| Themes > Science > Physics > Elementary particle physics > Elementary particle physics Today > Elementary particle physics Today > Electroweak Symmetry Breaking |
In contrast to quantum chromodynamics, the description of electroweak processes in the Standard Model has many arbitrary or free parameters. Most stem from the breaking of the underlying symmetry between electromagnetism and the weak interactions. This symmetry breaking provides mass for the W and Z, but leaves the photon massless. In the minimal Standard Model, electroweak symmetry is broken by the Higgs mechanism. This idea has its roots in condensed matter physics, where it was introduced in connection with the Landau-Ginsberg theory of superconductivity. In this scheme, a particle's mass depends on its interactions with the Higgs field, a medium that permeates all of space and time. The W and Z masses result from their couplings to this field. The photon and gluon have no such couplings, so they remain massless. Quark and lepton masses are determined by the strength of their couplings to the Higgs field. These couplings also determine the extent to which quarks can mix between generations. Even charge parity (CP) violation--a fundamental asymmetry between matter and antimatter that may be responsible for matter dominance and our place in the universe--is generated by couplings to the Higgs. Unfortunately, we do not understand the origin of these couplings, so they must be determined phenomenologically by experiments. Current theoretical models can accommodate a top quark mass 340,000 times that of the electron and the small degree of CP violation seen in nature, but we cannot explain them. |
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