Limits on the flux of tau neutrinos from 1 PeV to 3 EeV with the MAGIC telescopes

A search for tau neutrino induced showers with the MAGIC telescopes is presented. The MAGIC telescopes located at an altitude of 2200 m a.s.l. in the Canary Island of La Palma, can point towards the horizon or a few degrees below across an azimuthal range of about 80°. This provides a possibility to search for air showers induced by tau leptons arising from interactions of tau neutrinos in the Earth crust or the surrounding ocean. In this paper we show how such air showers can be discriminated from the background of very inclined hadronic showers by using Monte Carlo simulations. Taking into account the orography of the site, the point source acceptance and the event rates expected have been calculated for a sample of generic neutrino fluxes from photo-hadronic interactions in AGNs. The analysis of about 30 h of data taken towards the sea leads to a 90% C.L. point source limit for tau neutrinos in the energy range from 1.0 × 1015 eV to 3.0 × 1018 eV of about Eντ2 × ϕ(Eντ) < 2.0 ×10-4 GeV cm-2 s-1 for an assumed power-law neutrino spectrum with spectral index γ=-2. However, with 300 h and in case of an optimistic neutrino flare model, limits of the level down to Eντ 2 × ϕ(Eντ) < 8.4 ×10-6 GeV cm-2 s-1 can be expected.