The view from a neutrino telescope

The ANTARES scientific programme can be divided into three broad subject areas:

particle physics (neutrino oscillations),
particle astrophysics (searches for neutralino dark matter)
astronomy.

Astronomy and astrophysics

The two confirmed astrophysical neutrino sources are: the Sun and supernovae as exemplified by SN 1987A. Both of these produce low energy neutrinos which could not be tracked by ANTARES, although a nearby supernova could be detected through a transient increase in the overall singles rate. However, there are a number of candidate astrophysical sources of high-energy neutrinos which would be detected by ANTARES. Since pions produced by high-energy protons are the likeliest source of high-energy neutrinos, candidate astrophysical neutrino sources are the proton accelerators, which might explain the High Energy Cosmic Rays spectrum. High energy gamma-rays may be associated with high energy protons and their subsequent decays from pions, but they are also produced by synchrotron radiation of fast electrons in the presence of magnetic fields. The observation of neutrino sources would unambiguously discriminate between the two acceleration mechanisms.

Candidate sources can be identified both within the Galaxy--accreting binaries containing neutron stars or black holes, supernovae and young supernova remnants--and elsewhere, most notably active galactic nuclei (AGN) and gamma-ray bursters. This represents a rich spectrum of possible sources, both steady and transient, covering a wide range of neutrino energies.

It is worth noting that the history of astronomical observation suggests that the likeliest outcome of opening a new observational window is the discovery of a completely unexpected class of sources. Such discoveries are by definition difficult to anticipate, but one possible pointer is that the very highest-energy cosmic rays, those above 10²⁰ eV, remain difficult to explain in present models. This puzzle could be solved by the observation of their associated neutrinos.

Particle physics

Within the minimal standard model, neutrinos are strictly massless, but the need to incorporate non-zero masses has long been anticipated. Neutrino astronomy was instrumental in establishing non-zero neutrino masses as an important topic: the lower-than-predicted flux of electron neutrinos from the Sun (the Solar Neutrino Problem) is now generally believed to be explained by neutrino oscillations either in vacuum or more probably within the Sun itself, enhanced by the high electron density in the solar core (the MSW effect).

Neutrino oscillation solutions to the solar neutrino problem involve the conversion of electron neutrinos into some other flavour. The energies involved are not well matched to the detection capabilities of ANTARES. However, recent results from Super-Kamiokande appear to show a similar flux reduction occurring for muon neutrinos generated by cosmic-ray interactions at the top of the Earth's atmosphere. The effect is interpreted as evidence for the oscillation of into either $\nu_\tau$ or a so-called ``sterile'' neutrino, with a large mixing angle and a squared mass difference of around 10^-2 to 10^-3 eV². This is a result of major importance which urgently requires confirmation by an experiment with independent systematics. The proposed ANTARES configuration should be capable of exploring the region of parameter space favoured by the Super-Kamiokande data.

Cosmology and dark matter

In recent years it has become generally accepted by astrophysicists that most of the matter in the universe is non-luminous "dark matter''. The clearest evidence for this is the observed flatness of the rotation curves of disk galaxies, which imply a dynamical mass far in excess of that accounted for by the constituent stars and gas. Constraints from the observed abundances of light elements indicate that much of the dark matter in the cosmos must be non-baryonic. No presently known particle has the required properties, but a good theoretical candidate is the stable neutral particle expected in most versions of supersymmetry theory.

The detection and identification of a relic cosmological population of supersymmetric particles would be of immense importance to both cosmology and particle theory. Neutrino telescopes are not directly sensitive to a weakly interacting massive particle (WIMP). However, supersymmetric WIMPs will accumulate in the cores of the Sun and the Earth or in the centre of the Galaxy through gravitational capture. The resulting high space density leads to annihilation reactions, which will yield high-energy neutrinos through the decays of the gauge bosons and heavy particles produced. The proposed detector would be sensitive to these neutrinos over a useful range of WIMP masses. Compared to ongoing direct detection experiments, neutrino telescopes are generally more suitable for higher masses, although resonances in the Earth's capture cross-section enhance the signal strongly at certain lower masses, particularly around 56 GeV. But even a confirmation of a prior direct detection would provide useful information about the couplings of the WIMP, and thus help to constrain the parameters of the theory.