Inelastic Exponentiation and Classical Gravitational Scattering at One Loop

We calculate the inelastic $2\to 3$ one-loop amplitude for the scattering of two point-like, spinless objects with generic masses involving the additional emission of a single graviton. We focus on the near-forward, or classical, limit. Our results include the leading and subleading orders in the soft-region expansion, which captures all non-analytic contributions in the transferred momentum and in the graviton’s frequency. This allows us to check the first constraint arising from the inelastic exponentiation put forward in Refs. 2107.12891, 2112.07556, 2210.12118 and to calculate the $2\to 3$ one-loop matrix element of the $N$-operator, linked to the $S$-matrix by $S=e^{iN}$, showing that it is real, classical and free of infrared divergences. We discuss how our results feature in the calculation of the $\mathcal{O}(G^3)$ corrections to the asymptotic waveform.