Upper Limit on the Diffuse Flux of Ultrahigh Energy Tau Neutrinos from the Pierre Auger Observatory

The surface detector array of the Pierre Auger Observatory is sensitive to Earth-skimming tau neutrinos that interact in Earth’s crust. Tau leptons from ${\nu }_{\tau }$ charged-current interactions can emerge and decay in the atmosphere to produce a nearly horizontal shower with a significant electromagnetic component. The data collected between 1 January 2004 and 31 August 2007 are used to place an upper limit on the diffuse flux of ${\nu }_{\tau }$ at EeV energies. Assuming an $E_{\nu }^{-2}$ differential energy spectrum the limit set at 90% C.L. is $E_{\nu }^{2}d{N}_{{\nu }_{\tau }}/d{E}_{\nu }<1.3\times {10}^{-7}\mathrm{GeV}{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}{\mathrm{sr}}^{-1}$ in the energy range $2\times {10}^{17}\mathrm{eV}<E_{\nu }<2\times {10}^{19}\mathrm{eV}$.

Figure 1

FADC traces of stations at 1 km from the shower core for two real showers of 5 EeV. Top panel: electromagnetic component ($\theta \sim 22°$); bottom: muonic signal ($\theta \sim 80°$).

Figure 2

Distribution of discriminating variables for showers initiated by $\tau$s decaying in the atmosphere, generated by ${\nu }_{\tau }$s with energies sampled from an $E_{\nu }^{-2}$ flux (histogram), and for real events passing the “young shower” selection (points). Left: length/width ratio of the footprint of the shower on the ground; middle: average speed between pairs of stations; right: r.m.s. scatter of the speeds. See text for details.

Figure 3

Limits at 90% C.L. for a diffuse flux of ${\nu }_{\tau }$ from the Pierre Auger Observatory. Limits from other experiments [36, 37, 38, 39, 40, 41, 42, 43] are converted to a single flavor assuming a $1:1:1$ ratio of the 3 neutrino flavours and scaled to 90% C.L. where needed. Two different formats are used: differential (squares) and integrated (constant lines). The shaded curve shows the range of expected fluxes of GZK neutrinos from Refs. [10, 11], although predictions almost 1 order of magnitude lower and higher exist.