Space–time noncommutative field theories and unitarity Original Research Article

We study the perturbative unitarity of noncommutative scalar field theories. Field theories with space–time noncommutativity do not have a unitary S-matrix. Field theories with only space noncommutativity are perturbatively unitary. This can be understood from string theory, since space noncommutative field theories describe a low energy limit of string theory in a background magnetic field. On the other hand, there is no regime in which space–time noncommutative field theory is an appropriate description of string theory. Whenever space–time noncommutative field theory becomes relevant massive open string states cannot be neglected.