AMANDA Observations Constrain the Ultrahigh Energy Neutrino Flux

A number of experimental techniques are currently being deployed in an effort to make the first detection of ultrahigh energy cosmic neutrinos. To accomplish this goal, techniques using radio and acoustic detectors are being developed, which are optimally designed for studying neutrinos with energies in the PeV-EeV range and above. Data from the AMANDA experiment, in contrast, have been used to place limits on the cosmic neutrino flux at less extreme energies (up to $\sim 10\mathrm{PeV}$). In this Letter, we show that by adopting a different analysis strategy, optimized for much higher energy neutrinos, the same AMANDA data can be used to place a limit competitive with radio techniques at EeV energies. We also discuss the sensitivity of the IceCube experiment, in various stages of deployment, to ultrahigh energy neutrinos.

Figure 1

The effective volume of AMANDA to electron neutrinos as found using the cuts from the AMANDA cascade analysis [8] and using the $>300$ module cut adopted in this study.

Figure 2

The current experimental limits on the diffuse flux of high and ultrahigh energy neutrinos (summed over flavors in the ratio ${\varphi }_{{\nu }_e}:{\varphi }_{{\nu }_{\mu }}:{\varphi }_{{\nu }_{\tau }}=1:1:1$, after oscillations are considered). The AMANDA cascade, RICE, and ANITA-lite limits are found in Refs. 3, 8, 15, respectively. The AMANDA and ANITA-lite limits are shown at the 90% confidence level, while the RICE limit shown is at 95% confidence. For comparison, we also show the neutrino spectrum predicted from the propagation of ultrahigh energy cosmic ray protons (the cosmogenic flux) [16] and the Waxman-Bahcall bound [17]. The solid and dotted curves correspond to cosmogenic neutrino fluxes calculated assuming normal [$\propto (z+1{)}^3$] and strong [$\propto (z+1{)}^4$] source evolution, respectively.

Figure 3

The effective volumes (water equivalent) of various current and near future ultrahigh energy neutrino detectors. The volume of the AMANDA experiment has been calculated using the $>300$ module cut described in the text. A similar approach was taken for IceCube, requiring a number of modules be triggered equivalent to a volume of 300 modules in AMANDA (the module spacing in IceCube is greater than in AMANDA). The upper and lower RICE curves (dots) correspond to hadronic and electromagnetic showers, respectively. The ANITA, ANITA-lite, and Auger curves are shown for comparison with AMANDA, IceCube, and RICE in terms of $2\pi$ steridians equivalent. In the case of ANITA and ANITA-lite, the volumes have been scaled by a factor of $10/365$ to account for the 10 d duration of each balloon mission. See text for more details.