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photoinjectors with $10^{8}\\ \\text{to}\\ 10^{9}$ electrons per bunch, determine the contributions of intrabeam scattering, shot-noise-seeded microbunching instability and cathode emission to the slice energy spread $\\sigma_E$ (few keV) at injector exit and after bunch compression, and predict $\\sigma_E$ within $20\\%$ of measured values at several facilities.","problem_ref":"f7cab580d7739cf1f7bcdec9f6049a651c24e54a9edbf2c7b4e20adfd74cb281","references":"","settled_by":"High-resolution slice energy spread measurements versus bunch charge and laser-heater setting compared with a start-to-end model including intrabeam scattering.","status_note":"Measurements at FEL injectors in the early 2020s attributed much of the excess to intrabeam scattering; quantitative agreement across facilities is not established.","title":"What sets the uncorrelated energy spread of high-brightness electron 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