This work introduces a minimal and falsifiable experimental test for additional contributions to attosecond time delays. Starting from a general phase-based formulation, it is shown that any weak perturbation of the propagation constant leads, at leading order, to a linear contribution of the group delay with respect to the propagation length. This defines a simple observable characterized by a single parameter, directly accessible experimentally. The proposed test relies on measuring the dependence of group delay on multilayer thickness in XUV propagation. A differential measurement strategy between distinct multilayer systems is introduced to mitigate dominant systematic effects such as dispersion, absorption, and interface roughness. The analysis remains fully model-independent and does not rely on any specific microscopic mechanism. Instead, it provides a direct and experimentally implementable constraint on weak additional contributions that are only weakly dependent on material properties. An illustrative analysis of existing photoemission data is included, showing no evidence for a universal residual within current uncertainties, and motivating the need for a dedicated propagation-based measurement. This work defines a compact experimental protocol that can be directly tested with current attosecond spectroscopy techniques, providing a clear falsifiable framework for precision measurements in ultrafast physics.
Noel COPINET (Fri,) studied this question.