Null Vectors in Logarithmic Conformal Field Theory

The representation theory of the Virasoro algebra in the case of a logarithmic conformal field theory is considered. Here, indecomposable representations have to be taken into account, which has many interesting consequences. We study the generalization of null vectors towards the case of indecomposable representation modules and, in particular, how such logarithmic null vectors can be used to derive differential equations for correlation functions. We show that differential equations for correlation functions with logarithmic fields become inhomogeneous. During the last few years, so-called logarithmic conformal field theory (LCFT) established itself as a well-defined new animal in the zoo of conformal field theories in two dimensions [1]. By now, quite a number of applications have been pursued, and sometimes longstanding puzzles in the description of certain theoretical models could be resolved, e.g. the Haldane-Rezzayi state in the fractional quantum Hall effect [2], multifractality, etc. (see [3] for examples). However, the computation of correlation functions within an LCFT still remains difficult, and only in a few cases, four-point functions (or even higher-point functions) could be obtained explicitly. The main reason for this obstruction is that the representation theory of the Virasoro algebra is much more complicated in the LCFT case due to the fact that there exist indecomposable but non-irreducible representations (Jordan cells). This fact has many wide ranging implications. First of all, it is responsible for the appearance of logarithmic singularities in correlation functions. Furthermore, it makes it necessary to generalize almost every notion of (rational) conformal field theory, e.g. characters, highestweight modules, null vectors etc. Null vectors are the perhaps most impor∗Research supported by EU TMR network no. FMRXCT96-0012 and the DFG String network (SPP no. 1096), Fl 259/2-1. tant tool in conformal field theory (CFT) to explicitly calculate correlation functions. In certain CFTs, namely the so-called minimal models, a subset of highest-weight modules possess infinitely many null vectors which, in principle, allow to compute arbitrary correlation functions involving fields only out of this subset. It is well known that global conformal covariance can only fix the twoand three-point functions up to constants. The existence of null vectors makes it possible to find differential equations for higherpoint correlators, incorporating local conformal covariance as well. This paper will pursue the question, how this can be translated to the logarithmic case. For the sake of simplicity, we will concentrate on the case where the indecomposable representations are spanned by rank two Jordan cells with respect to the Virasoro algebra. The abbreviation LCFT will refer to this case. To each such highest-weight Jordan cell {|h; 1〉, |h; 0〉} belong two fields, and ordinary primary field Φh(z), and its logarithmic partner Ψh(z). In particular, one then has L0|h; 1〉 = h|h; 1〉 + |h; 0〉, L0|h; 0〉 = h|h; 0〉. Furthermore, the main scope will lie on the evaluation of four-point functions. 1. SL(2,C) Covariance In ordinary CFT, the four-point function is fixed Non-perturbative Quantum Effects 2000 Michael Flohr by global conformal covariance up to an arbitrary function F (x, x̄) of the harmonic ratio of the four points, x = z12z34 z14z32 with zij = zi − zj. As usual, we consider only the chiral half of the theory, although LCFTs are known not to factorize entirely in chiral and anti-chiral halfs. In LCFT, already the two-point functions behave differently, and the most surprising fact is that the propagator of two primary fields vanishes, 〈Φh(z)Φh′(w)〉 = 0. In particular, the norm of the vacuum, i.e. the expectation value of the identity, is zero. On the other hand, it can be shown [5] that all LCFTs possess a logarithmic field Ψ0(z) of conformal weight h = 0, such that with |0̃〉 = Ψ0(0)|0〉 the scalar product 〈0|0̃〉 = 1. More generally, we have 〈Φh(z)Ψh′(w)〉 = δhh′ A (z − w)h+h , (1.1) 〈Ψh(z)Ψh′(w)〉 = δhh′ − 2A log(z − w) (z − w)h+h , with A,B free constants. In an analogous way, the three-point functions can be obtained up to constants from the Ward-identities generated by the action of L±1 and L0. Note that the action of the Virasoro modes is non-diagonal in the case of an LCFT, Ln〈φ1(z1) . . . φn(zn)〉 = (1.2) ∑ i z [z∂i + (n+ 1)(hi + δhi)] 〈φ1(z1) . . . φn(zn)〉 where φi(zi) is either Φhi(zi) or Ψhi(zi) and the off-diagonal action is δhiΨhj(z) = δijΦhj (z) and δhiΦhj (z) = 0. Therefore, the action of the Virasoro modes yields additional terms with the number of logarithmic fields reduced by one. This action reflects the transformation behavior of a logarithmic field under conformal transformations,