Signals of Phase Transition in Atmospheric Showers

We 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. It is expected that the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC) will reach regions where the energy density will be higher than the critical value of a few GeV/fm 3 predicted by lattice QCD calculations. This critical energy density corresponds to the new phase(s) of deconfinement of the quark gluon plasma or (and) of restoration of chiral symmetry. The energies of RHIC (√ s = 200 AGeV) and LHC (√ =7000 AGeV) correspond, for Ag–Ag collisions, to laboratory energies slightly higher than 10 15 and 10 18 eV, which are in the range of typical very high energy cosmic rays. Unfortunately in cosmic rays projectile and target are not heavy nuclei. The target is always Air and the heaviest projectile is Iron. Nevertheless, it would be worth to investigate whether cosmic ray collisions in the atmosphere can produce an hadronic phase transition and which are the main consequences related to the development of cosmic ray cascades. Recently, it has been pointed out that the confined to deconfined phase transition could take place as a percolation of the strings formed in a nucleus-nucleus collision [1,2]. For a given collision and a given degree of the centrality of the collision there is a defined available total area in the impact parameter space. In this space, the strings exchanged between the partons of projectile and target are seen as circles of radius r inside the area. As the energy 1