A Spin on Radiation Reaction with Cuts

Abstract

We derive the radiation-reaction equations of motion for a spinning charged body, the $\sqrt{\text{Kerr}}$ particle, through quadratic order in its spin, generalising the Abraham--Lorentz--Dirac equation. Extending an argument of Coleman, we obtain radiation reaction equations by evaluating the self-field of the spinning body on its own worldline, and removing its divergences with dimensional regularisation. We use the resulting equations of motion to compute the electromagnetic waveform emitted in the scattering of the spinning body off a spectator charge, both at tree level and including the leading radiation-reaction correction, which enters at one loop. Then, we apply the construction introduced in a previous work by some of the authors -- where radiation reaction is encoded in unitarity cuts involving Compton amplitudes -- to spinning particles, and recompute the radiation-reaction contribution to the one-loop waveform. We find that the waveform calculated using classical and amplitude methods agree through quadratic order in spin, providing a check on our radiation-reaction equations. Along the way, we also reproduce known results for linear-in-spin self-force effects and for the spinless self-force in six dimensions.