  The rigorous diagnostics of experiments with warm dense matter (WDM) is
notoriously difficult. A key method is given by X-ray Thomson scattering
(XRTS), but the interpretation of XRTS measurements is usually based on
theoretical models that entail various approximations. Recently, Dornheim et
al. [arXiv:2206.12805] have introduced a new framework for temperature
diagnostics of XRTS experiments that is based on imaginary-time correlation
functions (ITCF). On the one hand, switching from the frequency- to the
imaginary-time domain gives one direct access to a number of physical
properties, which facilitates the extraction of the temperature of arbitrarily
complex materials without any models or approximations. On the other hand, the
bulk of theoretical works in dynamic quantum many-body theory is devoted to the
frequency-domain, and, to our knowledge, the manifestation of physics
properties within the ITCF remains poorly understood. In the present work, we
aim to change this unsatisfactory situation by introducing a simple,
semi-analytical model for the imaginary-time dependence of two-body
correlations within the framework of imaginary-time path integrals. As a
practical example, we compare our new model to extensive ab initio path
integral Monte Carlo results for the ITCF of a uniform electron gas, and find
excellent agreement over a broad range of wave numbers, densities, and
temperatures.
