X max - X 1 . It is this quantity which we expect the correlate with properties of the first interaction, since it is the distance after initiation which the shower takes to reach maximum. It turns out that the difference in between Sibyll and QGSjet is a consequence of the differing elasticity distributions produced by those models.
Elasticity is defined as the energy fraction of the most energetic secondary.
Figure 11 shows that the elasticity of the first interaction is highly correlated with . Both models appear to give the same relationship between elasticity and implying that elasticity is the primary factor controlling shower development. It is hence interesting to investigate elasticity further. Figure 12 shows some plots of elasticity for corsib and corqgs proton-air interactions. The two models give similar results.