Closed string radiation from moving D-branes

TL;DR

This paper calculates the radiation emitted as massless closed strings from moving D-branes, focusing on graviton and dilaton emissions, within various compactification schemes, revealing no axion emission and energy estimates for large separations.

Contribution

It provides a detailed computation of closed string radiation from moving D-branes, including world sheet propagators and emission amplitudes in different compactifications, which was not previously done.

Findings

No on-shell axion emission detected.

Graviton emission amplitude computed for large inter-brane distances.

Dilaton emission amplitude vanishes in the large separation limit.

Abstract

We compute the amplitude for the radiation of massless NS-NS closed string states from the interaction of two moving D-branes. We consider particle-like D-branes with reference to 4-dimensional spacetime, in toroidal and orbifold compactifications, and we work out the relevant world sheet propagators within the moving boundary state formalism. We find no on-shell axion emission. For large inter-brane separation, we compute the spacetime graviton emission amplitude and estimate the average energy radiated, whereas the spacetime dilaton amplitude is found to vanish in this limit. The possibility of emission of other massless states depends on the nature of the branes and of the compactification scheme.