Strange fireball as an explanation of the muon excess in Auger data

Abstract

We argue that ultrahigh energy cosmic ray collisions in the Earth atmosphere can probe the strange quark density of the nucleon. These collisions have center-of-mass energies \agt 10^{4.6} A GeV, where A \geq 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 \bar u and d \bar d pairs, resulting in gluon fragmentation mainly into s \bar 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 a \sim 40\% enhancement of the muon content in atmospheric cascades, in agreement with recent data reported by the Pierre Auger Collaboration.