Basics of CFT

Date: 2015-10-07; view: 501.

Relation to conformal field theory

The reader who already knows a little about CFT will have recognised the differential equations in Section 4 as being very similar in form to the BPZ equations [33] satisfied by the correlation functions of a _2,1 operator, corresponding to a highest weight representation of the Virasoro algebra with a level 2 null state.

For those readers for whom the above paragraph makes no sense, and in any case to make the argument self-contained, we first introduce the concepts of (boundary) conformal field theory (BCFT). We stress that these are heuristic in nature—they serve only as a guide to formulating the basic principles of CFT which can then be developed into a mathematically consistent theory. For a longer introduction to BCFT see [34] and, for a complete account of CFT [35].

We have at the back of our minds a euclidean field theory defined as a path integral over some set of fundamental fields {ψ (r)}. The partition function is Z = ∫ e^−S({ψ})[dψ] where the action is an integral over a local lagrangian density. These fields may be thought of as smeared-out continuum versions of the lattice degrees of freedom. As in any field theory, this continuum limit involves renormalisation. There are so-called local scaling operators which are particular functionals of the fundamental degrees of freedom, which have the property that we can define renormalised scaling operators whose correlators are finite in the continuum limit a → 0, that is

(41)

exists. The numbers x_j are called the scaling dimensions, and are related to the various critical exponents. They are related to the conformal weights by ; the difference is called the spin of _j, and describes its behaviour under rotations. There are also boundary operators, localised on the boundary, which have only a single conformal weight equal to their scaling dimension.

The theory is developed independently of any particular set of fundamental fields or lagrangian. An important role in this is played by the stress tensor T^μν (r), defined as the local response of the action to a change in the metric:

(42)

Invariance under local rotations and scale transformations usually implies that T^μν is symmetric and traceless: . This also implies invariance under conformal transformations, corresponding to δg_μν f (r)g_μν.

In two-dimensional flat space, infinitesimal coordinate transformations r^μ→r^′μ=r^μ+α^μ(r) correspond to infinitesimal transformations of the metric with δg^μν = −(∂^μα^μ + ∂^να^ν). It is important to realise that under these transformations the underlying lattice, or UV cut-off, is not transformed. Otherwise they would amount to a trivial reparametrisation. For a conformal transformation, α^μ (r) is given by an analytic function: in complex coordinates , , so α ^z ≡ α (z) is holomorphic. However, such a function cannot be small everywhere (unless it is constant), so it is necessary to consider coordinate transformations which are not everywhere conformal.

Consider therefore two concentric semicircles Γ₁ and Γ₂ in the upper half plane, centred on the origin, and of radii R₁ < R₂. For |r| < R₁ let α^μ be conformal, with α^z = α (z), while for |r| > R₂ take α^μ = 0. In between, α^μ is not conformal, but is differentiable, so that . This can be integrated by parts to give a term (which must vanish because α_ν is arbitrary in this region, implying that ∂_μT^μν = 0) and two surface terms. That on Γ₂ vanishes because α^μ = 0 there. We are left with

(43)

where dℓ^λ is the line element along Γ₁.

The fact that T^μν is conserved means, in complex coordinates, that , so that the correlations functions of T (z) ≡ T_zz are holomorphic functions of z, while those of are antiholomorphic. Eq. (43) may then be written

(44)

In any field theory with a boundary, it is necessary to impose some boundary condition. It can be argued that any translationally invariant boundary condition flows under the RG to conditions satisfying T_xy = 0, which in complex coordinates means that on the real axis. This means that the correlators of are those of T analytically continued into the lower half plane. The second term in (44) may then be dropped if the contour in the first term is around a complete circle.

The conclusion of all this is that the effect of an infinitesimal conformal transformation on any correlator of observables inside Γ₁ is the same as inserting a contour integral ∫ T (z)α (z) dz/2πi into the correlator.

Another important element of CFT is the operator product expansion (OPE) of the stress tensor with other local operators. Since T is holomorphic, this has the form

(45)

where the ⁽ⁿ⁾ are (possibly new) local operators. By taking α (z) z (corresponding to a scale transformation) we see that ⁽⁰⁾ = h , where h is its scaling dimension. Similarly, by taking α = const., ⁽⁻¹⁾ = ∂_x . Local operators for which ⁽ⁿ⁾ vanishes for n 1 are called primary. T itself is not primary: its OPE with itself takes the form

T(z)·T(0)=c/2z4+(2/z2)T(0)+(1/z)∂zT(0)+ ,

(46)

where c is the conformal anomaly number, ubiquitous in CFT. For example, the partition function on a long cylinder of length L and circumference ℓ behaves as exp(πcL/ℓ), cf. (10).