Branonium

TL;DR

This work introduces branonium, bound states of brane/antibrane systems, by treating a probe antibrane in the field of N source branes. The authors show the probe dynamics reduce to a relativistic central-force problem in the transverse space, enabling explicit solutions and revealing a remarkable case where fully relativistic D6-brane orbits reproduce exact nonrelativistic conic sections, notably ellipses, with no precession. They analyze stability against bending and decay channels, finding bulk radiation to be the dominant decay mechanism, and provide estimates for orbital lifetimes and energy loss, including quantum aspects for D6-branes and the impact of compactification. The results offer a rich framework for brane dynamics with potential cosmological applications, including branonium-like inflation and cyclic scenarios discussed in accompanying work. Overall, the paper demonstrates that brane bound states exhibit tractable classical/quantum structures and distinctive stability properties, with significant implications for brane-world cosmology and string-theoretic bound-state phenomena.

Abstract

We study the bound states of brane/antibrane systems by examining the motion of a probe antibrane moving in the background fields of N source branes. The classical system resembles the point-particle central force problem, and the orbits can be solved by quadrature. Generically the antibrane has orbits which are not closed on themselves. An important special case occurs for some Dp-branes moving in three transverse dimensions, in which case the orbits may be obtained in closed form, giving the standard conic sections but with a nonstandard time evolution along the orbit. Somewhat surprisingly, in this case the resulting elliptical orbits are exact solutions, and do not simply apply in the limit of asymptotically-large separation or non-relativistic velocities. The orbits eventually decay through the radiation of massless modes into the bulk and onto the branes, and we estimate this decay time. Applications of these orbits to cosmology are discussed in a companion paper.