Limit on ultrahigh energy neutrino flux from the Parkes Lunar Radio Cherenkov experiment

The first search for ultrahigh energy (UHE) neutrinos using a radio telescope was conducted by Hankins, Ekers & O'Sullivan. This was a search for nanosecond duration radio Cherenkov pulses from UHE neutrino interactions in the lunar regolith, and was made using a broad-bandwidth receiver fitted to the Parkes radio telescope, Australia. At the time, no simulations were available to calculate the experimental sensitivity and hence convert the null result into a neutrino flux limit. Proposed future experiments include the use of broad-bandwidth receivers, making the sensitivity achieved by the Parkes experiment highly relevant to the future prospects of this field. We have therefore calculated the effective aperture for the Parkes experiment and found that when pointing at the lunar limb, the effective aperture at all neutrino energies was superior to single-antenna, narrow-bandwidth experiments, and that the detection threshold was comparable to that of the double-antenna experiment at Goldstone. However, because only a small fraction of the observing time was spent pointing at the limb, the Parkes experiment places only comparatively weak limits on the UHE neutrino flux. Future efforts should use multiple telescopes and broad-bandwidth receivers.