Gravitational bremsstrahlung in ultra-planckian collisions

Abstract

Abstract A classical computation of gravitational bremsstrahlung in ultra-planckian collisions of massive point particles is presented in an arbitrary number d of toroidal or non-compact extra dimensions. Our method generalizes the post-linear formalism of General Relativity to the multidimensional case. The total emitted energy, as well as its angular and frequency distribution are discussed in detail. In terms of the gravitational radius r S of the collision energy, the impact parameter b and the Lorentz factor in the CM frame, the leading order radiation efficiency in the Lab frame is shown to be ϵ ~ (r S /b)3(d+1)γcm for d = 0, 1 and ϵ ~ $ {{\left( {{{{{r_S}}} \left/ {b} \right.}} \right)}^{{3\left( {d+1} \right)}}}\gamma_{\mathrm{cm}}^{2d-3 } $ for d ≥ 2, up to a known d-dependent coefficient and a ln γcm factor for d = 2, while the characteristic frequency of the radiation is ω ~ γ/b. The contribution of the low frequency part (ω < 1/b) of the radiation 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. Finally, it is shown that within the region of validity of our approach the efficiency can obtain unnatural values greater than one, which is interpreted to mean that the peripheral ultra-planckian collisions should be strongly radiation damped.