Transplanckian bremsstrahlung and black hole production

Classical gravitational bremsstrahlung in particle collisions at transplanckian energies is studied in ${\mathcal M}_4\times {\mathcal T}^d$. The radiation efficiency $ε\equiv E_{\rm rad}/E_{\rm initial}$ is computed in terms of the Schwarzschild radius $r_S(\sqrt{s})$, the impact parameter $b$ and the Lorentz factor $γ_{\rm cm}$ and found to be $ε=C_d (r_S/b)^{3d+3} γ_{\rm cm}^{2d+1}$, larger than previous estimates by many powers of $γ_{\rm cm}\gg 1$. The result is reliable for impact parameters in the overlap of $r_S<b<b_c {\rm and} b>λ_C$, with $b_c$ marking (for $d\neq 0$) the loss of the notion of classical trajectories and $λ_C\equiv \hbar/mc$ the Compton length of the scattered particles. The condition on $s$ and $m$ for extreme radiation damping and (presumably) no black hole production is also derived.

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