Very high energy proton-proton cross section

The recent Pierre Auger Observatory result suggesting a coincidence of extensive air showers arrival directions with 'nearby' active galactic nuclei and HiRes discovery of the Greisen-Zatsepin-Kuzmin cutoff indicates protons to be only or at least the strongly dominant component of primary extra galactic cosmic ray flux. However, showers initiated by these ultrahigh energy particles developed faster than predicted by the simulation calculations with conventional interaction models. This could be evidence of the substantial increase of the p-air cross section. The progress in understanding the proton-proton cross section description allows us to examine this possibility, and eventually reject it as an explanation of the ultrahigh energy cosmic ray 'pure proton' controversy.