Technicolor (physics)

Technicolor is a class of quantum field theories proposed as an alternative to the Standard Model Higgs mechanism for electroweak symmetry breaking. In the Standard Model, the Higgs boson, a fundamental scalar particle, is responsible for giving mass to the W and Z bosons, the mediators of the weak force, as well as to fermions (quarks and leptons). Technicolor theories attempt to achieve the same result dynamically, without invoking a fundamental scalar.

The central idea behind technicolor is to introduce a new gauge interaction, referred to as "technicolor," and associated new fermions, called "technifermions," that interact under this force. These technifermions, unlike the fundamental fermions in the Standard Model, would condense due to the strong technicolor force at a scale of roughly 1 TeV. This condensation spontaneously breaks the electroweak symmetry, analogous to how chiral symmetry breaking in Quantum Chromodynamics (QCD) gives mass to hadrons.

The massless bosons that arise from this symmetry breaking are then "eaten" by the W and Z bosons, giving them mass. This mechanism avoids the introduction of a fundamental scalar Higgs field and its associated hierarchy problem (the problem of explaining why the Higgs boson's mass is so much smaller than the Planck scale).

Early technicolor models predicted the existence of numerous new particles, called "technihadrons," at the TeV scale. These would be composite particles formed from the technifermions, analogous to the hadrons formed from quarks in QCD. However, precision electroweak measurements have placed stringent constraints on these models, as the predicted technihadrons have not been observed at the expected masses.

Several variations of technicolor have been proposed to address these issues. These include walking technicolor, which postulates a technicolor force that remains strong over a wider range of energy scales than ordinary QCD, and extended technicolor, which attempts to explain fermion masses by introducing additional gauge bosons that mediate interactions between technifermions and Standard Model fermions.

Despite these refinements, technicolor models face significant challenges. Generating realistic fermion masses without introducing excessive flavor-changing neutral currents (FCNCs) has proven difficult. Furthermore, the discovery of a Higgs-like particle at the Large Hadron Collider (LHC) with properties consistent with the Standard Model Higgs boson has further diminished the appeal of technicolor theories as the primary mechanism for electroweak symmetry breaking.

While largely disfavored as the sole explanation for electroweak symmetry breaking, technicolor-inspired dynamics may still play a role in more complex scenarios, such as composite Higgs models, where the observed Higgs boson is itself a composite particle.