We argue that ultrahigh-energy cosmic-ray collisions in Earth’s atmosphere can probe the strange quark density of the nucleon. These collisions have center-of-mass energies $\gtrsim 10^{4.6}A\mathrm{GeV}$, where $A\ge 14$ is the nuclear baryon number. We hypothesize the formation of a deconfined thermal fireball which undergoes a sudden hadronization. At production the fireball has a very high matter density and consists of gluons and two flavors of light quarks ($u$, $d$). Because the fireball is formed in the baryon-rich projectile fragmentation region, the high baryochemical potential damps the production of $u\overline{u}$ and $d\overline{d}$ pairs, resulting in gluon fragmentation mainly into $s\overline{s}$. The strange quarks then become much more abundant and upon hadronization the relative density of strange hadrons is significantly enhanced over that resulting from a hadron gas. Assuming the momentum distribution functions can be approximated by Fermi-Dirac and Bose-Einstein statistics, we estimate a kaon-to-pion ratio of about 3 and expect a similar (total) baryon-to-pion ratio. We show that, if this were the case, the excess of strange hadrons would suppress the fraction of energy which is transferred to decaying ${\pi }^0$’s by about 20%, yielding an $\sim 40\%$ enhancement of the muon content in atmospheric cascades, in agreement with recent data reported by the Pierre Auger Collaboration.

• Received 29 December 2016

DOI:https://doi.org/10.1103/PhysRevD.95.063005