Download ANTARES: a deep-sea neutrino telescope

ANTARES: a deep-sea neutrino telescope.
Where do cosmic rays come from? What are the mechanisms able to accelerate particles to
ultra-high energies? What happens in the surrounding of a super-massive black hole?
What kind of phenomenon is responsible for the mysterious gamma-ray bursts? Have dark
matter particles gathered in the centre of the earth or of the Galaxy? These questions and
many others motivate nowadays ambitious experiments in high energy astrophysics.
Astronomy with high-energy neutrinos may soon drastically change our view of the
Universe. The ANTARES project, a deep-sea neutrino telescope in the Mediterranean sea,
is expected to play a pioneering role in this new area of science.
The idea.
If one wants to understand the sources of high energy cosmic rays, one faces a fundamental
difficulty: at high energy, the traditional messenger, the photon, is absorbed by the radiation and
the matter it encounters along its path from the sources to the detectors. The Universe is actually
opaque to high energy gamma rays. Because they are electrically charged, primary cosmic rays
have suffered deviations by galactic and intergalactic magnetic fields and it is hard to identify
their sources. To look at the high energy sky, one is therefore obliged to use another messenger,
electrically neutral and basically insensitive to any obstacle along its path: the neutrino is that
ideal messenger.
But because of these essential qualities, neutrinos are also very difficult to detect; one needs a
gigantesque target to materialize them, much bigger than the already huge existing underground
detectors. One also needs an efficient shielding against the background induced by penetrating
cosmic rays. The adopted solution is to equip large volumes of a natural medium transparent to
visible light with an array of photo-detectors: these electronic eyes can then detect the wake of
Cherenkov light emitted by the charged particles induced by neutrino interactions in or near the
detector.
This almost forty years old idea triggered the aim to construct large cosmic neutrino observatories
in the polar ice or under the sea. A few projects are in progress at different stage (refer here to
Amanda, Baikal, Nestor, Nemo …).
Since 1996, the physicists and the engineers of the ANTARES project have assessed the
feasibility of a deep-sea telescope. Three years of intense R&D where indeed necessary to fully
understand the deep-sea environment and to solve the technical challenge of the deployment of a
complex and large piece of equipment in the depth of the Mediterranean sea. The collaboration1
has now started the construction of a 0.1 km² detector off the French Mediterranean shore.
High energy neutrinos as cosmic messengers:
The observation of solar neutrinos flux have led to the well known puzzle which involves both
the understanding of our star and of the neutrino nature. Until now and apart from a handful of
MeV neutrino events induced by the nearby supernova SN1987A, no neutrino of cosmic origin
could yet be identified. Candidate sources of high energy neutrinos include the cores or jets of
1
ANTARES is a CERN "Recognized Experiment". The ANTARES collaboration consist of 16 HEP and Astronomy
institutes and of 2 Sea Science institutes, from France, Italy, Netherlands, Russia, Spain and United Kingdom. See
http://antares.in2p3.fr/ for further information.
Active Galactic Nuclei (AGN), or the violent cosmic events which are responsible for the
frequent Gamma Ray Bursts (GRB). Whilst our understanding of these objects is constantly
improving, their activity at high energy remains a mystery: do they produce high energy cosmic
rays or are they simply gamma ray emitters? If high-energy protons are accelerated in these
sources, then production of high energy neutrinos is guaranteed. On the other hand, if these high
energy cosmic accelerators cannot fulfil the role of producing the observed cosmic ray spectrum,
then very high energy cosmic rays (and neutrinos) must be created by non-accelerating
mechanisms such as the decay of big-bang massive relic particles.
Furthermore, neutrinos could reveal the presence of dark matter in the form of neutralinos, the
lightest of the yet undiscovered particles predicted by supersymmetry theories. Neutralinos would
have been formed in the early Universe and have been accumulating since then in the cores of
stars, galaxies and planets. They would shine neutrinos from within the center of the earth, the
sun or the Milky Way.
Last but not least, as recent history has shown, astrophysics experiments allow sometimes to
explore domains of physical quantities out of reach of accelerator borne experiments: by studying
the fluxes of atmospheric neutrinos, the Super-Kamiokande and the Macro experiments have
been able to test very small values of the mass difference between two types of neutrinos. The
observation of the same phenomenon is also possible with large neutrino telescopes, using a
baseline up to the earth diameter and at higher energy thus with very different systematics. It
would thus be possible to confirm or rule out the recent evidence for neutrino masses, and to
measure accurately the oscillation parameters.
A large deep-sea telescope:
As neutrinos interact very weakly, a very massive target is required to detect them. The highenergy muon produced in the interaction is very penetrating and it’s trajectory almost aligned
with that of the neutrino. As it is electrically charged, it produces Cherenkov light in the ice or
water, which can be detected by sensitive light sensors. The measurement of the time
development of this luminous wake permits to reconstruct the direction of the muon and thus that
of the neutrino with a precision better than a fraction of a degree. It is then possible to locate on
the celestial sphere, the point from which the neutrino originated.
