  Wide field-of-view gamma-ray observatories must fight the overwhelming cosmic
ray background to identify very-high-energy astrophysical gamma-ray events.
This work introduces a novel gamma/hadron discriminating variable, $LCm$, which
quantifies the azimuthal non-uniformity of the particle distributions at the
ground. This non-uniformity, due to the presence of hadronic sub-showers, is
higher in proton-induced showers than in gamma showers. The discrimination
power of this new variable is then discussed, as a function of the air shower
array fill factor, in the energy range $10\,$TeV to $1\,$PeV, and compared to
the classical gamma/hadron discriminator based on the measurement of the number
of muons at the ground. The results obtained are extremely encouraging, paving
the way for the use of the proposed quantity in present and future large
ground-array gamma-ray observatories.
