Introduction

A new window on the universe has been opened with the development of neutrino telescopes. Detection of high energy (> 10¹² eV) cosmic neutrinos represents a unique opportunity to probe the distant universe. Photons of such high energy are likely to interact with the cosmic microwave background (CMB) and protons, being charged, will have their trajectories bent in galactic and intergalactic magnetic fields. Neutrinos will not interact with CMB photons and will point directly back to their source, giving essential information on these sources.

Arguably the best justification for neutrino telescopes is based on history - the opening of each new astronomical window has led to unanticipated discoveries. Although the most exciting discoveries made by neutrino telescopes may well be unexpected, these multi-purpose scientific instruments have scientific goals from the fields of particle physics, astronomy, cosmology and cosmic ray physics.

Active Galactic Nuclei (AGN) and Gamma Ray Bursts (GRB) must be considered as well motivated sources of high energy neutrinos since they are the source of the most energetic photons. The production of neutrinos in AGN and GRB requires the acceleration of hadrons as opposed to purely electron acceleration which is used in many models of AGN. The central engine and the jets associated with blazers have been identified as possible sources of high energy neutrino fluxes within AGN (e.g., Stecker et al 1991 and 1992, Stecker and Salamon 1996, Szabo and Protheroe 1992 and 1994, Protheroe 1997, Mannheim 1993 and 1995 and references within). Gaisser, Halzen and Stanev (1995) give a general discussion of the production of neutrinos in AGN. In their pioneering paper Stecker et al (1991) integrated theoretical results for generic AGN to estimate the diffuse neutrino flux from all the active galaxies in the universe. One of the goals of neutrino telescopes is to detect this flux. A recent review of the neutrino flux from AGN for several models is given in Protheroe (1998). Neutrino telescopes could distinguish between competing models of the underlying physics using the different energy dependence predicted for the flux. The requirements for a neutrino telescope based on radio receivers to achieve this are discussed by Frichter, Ralston and McKay (1996).

Correlation of ultra-high energy neutrino fluxes at earth with gamma-ray burst observations would provide essential information on the nature of these extraordinarily luminous sources, much as correlations between neutrinos with photons from SN1987A provided insight into the neutrino sector from supernovae. It has recently been suggested (Waxman and Bahcall 1997) that the highest energy cosmic rays and the highest energy gamma-ray bursts observed at earth have a common origin - the lack of temporal coincidence can be attributed to the greater path length that charged particles travel due to bending in magnetic fields. However since neutrinos experience no such deflection they offer the possibility of simultaneous observation with gamma-ray bursts.

The neutrinos referred to so far are produced by cosmic rays at their acceleration site. Neutrinos will also be produced when cosmic rays interact with the interstellar medium and the cosmic microwave background.

High energy neutrinos will be produced by cosmic ray interactions with interstellar gas. This diffuse galactic neutrino background should exist with an intensity comparable to the diffuse galactic gamma ray background. A survey of predictions is given by Gaisser, Halzen and Stanev (1995) and Protheroe (1998). At ultra-high energies, cosmic rays will interact with the photons of the cosmic microwave background radiation resulting in the Greissen, Zatsepin, Kuzmin (GZK) cutoff in the cosmic ray spectrum. Ultra-high energy neutrinos with an energy spectrum peaked around 10²⁰ eV will be produced in these interactions. Seckel and Frichter (1999) discuss detection of these neutrinos using a large radio array.

The indirect detection of dark matter is another goal of neutrino telescopes. Cold dark matter particles annihilate into neutrinos with massive cold dark matter particles producing high energy neutrinos which can be detected by high-energy neutrino telescopes. Additionally there is a great deal of particle physics information, in particular related to neutrino oscillation, which can be deduced from, for example, the angular distribution of upward-coming neutrino events.

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Introduction