Neutrino telescopes

SUPPLEMENTS Nuclear

ELSEVIER

Neutrino

Physics B (Proc. Suppl.)

113 (2002) 26-3 I

www.elsevier.com/localelnpe

Telescopes

John Carr a a Centre de Physique de Particules 163 Avenue de Luminy, Case 907, 13288 Marseille, Prance. E-mail: carrQcppm.in2p3.fr

de Marseille,

This review presents the scientific objectives and status of Neutrino Telescope Projects. The science program of these projects covers: neutrino astronomy, dark matter searches and measurements of neutrino oscillations. The two neutrino telescopes in operation: AMANDA and BAIKAL will be described together with the ANTARES neutrino telescope being built in the Mediterranean.

1. Introduction There are a number of possible techniques to detect high energy neutrinos from outer space, each observing a different form of radiation emitted from the interaction of the neutrino in matter. The most widely exploited method is the detection, in large water and ice volumes, of Cherenkov light from the muons and hadrons produced in the interactions. Water Cherenkov detectors (e.g. IMB, Kamiokande, Super-Kamiokande and SNO) are the only detectors so far to have observed neutrinos produced beyond the earth, these observations being of lo6 to lo7 eV neutrinos produced in the sun and supernova 1987a. Experiments based on detection of radio wave emission, detection of sound produced in the interactions together with detection of light emission from showers of neutrinos in the atmosphere are being developed. These techniques are possible for neutrinos with energies above 1016 eV. The present review concentrates on the large water and ice detectors aimed at detection of neutrinos in the lOlo to 1015 eV energy range. In this energy range there are numerous candidate sources in the cosmos which will be discussed later as well as the source of secondary neutrinos from cosmic ray interactions in the atmosphere of the earth. These atmospheric neutrinos form an irreducible background to the searches to primary cosmic neutrinos but also provide a means to 0920-5632/02/$ - see front matter 0 2002 Elsevier Science B.\! PII SO920-5632(02)018 19-4

measure the neutrino oscillation parameters. The detectors to be discussed are the AMANDA experiment in the glacial ice at the South Pole, the Lake Baikal experiment in Siberia and ANTARES being constructed in the Mediterranean. The major scientific objective of the neutrino telescopes is the discovery and understanding of the sites of acceleration of high energy particles in the universe. Since their original discovery by V. Hess one hundred years ago the origin of the high flux of charged cosmic rays arriving at the earth is unknown. Neutrinos give a unique possibility to trace cosmic rays back to their origin since, having neutral electric charge, they are unperturbed by magnetic fields and being weakly interacting they can pass through dense dust clouds which might surround the sources. An important secondary objective of neutrino telescopes is the search for dark matter in the form of neutralinos. In supersymmetric theories with R-parity conservation the relic neutralinos, formed in the big-bang, would concentrate in massive bodies such as the centres of the earth, sun and galaxy. In these sites neutralino annihilations and the subsequent decays of the resulting particles would yield neutrinos detectable in neutrino telescopes of the scale currently in operation and being constructed. Further objectives of some of the projects include the measurement of oscillations with atmospheric neutrinos, which is possible with the same All rights reserved.

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detectors, in the range of oscillation parameters indicated by the Super-Kamiokande experiment. Ancillary science possible with the experiments cover oceanography, seismology and biology for the sea based experiments and various other environmental science subjects for the ice and lake experiments. 2. Cosmic

