Abundance of primary protons at (1-3) x 10(15) eV inferred from a simulation of extensive air showers with gamma -ray families.

Abstract

A Monte Carlo simulation of extensive air showers (EAS's) was done under a particle-interaction model and was compared with the EAS's accompanied by \ensuremath{\gamma}-ray and hadronic families with total energy J${E}_{\ensuremath{\gamma}}$,H greater than 10 TeV obtained in the Mt. Norikura experiment (2780 m above sea level). The strong correlation between the shower size ${N}_{e}$ of EAS's with family and the primary energy, ${E}_{0}$, was obtained from the simulation as ${E}_{0}$/${N}_{e}$${=(2.0}_{\mathrm{\ensuremath{-}}0.7}^{+2.7}$) GeV with 95% width. The flux of the EAS's with family is sensitive to the chemical composition, especially to the fraction of protons in the primary cosmic-ray particles. The fraction of protons is inferred to be (20\ifmmode\pm\else\textpm\fi{}6)% and (23\ifmmode\pm\else\textpm\fi{}9)%, with one standard deviation (1\ensuremath{\sigma}), at energy (${1.4}_{\mathrm{\ensuremath{-}}0.5}^{+1.9}$)\ifmmode\times\else\texttimes\fi{}${10}^{15}$ eV and (${2.8}_{\mathrm{\ensuremath{-}}1.0}^{+3.8}$)\ifmmode\times\else\texttimes\fi{}${10}^{15}$ eV with 95% confidence interval, respectively, under the assumption of increasing cross section ${\ensuremath{\sigma}}_{p\mathrm{\ensuremath{-}}\mathrm{air}\mathrm{\ensuremath{\propto}}{E}^{0.055}}$ and approximate Feynman scaling in the fragmentation region. These values are in agreement with the values obtained from other mountain emulsion-chamber experiments within 2\ensuremath{\sigma}. The proton fraction thus obtained, around 20%, is much less than the normal abundance (\ensuremath{\sim}40%) expected from direct observations at lower energies around ${10}^{12}$ eV.