High Energy Neutrino Astronomy: Towards Kilometer-Scale Detectors

Of all high-energy particles, only neutrinos can directly convey astronomical information from the edge of the universe—and from deep inside the most cataclysmic high-energy processes. Copiously produced in high-energy collisions, travelling at the velocity of light, and not deflected by magnetic fields, neutrinos meet the basic requirements for astronomy. Their unique advantage arises from a fundamental property: they are affected only by the weakest of nature’s forces (but for gravity) and are therefore essentially unabsorbed as they travel cosmological distances between their origin and us. Many of the outstanding mysteries of astrophysics may be hidden from our sight at all wavelengths of the electromagnetic spectrum because of absorption by matter and radiation between us and the source. For example, the hot dense regions that form the central engines of stars and galaxies are opaque to photons. In other cases, such as supernova remnants, gamma ray bursters, and active galaxies, all of which may involve compact objects or black holes at their cores, the precise origin of the high-energy photons emerging from their surface regions is uncertain. Therefore, data obtained through a variety of observational windows—and especially through direct observations with neutrinos—may be of cardinal importance. The sun is an intense source of electron neutrinos (νe), albeit of relatively low energy. Solar neutrino astronomy began with first experiments in the mid-1960s; today there are five complementary neutrino detectors viewing the nuclear reactions in the core of the sun and, at the same time, studying the fundamental properties of neutrinos. The sun remained the only astronomical object studied with neutrinos until neutrinos from Supernova 1987A were observed. These are still the only two sources marking the astronomical neutrino spectrum. Suggestions to use a large volume of water for high-energy neutrino astronomy were made as early as the 1960s [1]. In this case, a muon neutrino (νμ) interacts with a hydrogen or oxygen nucleus in the water and produces a muon travelling in nearly the same direction as the neutrino. The blue C̆erenkov light emitted along the muon’s ∼kilometer-long trajectory is detected by strings of photomultiplier tubes deployed deep below the surface. DUMAND, a pioneering project located off the