NONPERTURBATIVE EFFECTS IN MATRIX MODELS AND VACUA OF 2-DIMENSIONAL GRAVITY

The most general large N eigenvalues distribution for the one-matrix model is shown to consist of tree-like structures in the complex plane. For the m = 2 critical point, such a solution describes the strong coupling phase of 2D quantum gravity (c = 0 non-critical string). It is obtained by taking combinations of complex contours in the matrix integral, and the relative weight of the contours is identified with the non-perturbative ''theta-parameter'' that fixes uniquely the solution of the string equation (Painleve 1). This allows to recover by instanton methods results on the non-perturbative effects obtained by the Isomonodromic Deformation Method, and to construct for each theta-vacuum the observables (the loop correlation functions) which satisfy the loop equations. The breakdown of analyticity of the large N solution is related to the existence of poles for the loop operators.