Scalar field theory
In theoretical physics, scalar field theory can refer to a classical or quantum theory of scalar fields. A field which is invariant under any Lorentz transformation is called a "scalar", in contrast to a vector or tensor field. The quanta of the quantized scalar field are spin-zero particles, and as such are bosons.
The only fundamental scalar field that has been observed in nature is the Higgs field. However, scalar fields appear in the effective field theory descriptions of many physical phenomena. An example is the pion, which is actually a "pseudoscalar", which means it is not invariant under parity transformations which invert the spatial directions, distinguishing it from a true scalar, which is parity-invariant. Because of the relative simplicity of the mathematics involved, scalar fields are often the first field introduced to a student of classical or quantum field theory.
In this article, the repeated index notation indicates the Einstein summation convention for summation over repeated indices. The theories described are defined in flat, D-dimensional Minkowski space, with (D-1) spatial dimension and one time dimension and are, by construction, relativistically covariant. The Minkowski space metric, , has a particularly simple form: it is diagonal, and here we use the + − − − sign convention.
- 1 Classical scalar field theory
- 2 Quantum scalar field theory
- 3 See also
- 4 References
- 5 Further reading
- 6 External links
Classical scalar field theory
Linear (free) theory
The most basic scalar field theory is the linear theory. It represents the normal modes of an infinity of coupled oscillators (see phonons). The action for the free relativistic scalar field theory is
This is an example of a quadratic action, since each of the terms is quadratic in the field, φ. The term proportional to m2 is sometimes known as a mass term, due to its subsequent interpretation, in the quantized version of this theory, in terms of particle mass.
The equation of motion for this theory is obtained by extremizing the action above. It takes the following form, linear in φ,
Nonlinear (interacting) theory
The most common generalization of the linear theory above is to add a scalar potential V(φ) to the equations of motion, where typically, V is a polynomial in φ of order 3 or more (often a monomial). Such a theory is sometimes said to be interacting, because the Euler-Lagrange equation is now nonlinear, implying a self-interaction. The action for the most general such theory is
The n! factors in the expansion are introduced because they are useful in the Feynman diagram expansion of the quantum theory, as described below.
The corresponding Euler-Lagrange equation of motion is now
Dimensional analysis and scaling
Physical quantities in these scalar field theories may have dimensions of length, time or mass, or some combination of the three.
However, in a relativistic theory, any quantity t, with dimensions of time, can be readily converted into a length, , by using the velocity of light, c. Similarly, any length l is equivalent to an inverse mass, ħ/mc, using Planck's constant, ħ. In natural units, one thinks of a time as a length, or either time or length as an inverse mass.
In short, one can think of the dimensions of any physical quantity as defined in terms of just one independent dimension, rather than in terms of all three. This is most often termed the mass dimension of the quantity. Knowing the dimensions of each quantity, allows one to uniquely restore conventional dimensions from a natural units expression in terms of this mass dimension, by simply reinserting the requisite powers of ħ and c required for dimensional consistency.
One conceivable objection is that this theory is classical, and therefore it is not obvious how Planck's constant should be a part of the theory at all. If desired, one could indeed recast the theory without mass dimensions at all: However, this would be at the expense of slightly obscuring the connection with the quantum scalar field. Given that one has dimensions of mass, Planck's constant is thought of here as an essentially arbitrary fixed reference quantity of action (not necessarily connected to quantization), hence with dimensions appropriate to convert between mass and inverse length.
The classical scaling dimension, or mass dimension, Δ, of φ describes the transformation of the field under a rescaling of coordinates:
The units of action are the same as the units of ħ, and so the action itself has zero mass dimension. This fixes the scaling dimension of the field φ to be
There is a specific sense in which some scalar field theories are scale-invariant. While the actions above are all constructed to have zero mass dimension, not all actions are invariant under the scaling transformation
The reason that not all actions are invariant is that one usually thinks of the parameters m and as fixed quantities, which are not rescaled under the transformation above. The condition for a scalar field theory to be scale invariant is then quite obvious: all of the parameters appearing in the action should be dimensionless quantities. In other words, a scale invariant theory is one without any fixed length scale (or equivalently, mass scale) in the theory.
For a scalar field theory with D spacetime dimensions, the only dimensionless parameter satisfies . For example, in D=4 only is classically dimensionless, and so the only classically scale-invariant scalar field theory in D=4 is the massless φ4 theory.
