Weinberg angle

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The Weinberg angle or weak mixing angle is a parameter in the Abdus-Salam theory of the electroweak force. It gives a relationship between the W- and Z-masses, as well as the ratio of Z-Boson mediated interaction which behaves like a photon, i.e. its mixing. The weak mixing angle is denoted by the symbol θ_W, but in practice the quantity sin²(θ_W) is far more frequently used than θ_W itself.

The value of θ_W varies as a function of the momentum transfer, Q, at which it is measured; this 'running' is a key prediction of the electroweak theory. The most precise measurements have been carried out in electron-positron collider experiments at a value of Q = 91.2 GeV/c, corresponding to the mass of the Z⁰ boson, M_Z. The 2004 best estimate of sin² θ_W(M_Z) in the MS-bar scheme is 0.23120(15). Atomic parity violation experiments yield values for sin² θ_W at smaller values of Q, below 0.01 GeV/c, but with much lower precision. In 2005 results were published from a study of parity violation in Møller scattering in which a value of sin² θ_W = 0.2397(13) was obtained at Q = 0.16 GeV/c, establishing experimentally the 'running' of the weak mixing angle.