Improving composition of ultrahigh energy cosmic rays with ground detector data

We show that the maximum shower depth (Xmax) distributions of ultrahigh energy cosmic rays (UHECRs), as measured by fluorescence telescopes, can be augmented by building a mapping to observables collected by surface detectors. The resulting statistical improvement of such an augmented dataset depends in a universal way on the strength of the correlation exhibited by the mapping. Building upon the publicly available data on "golden hybrid" events from the Pierre Auger Observatory, we project possible improvements in the inferred composition of UHECRs for a range of possible mappings with varying correlation strengths.