We know that there is a broken symmetry between the weak and electromagnetic forces because the theory that is based on this symmetry works—it makes a large number of successful predictions about the properties of the W and Z particles and about the forces they transmit. But we are not actually sure that the electroweak symmetry is broken by the vacuum value of some field in the theory or that there is a Higgs particle. Something has to be included in the electroweak theory to break this symmetry, but it is possible that the breakdown of the electroweak symmetry is due to indirect effects of
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