  Observational evidence from Auger and earlier experiments shows a deficit of
signal in a surface detector compared to predictions, which increases as a
function of zenith angle, when the energy of the event is fixed by fluorescence
measurements. We explore three potential explanations for this: the "Cronin
effect" (growth of high-transverse momentum cross sections with nuclear size),
the need for more particles at high transverse momentum in p- p collisions than
currently predicted by high energy hadronic models used for air shower
simulations, and the possibility that secondary interactions in the target air
nucleus produces additional soft pions not included in simulations. We report
here on the differences between Pythia and QGSJet II, especially for high Pt
particles. The possible impact of these effects on the predicted surface array
signal and attenuation with zenith angle are also reported.