Classical scale invariance, however, normally does not imply quantum scale invariance, because of the renormalization group involved−−see the discussion of the beta function below.
is said to be conformal if the transformation satisfies
for some function .
The conformal group contains as subgroups the isometries of the metric (the Poincaré group) and also the scaling transformations (or dilatations) considered above. In fact, the scale-invariant theories in the previous section are also conformally-invariant.
Massive theory illustrates a number of interesting phenomena in scalar field theory.
The Lagrangian density is
Spontaneous symmetry breaking
This Lagrangian has a symmetry under the transformation
If is positive, the potential has a single minimum, at the origin. The solution is clearly invariant under the symmetry. Conversely, if is negative, then one can readily see that the potential has two minima. This is known as a double well potential, and the lowest energy states (known as the vacua, in quantum field theoretical language) in such a theory are not invariant under the symmetry of the action (in fact it maps each of the two vacua into the other). In this case, the symmetry is said to be spontaneously broken.
The theory with a negative also has a kink solution, which is a canonical example of a soliton. Such a solution is of the form
where x is one of the spatial variables (φ is taken to be independent of t, and the remaining spatial variables). The solution interpolates between the two different vacua of the double well potential. It is not possible to deform the kink into a constant solution without passing through a solution of infinite energy, and for this reason the kink is said to be stable. For D>2, i.e. theories with more than one spatial dimension, this solution is called a domain wall.
Another well-known example of a scalar field theory with kink solutions is the sine-Gordon theory.
Complex scalar field theory
In a complex scalar field theory, the scalar field takes values in the complex numbers, rather than the real numbers. The action considered normally takes the form
This has a U(1), equivalently O(2) symmetry, whose action on the space of fields rotates , for some real phase angle α.
As for the real scalar field, spontaneous symmetry breaking is found if m2 is negative. This gives rise to Goldstone's Mexican hat potential which is a rotation of the double-well potential of a real scalar field by 2π radians about the V axis. The symmetry breaking takes place in one higher dimension, i.e. the choice of vacuum breaks a continuous U(1) symmetry instead of a discrete one. The two components of the scalar field are reconfigured as a massive mode and a massless Goldstone boson.
One can express the complex scalar field theory in terms of two real fields, and which transform in the vector representation of the internal symmetry. Although such fields transform as a vector under the internal symmetry, they are still Lorentz scalars. This can be generalised to a theory of N scalar fields transforming in the vector representation of the O(N) symmetry. The Lagrangian for an O(N)-invariant scalar field theory is typically of the form
using an appropriate -invariant inner product.
Quantum scalar field theory
In quantum field theory, the fields, and all observables constructed from them, are replaced by quantum operators on a Hilbert space. This Hilbert space is built on a vacuum state, and dynamics are governed by a quantum Hamiltonian, a positive-semidefinote operator which annihilates the vacuum. A construction of a quantum scalar field theory is detailed in the canonical quantization article, which relies on canonical commutation relations among the fields. Essentially, the infinity of classical oscillators repackaged in the scalar field as its (decoupled) normal modes, above, are now quantized in the standard manner, so the respective quantum operator field describes an infinity of quantum harmonic oscillators acting on a respective Fock space.
In brief, the basic variables are the quantum field φ and its canonical momentum π. Both these operator-valued fields are Hermitian. At spatial points at equal times, their canonical commutation relations are given by
while the free Hamiltonian is, similarly to above,
which resolve to annihilation and creation operators
These operators satisfy the commutation relations
The state annihilated by all of the operators a is identified as the bare vacuum, and a particle with momentum is created by applying to the vacuum. Applying all possible combinations of creation operators to the vacuum constructs the Hilbert space. This construction is called Fock space. The vacuum is annihilated by the Hamiltonian
Interactions can be included by adding an interaction Hamiltonian. For a φ4 theory, this corresponds to adding a Wick ordered term g:φ4:/4! to the Hamiltonian, and integrating over x. Scattering amplitudes may be calculated from this Hamiltonian in the interaction picture. These are constructed in perturbation theory by means of the Dyson series, which gives the time-ordered products, or n-particle Green's functions as described in the Dyson series article. The Green's functions may also be obtained from a generating function that is constructed as a solution to the Schwinger-Dyson equation.
