Particle cascades initiated by ultra-high energy (UHE) neutrinos in the lunar
regolith will emit an electromagnetic pulse with a time duration of the order
of nano seconds through a process known as the Askaryan effect. It has been
shown that in an observing window around 150 MHz there is a maximum chance for
detecting this radiation with radio telescopes commonly used in astronomy. In
50 hours of observation time with the Westerbork Synthesis Radio Telescope
array we have set a new limit on the flux of neutrinos, summed over all
flavors, with energies in excess of 4×1022 eV.Comment: Submitted to Phys. Rev. Let