Electroweak interaction

After the Wu experiment in 1956 discovered parity violation in the weak interaction, a search began for a way to relate the weak and electromagnetic interactions. Extending his doctoral advisor Julian Schwinger's work, Sheldon Glashow first experimented with introducing two different symmetries, one chiral and one achiral, and combined them such that their overall symmetry was unbroken. This did not yield a renormalizable theory, and its gauge symmetry had to be broken by hand as no spontaneous mechanism was known, but it predicted a new particle, the Z boson. This received little notice, as it matched no experimental finding.

In 1964, Salam and John Clive Ward^[6] had the same idea, but predicted a massless photon and three massive gauge bosons with a manually broken symmetry. Later around 1967, while investigating spontaneous symmetry breaking, Weinberg found a set of symmetries predicting a massless, neutral gauge boson. Initially rejecting such a particle as useless, he later realized his symmetries produced the electroweak force, and he proceeded to predict rough masses for the W and Z bosons. Significantly, he suggested this new theory was renormalizable.^[3] In 1971, Gerard 't Hooft proved that spontaneously broken gauge symmetries are renormalizable even with massive gauge bosons.

Mathematically, electromagnetism is unified with the weak interactions as a Yang–Mills field with an SU(2) × U(1) gauge group, which describes the formal operations that can be applied to the electroweak gauge fields without changing the dynamics of the system. These fields are the weak isospin fields W₁, W₂, and W₃, and the weak hypercharge field B. This invariance is known as electroweak symmetry.

The generators of SU(2) and U(1) are given the name weak isospin (labeled T) and weak hypercharge (labeled Y) respectively. These then give rise to the gauge bosons that mediate the electroweak interactions – the three W bosons of weak isospin (W₁, W₂, and W₃), and the B boson of weak hypercharge, respectively, all of which are "initially" massless. These are not physical fields yet, before spontaneous symmetry breaking and the associated Higgs mechanism.

In the Standard Model, the observed physical particles, the W^±
and Z⁰
bosons, and the photon, are produced through the spontaneous symmetry breaking of the electroweak symmetry SU(2) × U(1)_Y to U(1)_em,^[b] effected by the Higgs mechanism (see also Higgs boson), an elaborate quantum-field-theoretic phenomenon that "spontaneously" alters the realization of the symmetry and rearranges degrees of freedom.^{[8][9][10][11]}

The electric charge arises as the particular linear combination (nontrivial) of Y_W (weak hypercharge) and the T₃ component of weak isospin (${\displaystyle Q=T_{3}+{\tfrac {1}{2}}\,Y_{\mathrm {W} }}$) that does not couple to the Higgs boson. That is to say: the Higgs and the electromagnetic field have no effect on each other, at the level of the fundamental forces ("tree level"), while any other combination of the hypercharge and the weak isospin must interact with the Higgs. This causes an apparent separation between the weak force, which interacts with the Higgs, and electromagnetism, which does not. Mathematically, the electric charge is a specific combination of the hypercharge and T₃ outlined in the figure.

U(1)_em (the symmetry group of electromagnetism only) is defined to be the group generated by this special linear combination, and the symmetry described by the U(1)_em group is unbroken, since it does not directly interact with the Higgs.^[c]

The above spontaneous symmetry breaking makes the W₃ and B bosons coalesce into two different physical bosons with different masses – the Z⁰
boson, and the photon (γ),

${\displaystyle {\begin{pmatrix}\gamma \\Z^{0}\end{pmatrix}}={\begin{pmatrix}\cos \theta _{\text{W}}&\sin \theta _{\text{W}}\\-\sin \theta _{\text{W}}&\cos \theta _{\text{W}}\end{pmatrix}}{\begin{pmatrix}B\\W_{3}\end{pmatrix}},}$

where θ_W is the weak mixing angle. The axes representing the particles have essentially just been rotated, in the (W₃, B) plane, by the angle θ_W. This also introduces a mismatch between the mass of the Z⁰
and the mass of the W^±
particles (denoted as m_Z and m_W, respectively),

${\displaystyle m_{\text{Z}}={\frac {m_{\text{W}}}{\,\cos \theta _{\text{W}}\,}}~.}$

The W₁ and W₂ bosons, in turn, combine to produce the charged massive bosons W^±
:^[12]

