Cascade Ages, Similarity, Universality, and All That

A large number of particles continuously multiplying and changing their types form a complex stochastic process. Knowing the conservation laws and understanding the hierarchy of events allows to formulate the main characteristics of the process in in terms of collective variables. The most important of them is the age parameter s, which relates the penetration depth of particles to the energy of the primary particle initiating the shower. The concept of the “age” of a shower has been in use in the cosmic ray community for more than half a century.