Unstable magnetic fluxes in heterotic string theory

We study exact string backgrounds representing a constant magnetic fieldbackground in heterotic string theory. These backgrounds are obtained byKaluza-Klein reduction of a special class of plane wave solutions. For smallvalues of the magnetic field they possess localized closed string tachyonsanalogous to the Nielsen-Olesen instability of a constant magnetic field inSO(3) Yang-Mills theory. When the magnetic field is embedded in the SO(32)gauge group of the heterotic string it is possible to study the lowest leveltachyon in supergravity. We identify the closed string tachyons as fluctuationsof the supergravity fields about the background. We argue that the tachyonsignals the decay of the background to flat space. Our evidence rests on thestudy of the closed string tachyon potential, world sheet renormalization groupequations in the supergravity approximation and the S-dual of this system intype I theory. In the S-dual description, the closed string tachyons inheterotic string theory correspond to open string tachyons in type I theory.Finally we analyze the non-perturbative stability of these models representingconstant magnetic field backgrounds and show that it is stable to pairproduction of particles.Comment: Reference added, corrected some typo