Questions you may ask
Why is it important to survey the Universe with a neutrino telescope as opposed to telescopes which detect electromagnetic radiation or cosmic rays impinging on the Earth atmosphere? Why should a neutrino telescope be built at the Northern Hemisphere? Why should it be so big and why should it be located at the bottom of the Mediterranean Sea?
Let's try to answer these questions.
Astronomy with photons as cosmic messengers
Mankind has observed sources of light in the night sky for thousands of years. In recent decades, the observation of the sky has been dramatically extended from visible to 'non visible' electromagnetic radiation with the detection of radio, X-rays and gamma-rays, leading to multi-wavelength astronomy.
Recently, satellite experiments have shown the existence of galactic and extra-galactic sources of gamma rays up to several tens of GeV. Ground-based detectors have shown that some of these sources produce electromagnetic radiation with an energy spectrum that extends up to several tens of TeV.
The majority of the observed sources however become faint at these energies. This can be explained by the absorption of high-energy photons through interactions with the infrared or cosmic microwave background radiation. Detectable extragalactic sources with energies above a TeV should reside in the closest galaxies, as the cosmic microwave background radiation prevents us observing high-energy photons emitted by sources at larger distances.
Apart from the energy spectra, the observed sources show different time behaviour: constant (e.g. the Sun); periodic (e.g. pulsars, Cepheids, binary stars); impulsive (e.g. gamma ray bursts, (super)novae) or erratic (e.g. active galactic nuclei).
Astrophysics with cosmic rays as messengers
At the beginning of 20th century, the observation of increasing ionisation of the Earth's atmosphere at higher altitudes has led to the idea that charged particles from outer space impinge on Earth.
Since the 1940's, larger and larger detectors have been built to detect cosmic rays at higher and higher energies. However, at energies above 5x1019, eV cosmic rays are expected to have a limited range due to interactions with the cosmic microwave background. This implies that the highest energy cosmic rays observed so far, up to 3x1020 eV (50 Joules), should originate in the vicinity of our galaxy.
The present knowledge of physics does not describe any acceleration model capable explaining these observations.
At lower energies cosmic rays are deflected by the galactic and extra-galactic magnetic fields; their origin cannot be traced back. At extreme high energies, where the origin of cosmic rays could possibly be traced back, the huge Pierre Auger Observatory will be able to detect cosmic rays from extragalactic sources. In November 2008 they have published a correlation with Active Galactic Nuclei.
In any case, detection of cosmic rays, regardless of their energy, will not allow us to study their time behaviour, since the time of flight varies too much due to the intergalactic magnetic fields.
Astroparticle physics with neutrinos
The observations mentioned above motivate a survey of the Universe at large distance and energy scales using different cosmic messengers to better understand the physics of the relevant phenomena.
An alternative solution for the observation of sources at large distances is the detection of neutrinos. Neutrinos have the characteristics required to complement the photon and cosmic rays as cosmic messengers.
Indeed, the neutrino has no charge and is therefore not deflected by magnetic fields. It is stable and interacts only weakly. Thus it can travel from the most remote places in the Universe to the Earth. Neutrinos with energies between 1012 eV and 1020 eV represent unique messengers to sound the deep Universe.
High energy cosmic neutrinos have not been observed so far; their flux can only be evaluated using models. Generally these models assume that neutrinos originate predominantly as decay products of hadrons produced by interactions of high energy protons with photons or nuclei.
Several calculations based on different models have been made. Most calculations yield neutrino fluxes that require a neutrino telescope with a volume of at least a cubic kilometre: for active galactic nuclei and micro-quasars several hundred neutrinos could be detected per year while for gamma ray bursts the figure is several tens.
Neutrinos from gamma ray bursts
The signature of gamma ray burst events in a neutrino telescope should be very clean due to space and time correlation with optical observations; the background will be so small that a few events will be sufficient for the discovery of such a neutrino source. The correlated detection of photons and neutrinos will then constrain the gamma ray burst models.
Neutrinos from the centre of the Galaxy
The NAOS-CONICA device has measured the path of the S2 star through its pericentre. The results are consistent with the presence of a black hole in the centre of the Galaxy. The measurement of the light spectrum in the infrared region reveals signs of the presence of an accretion disk and of jets. It is therefore assumed that the Galactic Centre contains an active galactic nucleus.
Furthermore, the Cangaroo and HESS collaborations have reported observation of multi-TeV photon signalis from the Galactic Centre. These signals hints at the existence of an intense source of high-energy neutrinos in the Galactic Centre.
Dark matter search using neutrinos
Another subject of interest is the search for non baryonic dark matter. The direct search for dark matter is the subject of several present underground experiments.
An indirect search for dark matter can be made with a neutrino telescope able to detect neutrinos generated by the annihilation of neutralinos accumulated -since the origin of the Universe - in the centres of massive celestial bodies such as the Earth, the Sun and the Galactic Centre. Calculations suggest that the sensitivity of this detection method makes it complementary to direct search methods.
A neutrino telescope in the Mediterranean Sea will complement the IceCube neutrino telescope being built at the South Pole. A neutrino telescope located in the Mediterranean Sea will survey the larger part of the Galactic disc, including the Galactic Centre, which is barely visible with a neutrino telescope at the South Pole.
Detection principle and the Mediterranean Sea
The combination of the relatively low flux of high energy cosmic neutrinos and their weak interaction with matter implies the need for a very massive detector (1012 kg). One solution is to instrument a large volume of deep sea water with a three-dimensional array of optical modules, i.e. photomultiplier tubes housed in transparent pressure vessels. The neutrinos can then be detected indirectly through detection of Cerenkov light produced by charged particles (muons) emerging from neutrino interactions in the sea water or sea bed.
Photomultipliers can safely be operated below the maximum penetration depth of daylight (around 1000 m). Even at these depths the detector can be swamped by Cerenkov light from muons produced in cosmic ray interactions in the Earth's atmosphere (1010 per km2 per year). The greater the depth, the smaller the muon background.The Mediterranean Sea appears to be an ideal place for a neutrino telescope: it provides water of excellent optical properties at the right depth.