${\displaystyle W^{\pm }={\frac {1}{\sqrt {2\,}}}\,{\bigl (}\,W_{1}\mp iW_{2}\,{\bigr )}~.}$

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

${\displaystyle {\mathcal {L}}_{\mathrm {EW} }={\mathcal {L}}_{g}+{\mathcal {L}}_{f}+{\mathcal {L}}_{h}+{\mathcal {L}}_{y}~.}$

The ${\displaystyle {\mathcal {L}}_{g}}$ term describes the interaction between the three W vector bosons and the B vector boson,

${\displaystyle {\mathcal {L}}_{g}=-{\tfrac {1}{4}}W_{a}^{\mu \nu }W_{\mu \nu }^{a}-{\tfrac {1}{4}}B^{\mu \nu }B_{\mu \nu },}$

where ${\displaystyle W_{a}^{\mu \nu }}$ (${\displaystyle a=1,2,3}$) and ${\displaystyle B^{\mu \nu }}$ are the field strength tensors for the weak isospin and weak hypercharge gauge fields.

${\displaystyle {\mathcal {L}}_{f}}$ is the kinetic term for the Standard Model fermions. The interaction of the gauge bosons and the fermions are through the gauge covariant derivative,

${\displaystyle {\mathcal {L}}_{f}={\overline {Q}}_{j}iD\!\!\!\!/\;Q_{j}+{\overline {u}}_{j}iD\!\!\!\!/\;u_{j}+{\overline {d}}_{j}iD\!\!\!\!/\;d_{j}+{\overline {L}}_{j}iD\!\!\!\!/\;L_{j}+{\overline {e}}_{j}iD\!\!\!\!/\;e_{j},}$

where the subscript j sums over the three generations of fermions; Q, u, and d are the left-handed doublet, right-handed singlet up, and right handed singlet down quark fields; and L and e are the left-handed doublet and right-handed singlet electron fields. The Feynman slash ${\displaystyle D\!\!\!\!/}$ means the contraction of the 4-gradient with the Dirac matrices, defined as

${\displaystyle D\!\!\!\!/\equiv \gamma ^{\mu }\ D_{\mu },}$

and the covariant derivative (excluding the gluon gauge field for the strong interaction) is defined as

${\displaystyle \ D_{\mu }\equiv \partial _{\mu }-i\ {\frac {g'}{2}}\ Y\ B_{\mu }-i\ {\frac {g}{2}}\ T_{j}\ W_{\mu }^{j}.}$

Here ${\displaystyle \ Y\ }$ is the weak hypercharge and the ${\displaystyle \ T_{j}\ }$ are the components of the weak isospin.

The ${\displaystyle {\mathcal {L}}_{h}}$ term describes the Higgs field ${\displaystyle h}$ and its interactions with itself and the gauge bosons,

${\displaystyle {\mathcal {L}}_{h}=|D_{\mu }h|^{2}-\lambda \left(|h|^{2}-{\frac {v^{2}}{2}}\right)^{2}\ ,}$

where ${\displaystyle v}$ is the vacuum expectation value.

The ${\displaystyle \ {\mathcal {L}}_{y}\ }$ term describes the Yukawa interaction with the fermions,

${\displaystyle {\mathcal {L}}_{y}=-y_{u}^{ij}\epsilon ^{ab}\ h_{b}^{\dagger }\ {\overline {Q}}_{ia}u_{j}^{c}-y_{d}^{ij}\ h\ {\overline {Q}}_{i}d_{j}^{c}-y_{e}^{ij}\ h\ {\overline {L}}_{i}e_{j}^{c}+\mathrm {h.c.} ~,}$

and generates their masses, manifest when the Higgs field acquires a nonzero vacuum expectation value, discussed next. The ${\displaystyle \ y_{k}^{ij}\ ,}$ for ${\displaystyle \ k\in \{\mathrm {u,d,e} \}\ ,}$ are matrices of Yukawa couplings.

The Lagrangian reorganizes itself as the Higgs field acquires a non-vanishing vacuum expectation value dictated by the potential of the previous section. As a result of this rewriting, the symmetry breaking becomes manifest. In the history of the universe, this is believed to have happened shortly after the hot big bang, when the universe was at a temperature 159.5±1.5 GeV^[13] (assuming the Standard Model of particle physics).

Due to its complexity, this Lagrangian is best described by breaking it up into several parts as follows.

