy scale exists that can be explained dynamically similarly to the mass of the proton. Naturalness requires that new particles exist with mass around a TeV and these could be discovered at LHC or future experiments. As of 2018, no direct or indirect signs that the Higgs or other SM particles are composite has been detected. CHM were introduced in the early '80s as an extension of technicolor theories to allow for the presence of a physical Higgs boson. At the time this was not required by the data but recent discoveries have shown the necessity of a physical Higgs doublet to break the electro-weak symmetry. This differs from ordinary technicolor theories where strong dynamics directly breaks the electro-weak symmetry without the need of a physical Higgs boson. The first CHM proposed by Georgi and Kaplan were based on known gauge theory dynamics that produces the Higgs doublet as a Goldstone boson. These models are very constrained and it is difficult to include fermion masses. The subject remained dormant for several years until it was realized that this type of models naturally arise in 5 dimensional theories known as Randall–Sundrum scenarios. Within these constructions enough freedom exists to build realistic models at the price of abandoning a full microscopic description. It was soon understood that these scenarios can also be realized in hypothetical strongly coupled conformal field theories (CFT) also providing a dynamical explanation for the generation of hierarchies. The connection between Randall-Sundrum models and CFTs is rooted in the AdS-CFT correspondence. This spurred a great deal of activity in the field. At first the Higgs was a generic scalar bound state. In the highly influential work the Higgs as a Goldstone boson was realized in Randall-Sundrum models and in CFTs. Detailed phenomenological studies showed that within this framework agreement with experimental data can be obtained with a mild tuning of parameters.