Sources of High Energy

Neutri-

nos

The origin of the bulk of the high-energy cosmic rays observed on Earth is at present largely unknown. It is expected that the majority of cosmic rays with energies below about 10’seV have their origin in our own galaxy while those at higher energies come from extragalactic sources. Highenergy gamma rays have been observed from numerous sources and it would be natural to expect charged cosmic rays also to originate from these. However, for many sources observed in gamma rays, it is uncertain whether the primary accelerated particles are hadrons or electrons and the observation of neutrinos would clearly demonstrate the existence of accelerated hadrons. In the galaxy it is Supernova Remnants (SNR) which are most popularly predicted to be the source of charged cosmic rays. A supernova remnant comprises a shell of matter, emitted after a supernova explosion, which continues to expand at speeds of typically a few tenths of the speed of light for thousands of years. The catalogue of Green lists over two hundred galactic supernova remnants of which several correspond to optically observed supernova (SN). A handful of SNRs have central pulsars and are known as plerions; the most famous being the Crab Nebula (SN1054). Some of these sources are known to be powerful emitters of Tev gamma rays but shell SNR, without a central pulsar, are less easily visible in TeV gammas. Recently, there has been an observation of the SNR RX 51713.7-3946 by the Gamma Ray Cherenkov telescope CANGAROO [l] in which the measured energy spectrum of the gamma rays is interpreted as evidence for the acceleration of cosmic ray protons in the source. This data for the first evidence of a cosmic ray source is still controversial [2], however if inter-

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preted as such the expected rate of neutrinos is of the order of 40 events/km2 /year [3]. Active Galactic Nuclei (AGN) are also known sources of TeV gamma rays. These objects, where jets of matter are emitted from the galaxy nucleus, are possibly a stage in the evolution of the majority of galaxies. The distribution of AGNs peaks at red shifts around 2, distances around 10 Gpcs, with the closest observed in TeV gamma around 100 Mpcs. Existing data on energy spectra of gamma ray observation can be explained using acceleration models with only electrons. Observations of neutrinos from AGNs would demonstrate the presence of hadronic acceleration also in these sources. Microquasars are thought to have the structure of a small scale AGN. Since 1992 about a dozen microquasars have been observed in the galaxy [4]. Multi wavelength observations support the model of microquasars as black holes of a few solar masses surrounded by an accretion disk fed from a companion star. The episodes of emission of high energy radiation, seen as separating blobs in radio telescope images, are explained as being due to instabilities in the accretion disk where the inner few hundred kilometres of material falls into the central black hole, with some fraction of this material being ejected in backto-back jets. Calculations of fluxes of neutrinos from microquasars give easily detectable fluxes in the neutrino telescopes in construction [5], e.g. in ANTARES 6.5 and 4.3 events/year from the microquasars GX339-4 and SS433 respectively with a background from atmospheric neutrinos of 0.3 events/year in a 1“ cone around the source. Gamma Ray Bursts (GRB) are energetic sources observed to emit short bursts of gammas in the energy range of a few hundred MeV with burst durations between 1OOms and 100s. When it was operational the BATSE detector on the Compton Gamma Ray Observatory observed l-2 events per day. The distribution of the BATSE observed GRBs, is uniform in galactic coordinates giving an indication of extragalactic origin. For about 20 GRBs with long burst duration, the redshift of the after glow has been observed and all are measured to be extragalactic. Many theories exist for the nature of gamma ray bursts

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and more data is needed to distinguish between them with neutrino observations again giving vital information to distinguish between models. 3. Search

for Dark

Matter

The present knowledge of the composition of matter in the universe comes from observations of galaxy clusters, cosmic microwave background radiation, Supernovae type la and big bang nucleosynthesis. In the current picture [6], the total amount of matter is close to the critical density for a flat universe with Rtotal N 1 and the fraction of matter is 30% with the rest being the, as yet, little understood dark energy. The matter contribution consists of baryonic matter with Rb N 0.04 and cold dark matter with RCDM N 0.26. A detailed review of searches for dark matter is given by L. Bergstrom in reference [7]. Neutrino Telescopes can perform indirect searches for Dark Matter, in the form of neutralinos concentrated at the centres of massive celestial objects, by searches for the neutrinos emanating from annihilation reactions. While laboratory direct dark matter search experiments are sensitive to any particle capable of causing a nuclear recoil, the indirect searches possible with neutrino telescopes require a specific model as to the nature of the dark matter candidate in order to predict the expected rates in an experiment. Generally supersymmetric models are applied with the neutralino as the dark matter candidate particle and in the MSSM the neutralino-nucleon cross-sections can be predicted for given model parameters. These models also predict the annihilation cross-sections of neutralinos and so the rates in the various indirect searches for dark matter. Two different searches are made for neutralino annihilations; the first, annihilations occurring in the galactic halo, giving gamma rays of unique energies in reactions like xx -+ Zy,yy and the second, annihilations in regions of concentrations of neutralinos in massive bodies, in reactions such as xx + WW, f f with W or f decaying to neutrinos. The searches for gamma ray lines from annihilations in the halo are performed in satellite and ground based gamma ray telescopes. In these experiments the

