Systematic uncertainties in Monte Carlo simulations of the atmospheric muon flux in the 5-line ANTARES detector

The ANTARES detector was operated in a configuration with five lines for a period of 10 months from February until November 2007. The duty cycle was better than 80% during this period and almost 2×1072×107 atmospheric muon triggers were collected. This large sample was used to test Monte Carlo simulation programs and to evaluate possible systematic effects due to uncertainties on environmental parameters and detector description. First results are presented and discussed.