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The AIRES system for air shower simulations: An Update

A report on the characteristics of ultra-high energy air showers simulated with the current version of the AIRES program is presented. The AIRES system includes a fast simulating program, originally designed on the basis of the well-known MOCCA program, and progressively improved and tested. The AIRES algorithms are briefly described and some results coming from the simulations are analyzed. When an ultra high energy astroparticle interacts with an atom of the Earth's atmosphere, it produces a shower of sec- ondary particles that continue interacting and generating mo- re secondary particles that can eventually hit the Earth's sur- face. The study of the characteristics of such air showers initiated by ultra high energy cosmic rays is of central im- portance. This is due to the fact that presently such primary particles cannot be detected directly; instead, they must be studied from different measurements of the air showers they produce. Due to the complexity of the processes that take place dur- ing the development of an air shower, detailed studies of its characteristics are commonly made with the help of numer- ical simulations. The simulating algorithms must take into account all the processes that significantly affect the behav- ior of the shower. This includes electrodynamic interactions, hadronic collisions, photonuclear processes, particle decays, scattering, etc. Among all those processes, the hadronic and photonuclear reactions are, at present, the less understood. In the case of air showers initiated by ultra-high energy astroparticles (E 10 19 eV), the primary particles have energies that are several orders of magnitude larger than the maximum ener- gies attainable in experimental colliders. This means that the models used to rule the behavior of such energetic particles