Feynman Path Integral
The Feynman diagram expansion may be obtained also from the Feynman path integral formulation. The time ordered vacuum expectation values of polynomials in φ, known as the n-particle Green's functions, are constructed by integrating over all possible fields, normalized by the vacuum expectation value with no external fields,
All of these Green's functions may be obtained by expanding the exponential in J(x)φ(x) in the generating function
Normally, this is applied to the scattering of particles with fixed momenta, in which case, a Fourier transform is useful, giving instead
The standard trick to evaluate this functional integral is to write it as a product of exponential factors, schematically,
The second two exponential factors can be expanded as power series, and the combinatorics of this expansion can be represented graphically. The integral with λ = 0 can be treated as a product of infinitely many elementary Gaussian integrals, and the result may be expressed as a sum of Feynman diagrams, calculated using the following Feynman rules:
- Each field in the n-point Euclidean Green's function is represented by an external line (half-edge) in the graph, and associated with momentum p.
- Each vertex is represented by a factor -g.
- At a given order gk, all diagrams with n external lines and k vertices are constructed such that the momenta flowing into each vertex is zero. Each internal line is represented by a propagator 1/(q2 + m2), where q is the momentum flowing through that line.
- Any unconstrained momenta are integrated over all values.
- The result is divided by a symmetry factor, which is the number of ways the lines and vertices of the graph can be rearranged without changing its connectivity.
- Do not include graphs containing "vacuum bubbles", connected subgraphs with no external lines.
The last rule takes into account the effect of dividing by . The Minkowski-space Feynman rules are similar, except that each vertex is represented by -ig, while each internal line is represented by a propagator i/(q2-m2 + iε), where the ε term represents the small Wick rotation needed to make the Minkowski-space Gaussian integral converge.
The integrals over unconstrained momenta, called "loop integrals", in the Feynman graphs typically diverge. This is normally handled by renormalization, which is a procedure of adding divergent counter-terms to the Lagrangian in such a way that the diagrams constructed from the original Lagrangian and counter-terms is finite. A renormalization scale must be introduced in the process, and the coupling constant and mass become dependent upon it.
The dependence of a coupling constant g on the scale λ is encoded by a beta function, β(g), defined by
This dependence on the energy scale is known as "the running of the coupling parameter", and theory of this systematic scale-dependence in quantum field theory is described by the renormalization group.
Beta-functions are usually computed in an approximation scheme, most commonly perturbation theory, where one assumes that the coupling constant is small. One can then make an expansion in powers of the coupling parameters and truncate the higher-order terms (also known as higher loop contributions, due to the number of loops in the corresponding Feynman graphs).
The beta-function at one loop (the first perturbative contribution) for the theory is
The fact that the sign in front of the lowest-order term is positive suggests that the coupling constant increases with energy. If this behavior persists at large couplings, this would indicate the presence of a Landau pole at finite energy, or quantum triviality. The question can only be answered non-perturbatively, since it involves strong coupling.
A quantum field theory is trivial when the running coupling, computed through its beta function, goes to zero when the cutoff is removed. Consequently, the propagator becomes that of a free particle and the field is no longer interacting. Alternatively, the field theory may be interpreted as an effective theory, in which the cutoff is not removed, giving finite interactions but leading to a Landau pole at some energy scale. For a φ4 interaction, Michael Aizenman proved that the theory is indeed trivial for space-time dimension . For the triviality has yet to be proven rigorously, but lattice computations have confirmed this. (See Landau pole for details and references.) This fact is relevant as the Higgs field, for which Quantum triviality bounds are used to set limits on the Higgs mass, based on the new physics must enter at a higher scale (perhaps the Planck scale) to prevent the Landau pole from being reached.
- A general reference for this section is Ramond, Pierre (2001-12-21). Field Theory: A Modern Primer (Second Edition). USA: Westview Press. ISBN 0-201-30450-3.
- See the previous reference, or for more detail, Itzykson, Zuber; Zuber, Jean-Bernard (2006-02-24). Quantum Field Theory. Dover. ISBN 0-07-032071-3.
- Aizenman, M. (1981). "Proof of the Triviality of ϕ4
d Field Theory and Some Mean-Field Features of Ising Models for d>4". Physical Review Letters 47 (1): 1–4. Bibcode:1981PhRvL..47....1A. doi:10.1103/PhysRevLett.47.1.
- Peskin, M and Schroeder, D. ;An Introduction to Quantum Field Theory, Westview Press (1995)
- Weinberg, Steven ; The Quantum Theory of Fields, (3 volumes) Cambridge University Press (1995)
- Srednicki, Mark; Quantum Field Theory, Cambridge University Press (2007)
- Zinn-Justin, Jean ; Quantum Field Theory and Critical Phenomena, Oxford University Press (2002)