Cosmic Ray Composition Primary cosmic particles collide with atoms in the Earth's atmosphere and produce a cascade of short lived particles, which can decay into muons. Some muons may penetrate through the water and reach Antares.

Cosmic Ray energies in Antares Primary energies measured by Antares between 10^13 eV and 10^16 eV (knee region included)

Motivation ● Composition may offer information over cosmic ray origin from outer space ● Direct measurements of cosmic ray is statistical limited for primary energies above 100 TeV ● Indirect measurements with air shower experiments (large fluctuation)

Shower Multiplicity ● Primary composition of Corsika contains five different mass groups (p, He, N, Ma, Fe)

Pure proton primaries or pure iron primaries versus data ● No way to explain data with only proton or iron primaries

Why can we discriminate iron from proton primaries by measuring showers? ● Shower multiplicity is proportional to the muon multiplicity ● It seems that proton primaries produce less energetic muons than iron primaries Additional information in analysis-elog 227 and 228

Conclusion ● Antares can be used to check models of chemical compositions of cosmic rays around the knee ● Composition measurements are hadronic interaction model dependent. How stable is the shower multiplicity method under different hadronic interaction models? ● Can Antares compete with other experiments?