Status of Neutrino Astronomy: The Quest for Kilometer-Scale Instruments

This is a (very) personal attempt to summarize the status of neutrino astronomy: its scientific motivations, our understanding of natural water and ice as particle detectors and, finally, the detector technology. The observed photon spectrum of conventional astronomy extends to energies in excess of 10 TeV, perhaps as high as 50 TeV. Astronomy can be done, at much higher energy, by measuring the arrival directions of cosmic rays with energy in excess of 100 EeV where they point back at their sources. At such energies their gyroradii exceed the size of our galaxy. More than 6 orders of magnitude in photon energy, or wavelength, are left unexplored. Neutrino telescopes have been conceived to fill this gap. Given the history of astronomy, it is difficult to imagine that this will be done without making major, and most likely totally surprising, discoveries. The 18 orders of magnitude in wavelength, from radio-waves to GeV gamma rays, are indeed sprinkled with unexpected discoveries. Neutrinos have the further advantage that, unlike photons of TeV energy and beyond, they are not absorbed by interstellar light. They can, in principle, reach us from the edge of the Universe. If, for instance, the sources of the high energy cosmic rays are well beyond the Virgo cluster, the photon window for their exploration closes above 100 TeV. Speculations that the highest energy photons and protons are produced by cosmic accelerators powered by the supermassive black holes at the center of active galaxies can be used to estimate the effective volume of a neutrino telescope. The answer is that a 1 km 3 detector is required to detect the neutrinos accompanying the highest energy cosmic rays[1]. This estimate finds further support when studying the other diverse scientific missions of such an instrument which touch astronomy, astrophysics, cosmology, cosmic ray and particle physics. Kilometer-scale detectors are required, according to the best available estimates, to have a good chance of finding the accelerators of the highest energy cosmic rays, of improving significantly the searches for cold dark matter particles or WIMPs, and to search meaningfully for the cosmic sources of gamma ray bursts. In order to explore the plausible region of astronomical parameter space for these fascinating cosmic enigmas, a high energy neutrino telescope must contain several thousand optical detection modules in a volume of order 1 kilometer on a side. Model building suggests that some, and most likely the most exciting, …