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gamma ray line energy would be directly related to the neutralino mass and give a very clear signature. The neutrino telescopes search for the neutrino decay products in the annihilations in massive bodies. Concentration of neutralinos in massive bodies such as the Earth, Sun and Galactic Centre would build up since the early universe where the neutralino dark matter would naturally be a fossil of the big bang similar to the 3K relic photons. Given the matter density in the various bodies and the total dark matter content in the halo, calculations can be made as a function of MSSM parameters for the rates to be expected in the current and future experiments. 4. Operating

Neutrino

Telescopes

There are currently two operating neutrino telescopes: BAIKAL [8] at a depth of 1200 m in the water of Lake Baikal in Siberia and AMANDA [9] at a depth of 2000 m in the ice at the South Pole in Antarctica. These detectors are sensitive in the energy range N lOlo - 1015eV. The BAIKAL detector was operating in 1993 with 36 optical modules and was finished in 1998 with 192 optical modules. Each optical module contains a 15 inch photo-tube, QUASAR-370, developed specially for the experiment. The detector is located in the Southern part of Lake Baikal at a point where the lake has a depth of 1366m and the distance to the shore is 3.6 km. The light transmission properties of the lake water vary greatly depending on the season due to sedimentation from river in-flow. Typical light absorption lengths are 20m and light scattering lengths 15m. The optical modules are deployed on 8 strings arranged at the edges and centre of an equilateral heptagon supported from above by a rigid frame. The detector is deployed into the water using the platform of frozen surface ice during the winter months. The effective area of the detector is about 2000m’. AMANDA was installed in stages in holes in the glacial ice made with a hot water drilling technique. The first detector elements were deployed in 1993 at depths of 810 to 1000m; however, measurements of the ice transparency at those depths showed that the light scattering was unac-

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ceptable for operation of a detector. Subsequent strings were deployed at depths of 1500 to 2000m where the ice properties are better. In 1997 the AMANDA BlO detector had 300 optical modules on 10 strings and the data so far published from AMANDA comes from this detector. Since then extra strings have been added with improved signal readout technology. The present AMANDA II detector has 19 strings and about 700 optical modules with an effective area of 30,000m2. The signal readout on the new strings is performed by optical fibre links while the earlier strings have readout on twisted pair cables. The rise time of the signal pulses read out by the analogue optical links is improved to 7ns compared to that of 1OOns on the twisted pair readout. These changes to the AMANDA detector give a detected event rate increased by a factor 4 to 5, together with a very much larger angular acceptance [lo]. Limits on high-energy neutrino fluxes have been published from both the BAIKAL and AMANDA experiments. These limits start to be close to some model predictions [14] of fluxes of neutrinos from AGNs but are still an order of magnitude higher than some theoretical limits based on cosmic ray fluxes [15]. The AMANDA experiment has limits on point source fluxes using their data from 1997 [16]. This data limits the flux from objects in the northern visible sky to less than 10-7cm-2sec-1 at 90% confidence level. 5. Mediterranean

Neutrino

Telescope

Projects

A deep sea-water telescope has significant advantages over ice and lake-water experiments due to the better optical properties of the medium. However there are serious technological challenges to overcome to deploy and operate in the sea. The pioneer sea-water project, DUMAND which worked from 1980 to 1995 to deploy a detector off the coast of Hawaii, did not overcome these challenges and the project was cancelled. In contrast the projects AMANDA and BAIKAL which deploy from the solid glacial ice and frozen surface lake ice, respectively, have developed workable deployment systems. The advantages of sea-water

