Gomis, J; Mehen, T, *Space-time noncommutative field theories and unitarity*,
Nuclear Physics B, vol. 591 no. 1-2
(2000),
pp. 265-276 .
**Abstract:**

*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. © 2000 Elsevier Science B.V.*