Experimental validation of electron correlation models in warm dense matter

We report X-ray Thomson scattering measurements of warm dense aluminium at densities 3. 75-4. 5 g/cm³ and a temperature of approximately 0. 6 eV, performed at the HED-HiBEF instrument of the European XFEL using the DiPOLE-100X drive laser. By probing plasmon dispersion across momentum transfers k = 0. 99-2. 57 Angstrom^-1 with high statistical fidelity, we directly test competing theories of electron dynamics under extreme conditions. Time-dependent density functional theory (TDDFT) reproduces both the observed plasmon energies and spectral shapes across the full k range, whereas the random phase approximation (RPA) and static local-field-correction (LFC) models systematically overestimate the plasmon frequency, even for aluminium (a canonical uniform electron gas metal). Considering electron localisation around ions and the loss of crystalline symmetry due to liquid-state disorder, our measurements provide direct evidence that simple uniform electron gas models fail in warm dense matter and establish TDDFT as a reliable approach for electronic correlations in this regime.