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.
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 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 or could produce deviations from the SM predictions in low energy observables. 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 1012 eV.
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. 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 TeV and these could be discovered at LHC or future experiments. As of 2015, no direct or indirect signs that the Higgs or other SM particles are composite has been detected.
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