NEGATIVE SCREENINGS IN LIOUVILLE THEORY

We demonstrate how negative powers of screenings arise as a non-pertur bative effect within the operator approach to Liouville theory. This e xplains the origin of the corresponding poles in the exact Liouville t hree-point function proposed by Dorn!Otto and (Zamolodchikov)(2) (DOZZ ) and leads to a consistent extension of the operator approach to arbi trary integer numbers of screenings of both types. The general Liouvil le three-point function in this setting is computed without any analyt ic continuation procedure, and found to support the DOZZ conjecture. W e point out the importance of the concept of free-field expansions wit h adjustable monodromies - recently advocated by Petersen, Rasmussen a nd Yu - in the present context, and show that it provides a unifying i nterpretation for two types of previously constructed local observable s.