${\displaystyle {\mathcal {L}}_{\mathrm {EW} }={\mathcal {L}}_{\mathrm {K} }+{\mathcal {L}}_{\mathrm {N} }+{\mathcal {L}}_{\mathrm {C} }+{\mathcal {L}}_{\mathrm {H} }+{\mathcal {L}}_{\mathrm {HV} }+{\mathcal {L}}_{\mathrm {WWV} }+{\mathcal {L}}_{\mathrm {WWVV} }+{\mathcal {L}}_{\mathrm {Y} }~.}$

The kinetic term ${\displaystyle {\mathcal {L}}_{K}}$ 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)

${\displaystyle {\begin{aligned}{\mathcal {L}}_{\mathrm {K} }=\sum _{f}{\overline {f}}(i\partial \!\!\!/\!\;-m_{f})\ f-{\frac {1}{4}}\ A_{\mu \nu }\ A^{\mu \nu }-{\frac {1}{2}}\ W_{\mu \nu }^{+}\ W^{-\mu \nu }+m_{W}^{2}\ W_{\mu }^{+}\ W^{-\mu }\\\qquad -{\frac {1}{4}}\ Z_{\mu \nu }Z^{\mu \nu }+{\frac {1}{2}}\ m_{Z}^{2}\ Z_{\mu }\ Z^{\mu }+{\frac {1}{2}}\ (\partial ^{\mu }\ H)(\partial _{\mu }\ H)-{\frac {1}{2}}\ m_{H}^{2}\ H^{2}~,\end{aligned}}}$

where the sum runs over all the fermions of the theory (quarks and leptons), and the fields ${\displaystyle \ A_{\mu \nu }\ ,}$ ${\displaystyle \ Z_{\mu \nu }\ ,}$ ${\displaystyle \ W_{\mu \nu }^{-}\ ,}$ and ${\displaystyle \ W_{\mu \nu }^{+}\equiv (W_{\mu \nu }^{-})^{\dagger }\ }$ are given as

${\displaystyle X_{\mu \nu }^{a}=\partial _{\mu }X_{\nu }^{a}-\partial _{\nu }X_{\mu }^{a}+gf^{abc}X_{\mu }^{b}X_{\nu }^{c}~,}$

with ${\displaystyle X}$ to be replaced by the relevant field (${\displaystyle A,}$ ${\displaystyle Z,}$ ${\displaystyle W^{\pm }}$) and f ^abc by the structure constants of the appropriate gauge group.

The neutral current ${\displaystyle \ {\mathcal {L}}_{\mathrm {N} }\ }$ and charged current ${\displaystyle \ {\mathcal {L}}_{\mathrm {C} }\ }$ components of the Lagrangian contain the interactions between the fermions and gauge bosons,

${\displaystyle {\mathcal {L}}_{\mathrm {N} }=e\ J_{\mu }^{\mathrm {em} }\ A^{\mu }+{\frac {g}{\ \cos \theta _{W}\ }}\ (\ J_{\mu }^{3}-\sin ^{2}\theta _{W}\ J_{\mu }^{\mathrm {em} }\ )\ Z^{\mu }~,}$

where ${\displaystyle ~e=g\ \sin \theta _{\mathrm {W} }=g'\ \cos \theta _{\mathrm {W} }~.}$ The electromagnetic current ${\displaystyle \;J_{\mu }^{\mathrm {em} }\;}$ is

${\displaystyle J_{\mu }^{\mathrm {em} }=\sum _{f}\ q_{f}\ {\overline {f}}\ \gamma _{\mu }\ f~,}$

where ${\displaystyle \ q_{f}\ }$ is the fermions' electric charges. The neutral weak current ${\displaystyle \ J_{\mu }^{3}\ }$ is

${\displaystyle J_{\mu }^{3}=\sum _{f}\ T_{f}^{3}\ {\overline {f}}\ \gamma _{\mu }\ {\frac {\ 1-\gamma ^{5}\ }{2}}\ f~,}$

where ${\displaystyle T_{f}^{3}}$ is the fermions' weak isospin.^[d]

