Vector Bremsstrahlung by Ultrarelativistic Collisions in Higher Dimensions

Abstract

A classical computation of vector bremsstrahlung in ultrarelativistic gravitational-force collisions of massive point particles is presented in an arbitrary number d of extra dimensions. Our method adapts the post-linear formalism of General Relativity to the multidimensional case. The total emitted energy, as well as its angular and frequency distribution and characteristic values, are discussed in detail. For an electromagnetic mediation propagated in the bulk, the emitted energy $E_{em}$ of scattering with impact parameter b has magnitude $E_{em} \sim e^4 e'^2 \gamma^{d+2}/(m^2 b^{3d+3})$, with dominant frequency $\omega_{em} \sim \gamma^2/b$. For the gravitational force the charge emits via vector field, propagated in the bulk, energy $E_{rad}\sim[G_D m' e]^2 \gamma^{d+2}/b^{3d+3}$ for $d \geq 2$, with dominant frequency $\omega\sim\gamma^2/b$ and energy $E_{rad}\sim[G_5 m' e]^2\gamma^{3}\ln \gamma/b^{6}$ for $d=1$, with most of the energy coming from a wide frequency region $\omega \in [\gamma/b),\gamma^2/b] $. For the UED model with extra space volume $V=(2\pi R)^d$ the emitted energy is $E_{UED}\sim (b^{d}/V)^2 E_{rad}$. Finally, for the ADD model, including four dimensions, the electromagnetic field living on 3-brane, loses on emission the energy $E_{ADD}\sim[G_D m'e]^2\gamma^{3}/(V b^{2d+3})$, with characteristic frequency $\omega_{ADD}\sim\gamma/b$. The contribution of the low frequency part of the radiation (soft photons) to the total radiated energy is shown to be negligible for all values of d. The domain of validity of the classical result is discussed. The result is analyzed from the viewpoint of the deWitt - Brehme - Hobbs equation (and corresponding equations in higher dimensions).