2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/47882We discuss collective effects like percolation, quark gluon plasma, or string fusion and their effect on the longitudinal development of high energy showers in air. It is shown that iron-air showers could develop more slowly than expected and produce the observed change in the slope of $X_max$ at the highest energies.11 pages, 3 figuresHigh Energy Physics - PhenomenologyAstrophysicstext