Abstract:
We describe applications of (perturbed) conformal field theories to two-dimensional disordered systems. We present various methods of study~: (i) {\it A direct method} in which we compute the explicit disorder dependence of the correlation functions for any sample of the disorder. This method seems to be specific to two dimensions. The examples we use are disordered versions of the Abelian and non-Abelian WZW models. We show that the disordered WZW model over the Lie group $\CG$ at level $k$ is equivalent at large impurity density to the product of the WZW model over the coset space $\CG^C/\CG$ at level $(-2h^v)$ times an arbitrary number of copies of the original WZW model. (ii) {\it The supersymmetric method} is introduced using the random bond Ising model and the random Dirac theory as examples. In particular, we show that the relevent algebra is the affine $OSp(2N|2N)$ Lie superalgebra, an algebra with zero superdimension. (iii) {\it The replica method} is introduced using the random phase sine-Gordon model as example. We describe particularities of its renormalization group flow. (iv) {\it A variationnal approach} is also presented using the random phase sine-Gordon model as example. Lectures presented at the '95 Cargese Summer School on "Low dimensional application of quantum field theory".

Abstract:
The efforts to improve on the precision of the measurement and theoretical prediction of the anomalous magnetic moment of the muon a_mu have turned into a test of our understanding of the hadronic contribution to vacuum polarisation. I describe how recent measurements of hadron production in e+e- interactions with initial-state radiation provide precision measurements of the hadron cross section, and have improved on the contribution to the prediction of the value of a_mu that dominates the global uncertainty.

Abstract:
We discuss two possible scenario for the direct cascade in two dimensional turbulent systems in presence of friction which differ by the presence or not of enstrophy dissipation in the inviscid limit.They are distinguished by the existence or not of a constant enstrophy transfer and by the presence of leading anomalous scaling in the velocity three point functions. We also point out that the velocity statistics become gaussian in the approximation consisting in neglecting odd order correlations in front even order ones.

Abstract:
The random vector potential model describes massless fermions coupled to a quenched random gauge field. We study its abelian and non-abelian versions. The abelian version can be completely solved using bosonization. We analyse the non-abelian model using its supersymmetric formulation and show, by a perturbative renormalisation group computation, that it is asymptotically free at large distances. We also show that all the quenched chiral current correlation functions can be computed exactly, without using the replica trick or the supersymmetric formulation, but using an exact expression for the effective action for any sample of the random gauge field. These chiral correlation functions are purely algebraic.

Abstract:
In the first part, we introduce the notion of fractional statistics in the sense of Haldane. We illustrate it on simple models related to anyon physics and to integrable models solvable by the Bethe ansatz. In the second part, we describe the properties of the long-range interacting spin chains. We describe its infinite dimensional symmetry, and we explain how the fractional statistics of its elementary excitations is an echo of this symmetry. In the third part, we review recent results on the Yangian representation theory which emerged from the study of the integrable long-range interacting models.

Abstract:
We review some aspects of the quantum Yangians as symmetry algebras of two-dimensional quantum field theories. The plan of these notes is the following: 1 - The classical Heisenberg model: Non-Abelian symmetries; The generators of the symmetries and the semi-classical Yangians; An alternative presentation of the semi-classical Yangians; Digression on Poisson-Lie groups. 2 - The quantum Heisenberg chain: Non-Abelian symmetries and the quantum Yangians; The transfer matrix and an alternative presentation of the Yangians; Digression on the double Yangians. Talk given at the "Integrable Quantum Field Theories" conference held at Come, Italy , September 13-19, 1992.

Abstract:
We describe few aspects of the quantum symmetries of some massless two-dimensional field theories. We discuss their relations with recent proposals for the factorized scattering theories of the massless $PCM_1$ and $O(3)_{\theta=\pi}$ sigma models. We use these symmetries to propose massless factorized S-matrices for the $su(2)$ sigma models with topological terms at any level, alias the $PCM_k$ models, and for the $su(2)$-coset massless flows.

Abstract:
We review the recently developed relation between the traditional algebraic approach to conformal field theories and the more recent probabilistic approach based on stochastic Loewner evolutions. It is based on implementing random conformal maps in conformal field theories.