  The precise and efficient identification of the nature of the primary cosmic
rays on an event-by-event basis stands as a fundamental aspiration for any
cosmic ray observatory. In particular, the detection and characterization of
gamma ray events are challenged by their occurrence within an overwhelmingly
greater flux of charged cosmic rays spanning several orders of magnitude. The
intricacies of distinguishing between cosmic ray compositions and the inherent
uncertainties associated with hadronic interactions present formidable
challenges, which, if not properly addressed, can introduce significant sources
of systematic errors. This work introduces a novel composition discriminant
variable, $P_{tail}^{\alpha}$, which quantifies the number of Water Cherenkov
Detectors with a signal well above the mean signal observed in WCDs located at
an equivalent distance from the shower core, in events with approximately the
same energy at the ground. This new event variable is then shown to be, in the
reconstructed energy range $10\,{\rm TeV}$ to $1.6\,{\rm PeV}$, well correlated
with the total number of muons that hit, in the same event, all the observatory
stations located at a distance greater than $200\,{\rm m}$ from the shower
core. The two variables should thus have similar efficiencies in the selection
of high-purity gamma event samples and in the determination of the nature of
charged cosmic ray events.
