Energy Evolution of the Shower Age Parameter and Morphological Screening of Ultra-High-Energy Neutrino-Like Candidates in Inclined Extensive Air Showers

The identification of rare deeply developing extensive air showers is an important objective in astroparticle physics, particularly in searches for ultra-high-energy neutrinos. In this work, 2056 inclined extensive air shower (EAS) events recorded by the Pierre Auger Observatory within the zenith-angle range of [Formula: see text]–[Formula: see text] are investigated using a multi-parametric morphological identification approach. The shower properties are reconstructed using the Nishimura–Kamata–Greisen (NKG) lateral distribution function, from which the shower age parameter ([Formula: see text]) and electron shower size ([Formula: see text]) are inferred. Additional observables, including the analytically reconstructed [Formula: see text] proxy, footprint geometry ([Formula: see text]), and triggered station multiplicity, are combined to characterize shower development. The analysis reveals an energy-dependent decrease of the shower age parameter, with the mean value decreasing from [Formula: see text] to [Formula: see text] above [Formula: see text] eV. Applying the criterion [Formula: see text] selects 32 young shower events and yields a final sample of 9 deeply developing shower candidates.A Monte Carlo randomization test with 100000 realizations shows that the selected candidates form an unusual morphological subset, with a Gaussian-equivalent significance of [Formula: see text] ([Formula: see text]). These results demonstrate that combining shower-development and geometrical observables provides a promising framework for identifying statistically unusual deeply developing air showers. No direct neutrino identification is claimed; the selected events are interpreted as neutrino-like candidates requiring further validation through dedicated simulations.