The Standard Model of particle physics is a gauge quantum field theory containing the internal symmetries of the unitary product group . The theory is commonly viewed as describing the fundamental set of particles – the leptons, quarks, gauge bosons and the Higgs boson.
The Standard Model is renormalizable and mathematically self-consistent;In fact, there are mathematical issues regarding quantum field theories still under debate (see e.g. Landau pole), but the predictions extracted from the Standard Model by current methods are all self-consistent. For a further discussion see e.g. R. Mann, chapter 25. however, despite having huge and continued successes in providing experimental predictions, it does leave some unexplained phenomena. In particular, although the physics of special relativity is incorporated, general relativity is not, and the Standard Model will fail at energies or distances where the graviton is expected to emerge. Therefore, in a modern field theory context, it is seen as an effective field theory.
Quantum field theory
The standard model is a quantum field theory, meaning its fundamental objects are quantum fields, which are defined at all points in spacetime. QFT treats particles as excited states (also called quanta) of their underlying quantum fields, which are more fundamental than the particles. These fields are the fermion fields, , which account for "matter particles"; the electroweak boson fields , , , and ; the gluon field, ; and the Higgs field, . That these are quantum rather than classical fields has the mathematical consequence that they are operator-valued. In particular, values of the fields generally do not commute. As operators, they act upon a quantum state (ket vector).
Alternative presentations of the fields As is common in quantum theory, there is more than one way to look at things. At first the basic fields given above may not seem to correspond well with the "fundamental particles" in the chart above, but there are several alternative presentations that, in particular contexts, may be more appropriate than those that are given above.
Fermions Rather than having one fermion field , it can be split up into separate components for each type of particle. This mirrors the historical evolution of quantum field theory, since the electron component (describing the electron and its antiparticle the positron) is then the original field of quantum electrodynamics, which was later accompanied by and fields for the muon and tauon respectively (and their antiparticles). Electroweak theory added , and for the corresponding neutrinos. The quarks add still further components. In order to be four-spinors like the electron and other lepton components, there must be one quark component for every combination of flavor and color, bringing the total to 24 (3 for charged leptons, 3 for neutrinos, and 2·3·3 = 18 for quarks). Each of these is a four component Dirac spinor, for a total of 96 complex-valued components for the fermion field.
An important definition is the barred fermion field , which is defined to be , where denotes the Hermitian adjoint of , and is the zeroth gamma matrix. If is thought of as an matrix then should be thought of as a matrix.
A chiral theory An independent decomposition of is that into chirality components:
where is the fifth gamma matrix. This is very important in the Standard Model because left and right chirality components are treated differently by the gauge interactions.
In particular, under weak isospin SU(2) transformations the left-handed particles are weak-isospin doublets, whereas the right-handed are singlets – i.e. the weak isospin of is zero. Put more simply, the weak interaction could rotate e.g. a left-handed electron into a left-handed neutrino (with emission of a ), but could not do so with the same right-handed particles. As an aside, the right-handed neutrino originally did not exist in the standard model – but the discovery of neutrino oscillation implies that neutrinos must have mass, and since chirality can change during the propagation of a massive particle, right-handed neutrinos must exist in reality. This does not however change the (experimentally demonstrated) chiral nature of the weak interaction.
Furthermore, acts differently on and (because they have different weak hypercharges).
Mass and interaction eigenstates A distinction can thus be made between, for example, the mass and interaction eigenstates of the neutrino. The former is the state that propagates in free space, whereas the latter is the different state that participates in interactions. Which is the "fundamental" particle? For the neutrino, it is conventional to define the "flavor" (, , or ) by the interaction eigenstate, whereas for the quarks we define the flavor (up, down, etc.) by the mass state. We can switch between these states using the CKM matrix for the quarks, or the PMNS matrix for the neutrinos (the charged leptons on the other hand are eigenstates of both mass and flavor).
As an aside, if a complex phase term exists within either of these matrices, it will give rise to direct CP violation, which could explain the dominance of matter over antimatter in our current universe. This has been proven for the CKM matrix, and is expected for the PMNS matrix.
Positive and negative energies Finally, the quantum fields are sometimes decomposed into "positive" and "negative" energy parts: . This is not so common when a quantum field theory has been set up, but often features prominently in the process of quantizing a field theory.
Bosons
Due to the Higgs mechanism, the electroweak boson fields , , , and "mix" to create the states that are physically observable. To retain gauge invariance, the underlying fields must be massless, but the observable states can gain masses in the process. These states are:
The massive neutral (Z) boson:
The massless neutral boson:
The massive charged W bosons:
where is the Weinberg angle.
