An experimental object or measurement is fiducial if it corresponds kinematically to the phase space that is accessible to the experiment performing the measurement. In other words, it falls into a region where the experiment is trusted. The word fiducial in this sense roughly means "based on having trust" (in the detector).
The fiducial definition, also called fiducial region or fiducial phase space, is a phase space at particle level (or parton level) that is designed to correspond reasonably closely to what is experimentally accessible by the detector and reconstruction/identification algorithms. The exact definition of this phase space is somewhat arbitrary, but in any case it is designed to define the measurement in a maximally model-independent way, so that theory and experiment can be compared without needing to rely on extrapolation beyond the experimental acceptance.
Fiducial measurements have the advantage of being more model-independent than extrapolating ones.
Comparing fiducial measurements to theoretical predictions requires the ability to apply the fiducial definition to the theoretical calculation/simulation. If events can be generated based on the calculation, the fiducial definition can be applied for instance in the Rivet framework.
- Fiducial on Wiktionary, the free dictionary. Retrieved 28 November 2016.