Galactic Positrons as a Probe of Large Extra Dimensions

In the last decade, there has been increased interest in the possibility that the Universe contains large additional dimensions. In this article it is proposed that the Kaluza-Klein modes which are predicted to exist in such models will be trapped in the galactic halo, and that the subsequent decays of these modes in the present will generate an observable flux of positrons and $\gamma$-rays from the galactic core. In particular, by restricting the rate of production 511 keV photons to be below that observed by the INTEGRAL satellite, a new set of constraints are imposed on the ADD model.