Transplanckian bremsstrahlung and black hole production

Classical gravitational bremsstrahlung in particle collisions at transplanckian energies is studied in M4×Td. The radiation efficiency ϵ≡Erad/Einitial is computed in terms of the Schwarzschild radius rS(s), the impact parameter b and the Lorentz boost factor γcm and found to be ϵ=Cd(rS/b)3d+3γcm2d+1, larger than previous estimates by several powers of γcm≫1. This means that in the ultrarelativistic case radiation loss becomes significant already for b≫rS, so radiation damping must be taken into account in estimates of black hole production at transplanckian energies. The result is reliable for impact parameters in the overlap of γνrS