Electron-ion temperature relaxation rate in warm dense matter
In plain words
When electrons are heated first, as by a laser, they take time to share energy with the heavy ions. In warm dense matter different theories disagree by large factors on how long that takes.
Precise statement
For hydrogen and carbon at density 1 to 100 $\mathrm{g}/\mathrm{cm}^3$ and temperature 1 to 100 eV, where the ion coupling parameter is near 1 and the electron degeneracy parameter $T/E_{F}$ is near 1, determine the electron-ion energy exchange rate relative to the Landau-Spitzer value. Quantum kinetic theories, coupled-mode treatments and ab initio molecular dynamics differ by up to several times. An answer is a measured rate with 20 percent precision compared with theory.
What would settle it
Time-resolved X-ray scattering measurements of $T_{\mathrm{e}}$ and $T_{\mathrm{i}}$ relaxation in isochorically heated samples, matched by one theory.