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neutrino telescopes are significantly better angular resolution e.g. less than 0.3” for ANTARES compared to 3” for AMANDA as well as more uniform efficiency due to the homogeneous medium. A disadvantage of a sea-water detector is the higher optical background due to radioactive decay of 40K and light emission from living organisms: bioluminescence. These backgrounds can be overcome in the design of the detector by having a higher density of optical modules and by having high bandwidth data readout. In the Mediterranean Sea there are three sites under evaluation for Neutrino Telescope projects. The most advanced project is that of the ANTARES collaboration [17] which is building a detector with initially 900 optical modules and effective area of 50,000 m2 at a site off the south coast of France near Toulon. The NEMO collaboration is exploring a site off Sicily. Since 1990 the ANTARES and NEMO collaborations have been working together on the detector at the Toulon site with the intention to chose the best site for a future larger telescope. The NESTOR collaboration [18] intends to build a detector with 168 optical modules and around 20,000 m2 effective area at a site near Pylos off the coast of Greece. The ANTARES collaboration started in 1996 to explore sites off the French coast. The site chosen is at location 42”50N 6OlOE with a depth of 2400m. The first phase of the ANTARES project has been to fully evaluate this site in terms of water quality, sedimentation rate and geological stability. The absorption length light at the site has been measured to be 45-60 m in the blue and 25-30 m in the ultra-violet, the scattering length for large angle scatters is greater than 1OOm and the loss of light transmission through the glass housings of the optical modules has been evaluated in measurements lasting 8 months to be less than 2% / year. Extensive studies of the bioluminescence rate at the site have been carried out and lead to the conclusion that this background will give a dead time of less than 5% in the photo-multipliers given the electronics design of the detector. The design of the ANTARES detector array is to have optical modules suspended on individ-

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ual mooring lines, with readout via cables connected to the bottom of the lines. This technology is similar to the solution originally chosen by the DUMAND collaboration. As with DUMAND, the ANTARES detector requires connections made on the seabed by underwater vehicles. However, in the last 10 years the relevant underwater technology has advanced dramatically due to the needs of the offshore-oil industry, facilitating the realization of the ANTARES instrumentation. Currently a wide range of suitable deepsea connectors is available and extensively used in industry, including electro-optical connectors wet mateable on the site. Many commercial underwater vehicles exist capable of making these connections. The ANTARES readout design maximizes the reliability of the detector by dividing the system into independent sections such that there is no single active component in the sea whose failure causes the loss of the whole detector. The detector signals are digitized in local electronics in the sea and than transmitted to the shore on high bandwidth optical links. On the shore, a computer farm makes the trigger decisions to decide which data is recorded on tape. A major aspect of the ANTARES approach is the possibility to recover and repair all elements of the detector deployed in the sea. The NESTOR detector is planned to be installed at a depth of 3,800 m. An important concept of the NESTOR project, and a significant difference with ANTARES, is to arrange the optical modules on a tower structure with all internal connections made on the surface during deployment and to so avoid the need for submarine connections. The towers have 12 hexagonal floors of 16 m in radius with photo-multipliers looking both upward and downward. Test deployments have been performed and many detector elements exist, including a cable connection from the site to the shore.

agency and government. The detector will consist of 80 strings with a total of 4800 optical modules. The detector will have an instrumented volume of about 1 km3 with a threshold of 0.5 - 1.1012 eV. Construction could start in 2003 with the completed detector expected around 2010. For Northern Hemisphere neutrino water Cherenkov detectors there are a number of projects. The BAIKAL group intends to increase significantly the size of their detector for very high energy v, by adding three outrigger strings. Northern Hemisphere neutrino detectors will complement the sky coverage of the South Pole detectors ray. With the assumption that the detector are 100% efficient in the lower hemisphere, a detector at the South Pole observes half the sky all the time, while a detector at 43” north, such as ANTARES, observes part of the sky all the time and part of the sky a fraction of the time. The Northern and Southern detectors together observe all the sky with a significant overlap, however only the Northern detector can observe the centre of the galaxy and the majority of the galaxy disk. 7. Conclusions The existing and upcoming neutrino telescopes promise to open up a new field of astronomy leading to exiting insights into the nature of the most extreme radiation sources in the universe. The same detectors have also many possibilities for original discoveries in searches for dark matter in the form of neutralinos as well as discoveries of other exotic particle species. The Mediterranean deep sea detectors, such as ANTARES, will start giving data within two years and complement in many ways the existing detectors. REFERENCES 1.

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