Physics:Weinberg angle

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Short description: Angle characterizing electroweak symmetry breaking
Weinberg angle θW, and relation between couplings g, g′, and e = g sin θW. Adapted from Lee (1981).[1]
The pattern of weak isospin, T3, and weak hypercharge, YW, of the known elementary particles, showing electric charge, Q,[lower-alpha 1] along the Weinberg angle. The neutral Higgs field (upper left, circled) breaks the electroweak symmetry and interacts with other particles to give them mass. Three components of the Higgs field become part of the massive W and Z bosons.

The weak mixing angle or Weinberg angle[2] is a parameter in the Weinberg–Salam theory of the electroweak interaction, part of the Standard Model of particle physics, and is usually denoted as θW. It is the angle by which spontaneous symmetry breaking rotates the original W0 and B0 vector boson plane, producing as a result the Z0 boson, and the photon.[3] Its measured value is slightly below 30°, but also varies, very slightly increasing, depending on how high the relative momentum of the particles involved in the interaction is that the angle is used for.[4]

Details

The algebraic formula for the combination of the W0 and B0 vector bosons (i.e. 'mixing') that simultaneously produces the massive Z0 boson and the massless photon (γ) is expressed by the formula

[math]\displaystyle{ \begin{pmatrix} \gamma \\ \textsf{Z}^0 \end{pmatrix} = \begin{pmatrix} \cos \theta_\textsf{W} & \sin \theta_\textsf{W} \\ -\sin \theta_\textsf{W} & \cos \theta_\textsf{W} \end{pmatrix} \begin{pmatrix} \textsf{B}^0 \\ \textsf{W}^0 \end{pmatrix} . }[/math][3]

The weak mixing angle also gives the relationship between the masses of the W and Z bosons (denoted as mW and mZ),

[math]\displaystyle{ m_\textsf{Z} = \frac{m_\textsf{W}}{\,\cos\theta_\textsf{W}} \,. }[/math]

The angle can be expressed in terms of the SU(2)L and U(1)Y couplings (weak isospin g and weak hypercharge g′, respectively),

[math]\displaystyle{ \cos \theta_\textsf{W} = \frac{g}{\,\sqrt{g^2+g'^2\,}\,} \qquad }[/math] and [math]\displaystyle{ \qquad \sin\theta_\textsf{W} = \frac{g'}{\,\sqrt{g^2+g'^2\,}\,} . }[/math]

The electric charge is then expressible in terms of it, e = g sin θW = g′ cos θW (refer to the figure).

Because the value of the mixing angle is currently determined empirically, in the absence of any superseding theoretical derivation it is mathematically defined as

[math]\displaystyle{ \cos \theta_\textsf{W} = \frac{\,m_\textsf{W}\,}{m_\textsf{Z}} . }[/math][5]

The value of θW varies as a function of the momentum transfer, ∆q, at which it is measured. This variation, or '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, mZ.

In practice, the quantity sin2 θW is more frequently used. The 2004 best estimate of sin2 θW, at ∆q = 91.2 GeV/c, in the MS scheme is 0.23120±0.00015, which is an average over measurements made in different processes, at different detectors. Atomic parity violation experiments yield values for sin2 θW 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 θW = 0.2397±0.0013 was obtained at ∆q = 0.16 GeV/c, establishing experimentally the so-called 'running' of the weak mixing angle. These values correspond to a Weinberg angle varying between 28.7° and 29.3° ≈ 30°. LHCb measured in 7 and 8 TeV proton–proton collisions an effective angle of sin2 θeffW = 0.23142,[6] though the value of ∆q for this measurement is determined by the partonic collision energy, which is close to the Z boson mass.

CODATA 2018[4] gives the value

[math]\displaystyle{ \sin^2 \theta _\textsf{W} = 1 - \left( m_\textsf{W}/m_\textsf{Z} \right)^2 = 0.222 90(30) . }[/math][lower-alpha 2]

The massless photon (γ) couples to the unbroken electric charge, Q = T3 + YW/2, while the Z0 boson couples to the broken charge T3 − sin2 θW Q.

Footnotes

  1. ↑ The electric charge Q is distinct from the similar-appearing symbol occasionally used for momentum-transfer ∆Q. This article uses ∆q, but upper case is common and may occur in some graphs.
  2. ↑ Note that at present, there is no generally accepted theory that explains why the measured value θW ≈ 29° should be what it is. The specific value is not predicted by the Standard Model: The Weinberg angle θW is an open, free parameter, although it is constrained and predicted through other measurements of Standard Model quantities.

References

  1. ↑ Lee, T.D. (1981). Particle Physics and Introduction to Field Theory. 
  2. ↑ Glashow, Sheldon (February 1961). "Partial-symmetries of weak interactions". Nuclear Physics 22 (4): 579–588. doi:10.1016/0029-5582(61)90469-2. Bibcode: 1961NucPh..22..579G. 
  3. ↑ 3.0 3.1 Cheng, T.P.; Li, L.F. (2006). Gauge Theory of Elementary Particle Physics. Oxford University Press. pp. 349–355. ISBN 0-19-851961-3. 
  4. ↑ 4.0 4.1 "Weak mixing angle". National Institute of Standards and Technology. 20 May 2019. http://physics.nist.gov/cgi-bin/cuu/Value?sin2th. 
  5. ↑ Okun, L.B. (1982). Leptons and Quarks. North-Holland Physics Publishing. p. 214. ISBN 0-444-86924-7. 
  6. ↑ Aaij, R.; Adeva, B.; Adinolfi, M.; Affolder, A.; Ajaltouni, Z.; Akar, S. et al. (2015-11-27). "Measurement of the forward-backward asymmetry in Z/γ∗ → μ+μ− decays and determination of the effective weak mixing angle". Journal of High Energy Physics 2015 (11): 190. doi:10.1007/JHEP11(2015)190. ISSN 1029-8479. 



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