High energy cosmic rays which bombard the upper atmosphere generate a flow of high energy
and highly penetrating muons which constitute a dominant background, even under a significant
shielding of several thousand of metres of water. This background can be further reduced if only
up-going tracks are selected, since neutrinos alone can cross the earth and produce an up going
muon under the detector. The earth is used as a target for the neutrinos as well as a shielding to
filter background muons. The detectors will therefore look downwards. A cosmic neutrino
telescope will consist of a three-dimensional array of light sensors (photomultiplier tubes)
watching at a volume of the order of a cubic kilometre at depth of a few thousands meters. In a
first stage, a smaller telescope with an effective area of 0.1km², will give the first indications on
cosmic neutrino sources and fluxes and will be used to validate the methods and the techniques of
this new type of astronomy.
Testing the feasibility and studying the environment:
However, before reaching this goal, one needs to study the design and the deployment of such a
large deep-sea detector and to learn about the unusual environment. To this end, a three years
intensive R&D programme was initiated by the ANTARES collaboration in 1996.
The aims of this first phase, which is now ending, were to measure the deep-sea water properties,
to find a convenient detector site, and to master the sea technologies required to deploy, connect
and run large and complex pieces of apparatus offshore in a deep-sea environment. In parallel,
studies were carried out to optimise the detector performances.
Instrumented mooring lines were designed and used during numerous on site campaigns in order
to study the properties of the deep-sea waters, including the transparency and light scattering, the
optical background due to natural radio-activity and to living organisms and the bio-fouling of
optical surfaces. The deep-sea waters happen to be of very good quality and light can propagate
over more than 50 m without noticeable alteration. This result is very encouraging and insures
excellent performances for a submarine detector. In particular the small amount of large angle
scattering indicates that a very good angular resolution can be achieved.
A running prototype connected to the shore:
In order to study mechanical options, and to test deployment and recovery procedures, a real size
detector line has been built and was immersed and recovered several times in 1998. This line was
350m high and consisted of 32 glass spheres aimed to house and protect the photo-multipliers
against the water pressure. The line was also instrumented with acoustic and electronic sensors to
provide position measurements within less than 10cm. More recently, in November 1999, the
same line was equipped with 8 photo-multipliers and was re-immersed at 1100m depth off the
Marseilles coast. It had been previously connected to a submarine electro-optical cable 37 km
long running to the shore. The photo-multipliers signals as well as the electrical power supply and
the control and monitoring of the system pass along this cable. This line is the first long term
running realisation of a deep-sea cosmic ray detector. The first data on atmospheric muons have
been taken and their analysis is underway. About one muon every 10 seconds of acquisition is
recorded and successfully reconstructed. The analysis of the position monitors indicates that an
accuracy of a few centimetres on the reconstructed position of individual photomultipliers has
been achieved.
The installation of the future telescope will need to connect individual lines to a central node in
situ. Deep-sea connection tests have therefore been carried out with the "Nautile", a deep-sea
submarine belonging to IFREMER, an oceanographic collaborator in the project. In December
1998, connections were successfully made at a depth of 2400m.
A 0.1km² detector, a step toward a km² telescope:
With the site quality and the technical feasibility having been established, the project is now
entering a new phase: in three years from now, a detector consisting of one thousand of photodetectors on 13 lines, 400 m high and distributed on a 150 m radius surface will be installed. It
will be deployed at a depth of 2400m off the coast of southern France near Toulon. The expected
performances can be summarized by the following features: an effective area after reconstruction
of 0.1km² for 10 TeV muons (the effective area increases with energy), 0.2 degree of median
angular resolution for > 10 TeV muons, the possibility to reconstruct direction and energy of
muons from neutrinos in a wide range of energy, including those of low energy (few GeV) events
useful to the study of neutrino oscillations.
This detector will open a new era of cosmic neutrino astronomy, will contribute to the first
measurements of the cosmic neutrino fluxes, find the brightest point-like sources in the heavens,
search for neutralinos in the centres of the earth, sun and Galaxy and measure atmospheric
neutrino oscillations parameters with unprecedented precision.
It will also provide practical experience and expertise, which will be invaluable for the realisation
of a future kilometre-scale detector capable of conducting a search for astronomical sources.
François Montanet.
Figures and captions:
A virtual reality image of what the 0.1 km2 ANTARES observatory will look like in 2003 when
its 13 lines will be immersed in the Mediterranean near Toulon. A submarine will be used to
connect each line to the junction box linked to the shore via a 40km electro-optical cable
http://antares.in2p3.fr/Publications/papers/CERNCourier/antares_pub.jpg
Immersion of a prototype module for the ANTARES underwater neutrino observatory
http://antares.in2p3.fr/Publications/papers/CERNCourier/phototheque-315.jpg
A prototype ANTARES detector line safely back on board after underwater tests.
http://antares.in2p3.fr/Publications/papers/CERNCourier/phototheque-318.jpg
Launching the submarine to test the ANTARES underwater connections
http://antares.in2p3.fr/Publications/papers/CERNCourier/phototheque-314.jpg
Figure 1 A prototype ANTARES detector line safely back on
board after underwater tests.