Effect of electron-beam energy chirp on signatures of radiation reaction in laser-based experiments

Current experiments investigating radiation reaction employ high energy electron beams together with tightly focused laser pulses in order to reach the quantum regime, as expressed through the quantum nonlinearity parameter χ. Such experiments are often complicated by the large number of latent variables, including the precise structure of the electron bunch. Here we examine a correlation between the electron spatial and energy distributions, called an energy chirp, investigate its significance to the laser-electron beam interaction and show that the resulting effect cannot be trivially ignored when analyzing current experiments. In particular, we show that the energy chirp has a large effect on the second central moment of the electron energy, but a lesser impact on the first electron energy moment or the photon critical energy. These results show the importance of improved characterization and control over electron bunch parameters on a shot-to-shot basis in such experiments.