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Determine which mechanism (field-induced broadening of the quasiparticle density of states with a nonequilibrium quasiparticle distribution, reduced two-level-system losses, or other) reproduces $R_{\\mathrm{BCS}}(H, f, T, \\text{mean free path})$ quantitatively.","problem_ref":null,"references":"","settled_by":"A single model that fits $R_{\\mathrm{BCS}}(H, f, T)$ for cavities at three or more frequencies with independently measured mean free path and no per-cavity fit parameters.","status_note":"","title":"Origin of the anti-Q-slope in nitrogen-doped 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