Odd Tachyons in Compact Extra Dimensions

We consider a real scalar field with an arbitrary negative bulk mass term in a general 5D setup, where the extra spatial coordinate is a warped interval of size $\pi R$. When the 5D field verifies Neumann conditions at the boundaries of the interval, the setup will always contain at least one tachyonic KK mode. On the other hand, when the 5D scalar verifies Dirichlet conditions, there is always a critical (negative) mass $M_{c}^2$ such that the Dirichlet scalar is stable as long as its (negative) bulk mass $\mu^2$ verifies $M^2_{c}<\mu^2$. Also, if we fix the bulk mass $\mu^2$ to a sufficiently negative value, there will always be a critical interval distance $\pi R_c$ such that the setup is unstable for $R>R_c$. We point out that the best mass (or distance) bound is obtained for the Dirichlet BC case, which can be interpreted as the generalization of the Breitenlohner-Freedman (BF) bound applied to a general compact 5D warped spacetime. In particular, in a slice of $AdS_5$ the critical mass is $M^2_{c}=-4k^2 -1/R^2$ and the critical interval distance is given by $1/R_c^2=|\mu^2|-4k^2$, where $k$ is the $AdS_5$ curvature (the 5D flat case can be obtained in the limit $k\to 0$, whereas the infinite $AdS_5$ result is recovered in the limit $R\to \infty$).