The charged current part of the Lagrangian is given by

${\displaystyle {\mathcal {L}}_{\mathrm {C} }=-{\frac {g}{\ {\sqrt {2\;}}\ }}\ \left[\ {\overline {u}}_{i}\ \gamma ^{\mu }\ {\frac {\ 1-\gamma ^{5}\ }{2}}\;M_{ij}^{\mathrm {CKM} }\ d_{j}+{\overline {\nu }}_{i}\ \gamma ^{\mu }\;{\frac {\ 1-\gamma ^{5}\ }{2}}\;e_{i}\ \right]\ W_{\mu }^{+}+\mathrm {h.c.} ~,}$

where ${\displaystyle \ \nu \ }$ is the right-handed singlet neutrino field, and the CKM matrix ${\displaystyle M_{ij}^{\mathrm {CKM} }}$ determines the mixing between mass and weak eigenstates of the quarks.^[d]

${\displaystyle {\mathcal {L}}_{\mathrm {H} }}$ contains the Higgs three-point and four-point self interaction terms,

${\displaystyle {\mathcal {L}}_{\mathrm {H} }=-{\frac {\ g\ m_{\mathrm {H} }^{2}\,}{\ 4\ m_{\mathrm {W} }\ }}\;H^{3}-{\frac {\ g^{2}\ m_{\mathrm {H} }^{2}\ }{32\ m_{\mathrm {W} }^{2}}}\;H^{4}~.}$

${\displaystyle {\mathcal {L}}_{\mathrm {HV} }}$ contains the Higgs interactions with gauge vector bosons,

${\displaystyle {\mathcal {L}}_{\mathrm {HV} }=\left(\ g\ m_{\mathrm {HV} }+{\frac {\ g^{2}\ }{4}}\;H^{2}\ \right)\left(\ W_{\mu }^{+}\ W^{-\mu }+{\frac {1}{\ 2\ \cos ^{2}\ \theta _{\mathrm {W} }\ }}\;Z_{\mu }\ Z^{\mu }\ \right)~.}$

${\displaystyle {\mathcal {L}}_{\mathrm {WWV} }}$ contains the gauge three-point self interactions,

${\displaystyle {\mathcal {L}}_{\mathrm {WWV} }=-i\ g\ \left[\;\left(\ W_{\mu \nu }^{+}\ W^{-\mu }-W^{+\mu }\ W_{\mu \nu }^{-}\ \right)\left(\ A^{\nu }\ \sin \theta _{\mathrm {W} }-Z^{\nu }\ \cos \theta _{\mathrm {W} }\ \right)+W_{\nu }^{-}\ W_{\mu }^{+}\ \left(\ A^{\mu \nu }\ \sin \theta _{\mathrm {W} }-Z^{\mu \nu }\ \cos \theta _{\mathrm {W} }\ \right)\;\right]~.}$

${\displaystyle {\mathcal {L}}_{\mathrm {WWVV} }}$ contains the gauge four-point self interactions,

${\displaystyle {\begin{aligned}{\mathcal {L}}_{\mathrm {WWVV} }=-{\frac {\ g^{2}\ }{4}}\ {\Biggl \{}\ &{\Bigl [}\ 2\ W_{\mu }^{+}\ W^{-\mu }+(\ A_{\mu }\ \sin \theta _{\mathrm {W} }-Z_{\mu }\ \cos \theta _{\mathrm {W} }\ )^{2}\ {\Bigr ]}^{2}\\&-{\Bigl [}\ W_{\mu }^{+}\ W_{\nu }^{-}+W_{\nu }^{+}\ W_{\mu }^{-}+\left(\ A_{\mu }\ \sin \theta _{\mathrm {W} }-Z_{\mu }\ \cos \theta _{\mathrm {W} }\ \right)\left(\ A_{\nu }\ \sin \theta _{\mathrm {W} }-Z_{\nu }\ \cos \theta _{\mathrm {W} }\ \right)\ {\Bigr ]}^{2}\,{\Biggr \}}~.\end{aligned}}}$

${\displaystyle \ {\mathcal {L}}_{\mathrm {Y} }\ }$ contains the Yukawa interactions between the fermions and the Higgs field,

${\displaystyle {\mathcal {L}}_{\mathrm {Y} }=-\sum _{f}\ {\frac {\ g\ m_{f}\ }{2\ m_{\mathrm {W} }}}\;{\overline {f}}\ f\ H~.}$

• Electroweak star
• Fundamental forces
• History of quantum field theory
• Mathematical formulation of the Standard Model
• Unitarity gauge
• Weinberg angle
• Yang–Mills theory