The field is the photon, which corresponds classically to the well-known electromagnetic four-potential – i.e. the electric and magnetic fields. The field actually contributes in every process the photon does, but due to its large mass, the contribution is usually negligible.
Perturbative QFT and the interaction picture Much of the qualitative descriptions of the Standard Model in terms of "particles" and "forces" comes from the perturbative quantum field theory view of the model. In this, the Lagrangian is decomposed as into separate free field and interaction Lagrangians. The free fields care for particles in isolation, whereas processes involving several particles arise through interactions. The idea is that the state vector should only change when particles interact, meaning a free particle is one whose quantum state is constant. This corresponds to the interaction picture in quantum mechanics.
In the more common Schrödinger picture, even the states of free particles change over time: typically the phase changes at a rate that depends on their energy. In the alternative Heisenberg picture, state vectors are kept constant, at the price of having the operators (in particular the observables) be time-dependent. The interaction picture constitutes an intermediate between the two, where some time dependence is placed in the operators (the quantum fields) and some in the state vector. In QFT, the former is called the free field part of the model, and the latter is called the interaction part. The free field model can be solved exactly, and then the solutions to the full model can be expressed as perturbations of the free field solutions, for example using the Dyson series.
It should be observed that the decomposition into free fields and interactions is in principle arbitrary. For example, renormalization in QED modifies the mass of the free field electron to match that of a physical electron (with an electromagnetic field), and will in doing so add a term to the free field Lagrangian which must be cancelled by a counterterm in the interaction Lagrangian, that then shows up as a two-line vertex in the Feynman diagrams. This is also how the Higgs field is thought to give particles mass: the part of the interaction term that corresponds to the nonzero vacuum expectation value of the Higgs field is moved from the interaction to the free field Lagrangian, where it looks just like a mass term having nothing to do with the Higgs field.
Free fields Under the usual free/interaction decomposition, which is suitable for low energies, the free fields obey the following equations: The fermion field satisfies the Dirac equation; for each type of fermion. The photon field satisfies the wave equation . The Higgs field satisfies the Klein–Gordon equation. The weak interaction fields satisfy the Proca equation. These equations can be solved exactly. One usually does so by considering first solutions that are periodic with some period along each spatial axis; later taking the limit: will lift this periodicity restriction.
In the periodic case, the solution for a field (any of the above) can be expressed as a Fourier series of the form
where: is a normalization factor; for the fermion field it is , where is the volume of the fundamental cell considered; for the photon field it is . The sum over is over all momenta consistent with the period , i.e., over all vectors where are integers. The sum over covers other degrees of freedom specific for the field, such as polarization or spin; it usually comes out as a sum from to or from to . is the relativistic energy for a momentum quantum of the field, when the rest mass is . and are annihilation and creation operators respectively for "a-particles" and "b-particles" respectively of momentum ; "b-particles" are the antiparticles of "a-particles". Different fields have different "a-" and "b-particles". For some fields, and are the same. and are non-operators that carry the vector or spinor aspects of the field (where relevant). is the four-momentum for a quantum with momentum . denotes an inner product of four-vectors. In the limit , the sum would turn into an integral with help from the hidden inside . The numeric value of also depends on the normalization chosen for and .
Technically, is the Hermitian adjoint of the operator in the inner product space of ket vectors. The identification of and as creation and annihilation operators comes from comparing conserved quantities for a state before and after one of these have acted upon it. can for example be seen to add one particle, because it will add to the eigenvalue of the a-particle number operator, and the momentum of that particle ought to be since the eigenvalue of the vector-valued momentum operator increases by that much. For these derivations, one starts out with expressions for the operators in terms of the quantum fields. That the operators with are creation operators and the one without are annihilation operators is a convention, imposed by the sign of the commutation relations postulated for them.
An important step in preparation for calculating in perturbative quantum field theory is to separate the "operator" factors and above from their corresponding vector or spinor factors and . The vertices of Feynman graphs come from the way that and from different factors in the interaction Lagrangian fit together, whereas the edges come from the way that the s and s must be moved around in order to put terms in the Dyson series on normal form.
Interaction terms and the path integral approach The Lagrangian can also be derived without using creation and annihilation operators (the "canonical" formalism) by using a path integral formulation, pioneered by Feynman building on the earlier work of Dirac. Feynman diagrams are pictorial representations of interaction terms. A quick derivation is indeed presented at the article on Feynman diagrams.
Lagrangian formalism