Inparticle physics, theelectroweak interaction orelectroweak force is theunified description of two of thefundamental interactions of nature:electromagnetism (electromagnetic interaction) and theweak 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 theunification energy, on the order of 246 GeV,[a] they would merge into a single force. Thus, if the temperature is high enough – approximately 1015K – then the electromagnetic force and weak force merge into a combined electroweak force.
In 1964,Salam andJohn Clive Ward[6] had the same idea, but predicted a masslessphoton and three massivegauge bosons with a manually broken symmetry. Later around 1967, while investigatingspontaneous symmetry breaking, Weinberg found a set of symmetries predicting a massless, neutralgauge 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 theW 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.
Weinberg's weak mixing angleθW, and relation between coupling constantsg, g′, ande. Adapted from Lee (1981).[7]The pattern ofweak isospin,T3, andweak hypercharge,YW, of the known elementary particles, showing the electric charge,Q, along theweak 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 massiveW andZ bosons.
Mathematically, electromagnetism is unified with the weak interactions as aYang–Mills field with anSU(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 fieldsW1,W2, andW3, and the weak hypercharge fieldB.This invariance is known aselectroweak symmetry.
Thegenerators ofSU(2) andU(1) are given the nameweak isospin (labeledT) andweak hypercharge (labeledY) respectively. These then give rise to the gauge bosons that mediate the electroweak interactions – the threeW bosons of weak isospin (W1,W2, andW3), and theB boson of weak hypercharge, respectively, all of which are "initially" massless. These are not physical fields yet, beforespontaneous symmetry breaking and the associatedHiggs mechanism.
The electric charge arises as the particular linear combination (nontrivial) ofYW (weak hypercharge) and theT3 component of weak isospin () that doesnot couple to theHiggs 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 anyother 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 andT3 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 theU(1)em group is unbroken, since it does notdirectly interact with the Higgs.[c]
The above spontaneous symmetry breaking makes theW3 andB bosons coalesce into two different physical bosons with different masses – theZ0 boson, and the photon (γ),
whereθW is theweak mixing angle. The axes representing the particles have essentially just been rotated, in the (W3,B) plane, by the angleθW. This also introduces a mismatch between the mass of theZ0 and the mass of theW± particles (denoted asmZ andmW, respectively),
TheW1 andW2 bosons, in turn, combine to produce the charged massive bosonsW± :[12]
where the subscriptj sums over the three generations of fermions;Q,u, andd are the left-handed doublet, right-handed singlet up, and right handed singlet down quark fields; andL ande are the left-handed doublet and right-handed singlet electron fields.TheFeynman slash means the contraction of the 4-gradient with theDirac matrices, defined as
and the covariant derivative (excluding thegluon gauge field for thestrong interaction) is defined as
Here is the weak hypercharge and the are the components of the weak isospin.
The term describes theHiggs field and its interactions with itself and the gauge bosons,
and generates their masses, manifest when the Higgs field acquires a nonzero vacuum expectation value, discussed next. The for 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 temperature159.5±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.
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
with to be replaced by the relevant field () andf abc by the structure constants of the appropriate 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 is
where is the fermions' electric charges. The neutral weak current is
^Note thatU(1)Y andU(1)em are distinct instances of genericU(1): Each of the two forces gets its own, independent copy of the unitary group.
^Although electromagnetism – e.g. the photon – does notdirectly interact with theHiggs boson, it does interactindirectly, throughquantum fluctuations.
^ab Note the factors in the weak coupling formulas: These factors are deliberately inserted to expunge any left-chiral components of the spinor fields. This is why electroweak theory is said to be a 'chiral theory'.