Electroweak interaction

fundamental interaction

In particle physics, the electroweak interaction is the unified description of two of the four known fundamental interactions of nature: electromagnetism and the weak interaction. Although these two forces appear very different at everyday low energies, the theory models them as two different aspects of the same force. Above the unification energy, on the order of 100 GeV, they would merge into a single electroweak force. Thus, if the universe is hot enough (approximately 1015 K, a temperature exceeded until shortly after the Big Bang), then the electromagnetic force and weak force merge into a combined electroweak force. During the electroweak epoch, the electroweak force separated from the strong force. During the quark epoch, the electroweak force split into the electromagnetic and weak force.

Sheldon Glashow, Abdus Salam, and Steven Weinberg were awarded the 1979 Nobel Prize in Physics for their contributions to the unification of the weak and electromagnetic interaction between elementary particles.[1][2] The existence of the electroweak interactions was experimentally established in two stages, the first being the discovery of neutral currents in neutrino scattering by the Gargamelle collaboration in 1973, and the second in 1983 by the UA1 and the UA2 collaborations that involved the discovery of the W and Z gauge bosons in proton–antiproton collisions at the converted Super Proton Synchrotron. In 1999, Gerardus 't Hooft and Martinus Veltman were awarded the Nobel prize for showing that the electroweak theory is renormalizable.



Weinberg's weak mixing angle θW, and relation between coupling constants g, g', and e. Adapted from T D Lee's book Particle Physics and Introduction to Field Theory (1981).
The pattern of weak isospin, T3, and weak hypercharge, YW, of the known elementary particles, showing the electric charge, Q, along the weak mixing angle. The neutral Higgs field (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.

Mathematically, the unification is accomplished under an SU(2) × U(1) gauge group. The corresponding gauge bosons are the three W bosons of weak isospin from SU(2) (W1, W2, and W3), and the B boson of weak hypercharge from U(1), respectively, all of which are massless.

In the Standard Model, the
, and the photon, are produced by the spontaneous symmetry breaking of the electroweak symmetry from SU(2) × U(1)Y to U(1)em, caused by the Higgs mechanism (see also Higgs boson).[3][4][5][6] U(1)Y and U(1)em are different copies of U(1); the generator of U(1)em is given by Q = Y/2 + I3, where Y is the generator of U(1)Y (called the weak hypercharge), and I3 is one of the SU(2) generators (a component of weak isospin).

The spontaneous symmetry breaking makes the W3 and B bosons coalesce into two different bosons – the
boson, and the photon (γ),


Where θW is the weak mixing angle. The axes representing the particles have essentially just been rotated, in the (W3, B) plane, by the angle θW. This also introduces a mismatch between the mass of the
and the mass of the
particles (denoted as MZ and MW, respectively),


The W1 and W2 bosons, in turn, combine to give massive charged bosons


The distinction between electromagnetism and the weak force arises because there is a (nontrivial) linear combination of Y and I3 that vanishes for the Higgs boson (it is an eigenstate of both Y and I3, so the coefficients may be taken as −I3 and Y): U(1)em is defined to be the group generated by this linear combination, and is unbroken because it does not interact with the Higgs.


Before electroweak symmetry breakingEdit

The Lagrangian for the electroweak interactions is divided into four parts before electroweak symmetry breaking


The   term describes the interaction between the three W particles and the B particle.


where   ( ) and   are the field strength tensors for the weak isospin and weak hypercharge fields.

  is the kinetic term for the Standard Model fermions. The interaction of the gauge bosons and the fermions are through the gauge covariant derivative.


where the subscript   runs over the three generations of fermions,  ,  , and   are the left-handed doublet, right-handed singlet up, and right handed singlet down quark fields, and   and   are the left-handed doublet and right-handed singlet electron fields.

The h term describes the Higgs field F.


The y term gives the Yukawa interaction that generates the fermion masses after the Higgs acquires a vacuum expectation value.


After electroweak symmetry breakingEdit

The Lagrangian reorganizes itself after the Higgs boson acquires a vacuum expectation value. Due to its complexity, this Lagrangian is best described by breaking it up into several parts as follows.


The kinetic term   contains all the quadratic terms of the Lagrangian, which include the dynamic terms (the partial derivatives) and the mass terms (conspicuously absent from the Lagrangian before symmetry breaking)


where the sum runs over all the fermions of the theory (quarks and leptons), and the fields  ,  ,  , and   are given as

 , (replace X by the relevant field, and fabc with the structure constants for the gauge group).

The neutral current   and charged current   components of the Lagrangian contain the interactions between the fermions and gauge bosons.


where the electromagnetic current   and the neutral weak current   are




  and   are the fermions' electric charges and weak isospin.

The charged current part of the Lagrangian is given by


  contains the Higgs three-point and four-point self interaction terms.


  contains the Higgs interactions with gauge vector bosons.


  contains the gauge three-point self interactions.


  contains the gauge four-point self interactions


and   contains the Yukawa interactions between the fermions and the Higgs field.


Note the   factors in the weak couplings: these factors project out the left handed components of the spinor fields. This is why electroweak theory (after symmetry breaking) is commonly said to be a chiral theory.

See alsoEdit


  1. ^ S. Bais (2005). The Equations: Icons of knowledge. p. 84. ISBN 0-674-01967-9. 
  2. ^ "The Nobel Prize in Physics 1979". The Nobel Foundation. Retrieved 2008-12-16. 
  3. ^ F. Englert; R. Brout (1964). "Broken Symmetry and the Mass of Gauge Vector Mesons". Physical Review Letters. 13 (9): 321–323. Bibcode:1964PhRvL..13..321E. doi:10.1103/PhysRevLett.13.321. 
  4. ^ P.W. Higgs (1964). "Broken Symmetries and the Masses of Gauge Bosons". Physical Review Letters. 13 (16): 508–509. Bibcode:1964PhRvL..13..508H. doi:10.1103/PhysRevLett.13.508. 
  5. ^ G.S. Guralnik; C.R. Hagen; T.W.B. Kibble (1964). "Global Conservation Laws and Massless Particles". Physical Review Letters. 13 (20): 585–587. Bibcode:1964PhRvL..13..585G. doi:10.1103/PhysRevLett.13.585. 
  6. ^ G.S. Guralnik (2009). "The History of the Guralnik, Hagen and Kibble development of the Theory of Spontaneous Symmetry Breaking and Gauge Particles". International Journal of Modern Physics A. 24 (14): 2601–2627. arXiv:0907.3466 . Bibcode:2009IJMPA..24.2601G. doi:10.1142/S0217751X09045431. 

General readersEdit

  • B. A. Schumm (2004). Deep Down Things: The Breathtaking Beauty of Particle Physics. Johns Hopkins University Press. ISBN 0-8018-7971-X.  Conveys much of the Standard Model with no formal mathematics. Very thorough on the weak interaction.