Magnetic fields, branes and noncommutative geometry

We construct a simple physical model of a particle moving on the infinite noncommutative 2-plane. The model consists of a pair of opposite charges moving in a strong magnetic field. In addition, the charges are connected by a spring. In the limit of large magnetic field, the charges are frozen into the lowest Landau level. Interaction of such particles include Moyal bracket phases characteristics of field theory on noncommutative space. The simple system arises in lightcone quantization of open strings attached to D-branes in a.s. tensor background. We use the model to work out the general form of lightcone vertices from string splitting. We then consider Feynman diagrams in uncompactified NC YM theories and find that for all planar diagrams the comm. and noncomm. theories are the same. This means large N theories are equivalent in the 't Hooft limit. Non planar diagrams convergence is improved.