Atmospheric muon and neutrino fluxes at very high energy

The observation of astrophysical neutrinos requires a detailed understanding of the atmospheric neutrino background. Since neutrinos are produced in meson decays together with a charged lepton, important constraints on this background can be obtained from the measurement of the atmospheric muon flux. Muons, however, can also be produced as μ+μ− pairs by purely electromagnetic processes. We use the Z-moment method to study and compare the contributions to the atmospheric muon and neutrino fluxes from different sources (π/K decay, charmed and unflavored hadron decay, and photon conversion into a muon pair). We pay special attention to the contribution from unflavored mesons (η, η′, ρ∘, ω and ϕ). These mesons are abundant in air showers, their lifetimes are much shorter than those of charged pions or kaons, and they have decay branching ratios of order 10−4 into final states containing a muon pair. We show that they may be the dominant source of muons at Eμ ≳ 103 TeV.