Large Natural Cherenkov Detectors : Water and Ice

In this review we rst address 2 questions: why do we need kilometer-scale muon and neutrino detectors? what do we learn from the operating Baikal and AMANDA detectors about the construction of kilometer-scale detectors? I will subsequently discuss the challenges for building the next-generation detectors. The main message is that these are diierent, in fact less ominous, than for commissioning the present, relatively small, detectors which must reconstruct events far outside their instrumented volume in order to achieve large eeective telescope area. High energy neutrino telescopes are multipurpose instruments; their science mission covers astronomy and astrophysics, cosmology, particle physics and cosmic ray physics. Their deployment creates new opportunities for glaciology and oceanography, possibly geologyy1]. The observations of astronomers span 19 decades in energy or wavelength, from radio-waves to the high energy gamma rays detected with satellite-borne detectorss2]. Major discoveries have been historically associated with the introduction of techniques for exploring new wavelengths. The important discoveries were surprises. In this spirit, the primary motivation for commissioning neu-trino telescopes is to cover uncharted territory: wavelengths smaller than 10 ?14 cm, or energies in excess of 10 GeV. This exploration has already been launched by truly pioneering observations using air Cherenkov telescopess3]. Larger cosmic ray arrays with sensitivity above 10 7 TeV, an energy where charged protons may point back at their sources with minimal deeection by the galactic magnetic eld, will be pursuing similar goalss4]. Could the high energy skies be devoid of particles? No, cosmic rays with energies exceeding 10 8 TeV have been recordedd4]. Between GeV gamma rays and the most energetic cosmic rays, there is uncharted territory spanning some eight orders of magnitude in wavelength. Exploring this energy region with neutrinos does have the deenite advantage that they can, unlike high energy photons and nuclei, reach us, essentially without attenuation in ux, from the largest red-shifts. The challenge is that neutrinos are diicult to detect: the small interaction cross sections that enable them to travel without attenuation over a Hubble radius, are also the reason why kilometer-scale detectors are required in order to capture them in suucient numbers to do astronomyy5]. There is nothing magical about this result | I will explain this next. Cosmic neutrinos, just like accelerator neutri-nos, are made in beam dumps. A beam of accelerated protons is dumped into a target where they produce pions in collisions with nuclei. Neutral pions decay into gamma rays …