We define canonical bases of the higher-level q-deformed Fock space modules of the affine Lie algebra sl n generalizing the result of Leclerc and Thibon for the case of level 1. We express the transition matrices between the canonical bases and the natural bases of the Fock spaces in terms of certain affine Kazhdan-Lusztig polynomials.

We consider a special correlation function in the isotropic spin-1 2 Heisenberg antiferromagnet. It is the probability of finding a ferromagnetic string of (adjacent) spins in the antiferromagnetic ground state. We give two different representations for this correlation function. Both of them are exact at any distance, but one becomes more effective for the description of long distance behaviour, the other for the description of short distance behaviour.

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