  The PYTHIA 6 Monte Carlo (MC) event generator, commonly used in collider
physics, is interfaced for the first time with a fast transport simulation of a
hydrogen atmosphere, with the same density as air, in order to study the
properties of extended atmospheric showers (EAS) produced by cosmic ray protons
with energies E$_{CR}\approx 10^{14}$--$10^{20}$ eV. At variance with the
hadronic MC generators (EPOS-LHC, QGSJET, and SIBYLL) commonly used in
cosmic-rays physics, PYTHIA includes the generation of harder hadronic jets and
heavy (charm and bottom) quarks, thereby producing higher transverse momentum
final particles, that could explain several anomalies observed in the data. The
electromagnetic, hadronic, and muonic properties of EAS generated with various
settings of PYTHIA 6, tuned to proton-proton data measured at the LHC, are
compared to those from EPOS-LHC, QGSJET 01, QGSJET II, and SIBYLL 2.1. Despite
their different underlying parton dynamics, the characteristics of the EAS
generated with PYTHIA 6 are in between those predicted by the rest of MC
generators. The only exceptions are the muonic components at large transverse
distances from the shower axis, where PYTHIA predicts more activity than the
rest of the models. Heavy-quark production, as implemented in this study for a
hydrogen atmosphere, does not seem to play a key role in the EAS muon
properties, pointing to nuclear effects as responsible of the muon anomalies
observed in the air-shower data.
