ANTARES and other Neutrino Telescopes in the Northern Hemisphere.

ANTARES and other Neutrino Telescopes in the Northern Hemisphere

Abstract

Several projects are concentrating their efforts on opening the high energy neutrino window on the Universe with km-scale detectors. The detection principle relies on the observation, using photomultipliers, of the Cherenkov light emitted by charged leptons induced by neutrino interactions in the surrounding detector medium.
In the Northern hemisphere, while the pioneering Baikal telescope, has been operating for 10 years, most of the activity now concentrates in the Mediterranean sea. Recently, the Antares collaboration has completed the construction of a 12 line array comprising 900 photomultipliers.
In this paper we will review the main results achieved with the detectors currently in operation in the Northern hemisphere, as well as the R&D efforts towards the construction of a large volume neutrino telescope in the Mediterranean.

1 Scientific motivations

Neutrino astronomy has a key role to play in the present context towards a multi-messenger coverage of the high energy sky. Neutrinos are indeed special probes since they can escape from the core of the sources and travel with the speed of light through magnetic fields and matter without being deflected or absorbed. They can therefore deliver direct information about the processes taking place in the production sites and reveal the existence of undetected sources.
High energy neutrinos are produced in a beam dump scenario in dense matter via meson (mainly pion) decay, when the accelerated hadrons interact with ambient matter or dense photon fields:

As reported by the Auger experiment [1] , good candidates for high energy neutrino production are active galactic nuclei (AGN) where the accretion of matter by a supermassive black hole may lead to relativistic ejecta [2]. Other potential sources of extra-galactic high energy neutrinos are transient sources like gamma ray bursters (GRB). As many models [3] for GRBs involves the collapsing of a star, acceleration of hadrons follows naturally. The diffuse flux of high energy neutrinos from GRBs [4] is lower than the one expected from AGNs, but the background can be dramatically reduced by requiring a spatial and temporal coincidence with the short electromagnetic bursts detected by a satellite.

High Energy activity from our Galaxy has also been reported by ground based gamma-ray telescopes [5]. Many of these galactic sources [6] are candidates to accelerate hadrons and subsequently produce neutrinos. As explained in section 2, such sources could only be observed by a northern neutrino telescope.

2 Detection principle

The neutrino’s advantage (for astronomy), the weak coupling to matter, is at the same time a big disadvantage (for detection). Huge volumes need to be monitored to compensate for the feeble signal expected from the cosmic neutrino sources. In this context, the water Cherenkov technique offers both a cheap and reliable option.

The detection principle relies on the observation, using a 3 dimensional array of photodetectors, of the Cherenkov light emitted, in a transparent medium, by charged leptons induced by charged-current neutrino interactions in the surrounding detector medium.

Thanks to the large muon path length, the effective detection volume in the muon channel is substantially higher than for other neutrino flavors. The higher the neutrino energy the smaller the deviation between the muon and the neutrino (typically ), thus enabling to point back to the source with a precision close to the one achieved by gamma-ray telescopes. Muon trajectories are reconstructed using the time and amplitude from the photodetector signals. The energy estimate (by a factor of 2-3) of the event can be achieved thanks to the energy deposited in the detector. Cosmic particles penetrating the atmosphere undergo a cascade of many secondary particles. Among them, high energy muons can reach the detector and constitute a very intense source of background. To suppress this background the detector concentrates on upward detection. As a result, the field of view is restricted to one half of the celestial sky, below horizon. Severe quality cuts criteria are then applied to the reconstruction to remove remaining mis-reconstructed muons. Atmospheric neutrinos produced in the atmospheric cascades can travel through the Earth and interact in the detector vicinity. To claim for an extraterrestrial discovery, one has then to search for an excess of events above a certain energy (diffuse flux) or in a given direction (point sources).

3 Current projects and detectors

Figure 1: Worldwild measurements of the vertical down-going muon flux, including the latest result by Antares [22].

The first attempt to build a NT was made by the Dumand collaboration [7], off the Hawaiian coast. In 1987, the collaboration deployed a prototype string tied to a ship and performed some first measurements. After this success a proposal for an array of 9 strings anchored to the sea bed at 4800 m from the surface was submitted. Only the first line could be deployed. It was operational for several hours only before a leak occurred. The project was finally canceled in 1995. New projects were born since then, some in the ice (Amanda and Icecube [8]) providing the detector with mechanical stability and avoiding leakage problems, others persevering with water. Figure 1 summarizes the muon flux measurements of most of these projects.

3.1 The Baikal neutrino telescope

The Baikal NT (NT-200) is located at 1100 m depth in Lake Baikal, Siberia. Deployment and repairs are made in winter, when the lake is covered by an ice shield. The detector with a diameter of 40 m, consists of 192 OMs on eight 72 m long strings (see figure 2). It is running in permanent regime since April , 1998.

The best limit on the flux of diffuse extragalactic neutrinos is obtained with a data sample of 1038 live days [9](see figure 3). Since directional information is not of primary importance for diffuse fluxes, the signal expected is an upward moving light front induced by isolated cascades originated by any of the 3 flavored neutrinos. To increase the sensitivity, no containment in the detector is required. The associated background is mainly bremsstrahlung showers initiated by high energy downward atmospheric muons. The final rejection criteria relies on the energy estimate of the event. A total of 372 upward going neutrino candidates were selected while 385 where expected by Monte Carlo simulations, with a median muon-angular resolution of .

Figure 2: From Left to right: conceptual layouts of the Baikal, Nestor and Nemo NT.
Figure 3: Current limits (plain line), sensitivities (dash-line) and prediction (W&B) [10] on high energy diffuse neutrino fluxes.

Benefiting from a low energy threshold, close to 10  GeV (because of the high OM density), the Baikal collaboration also reports interesting limits regarding WIMP as possible cold dark matter. Vertical neutrino events eventually produced by neutralino annihilations in the centre of the Earth have been looked for. In absence of excess, a limit on the muon flux as a function of the neutralino mass has been placed close to for a neutralino mass of 100 GeV/ (1 TeV/).

An extension of the detector called NT-200+ has been proposed and put into operation in April 2005. It consists of 36 additional OMs on 3 further 140 m long strings. It is expected that a factor of 4 in sensitivity can be reached by this modest extension. The construction of NT200+ is a first step towards the deployment of a km-scale (Gton) neutrino telescope. Such a detector could be made of similar detection units as NT200+, replacing the NT200 core by a single string. The detection units would be around 100 m distant from each other. The final detector would contain 1300-1700 OMs arranged on 90-100 strings with 12-16 OMs each. The thresholds for muons would then become 20-50 TeV while for 100 TeV cascades the effective volume would reach 0.5-0.8 . Such an extension requires new R&D efforts. A prototype string meant to test the future technology (photomultipliers, electronics, acquisition system, trigger strategy, calibration elements) has been installed in April 2008 as part of the NT200+ array.

3.2 The Nestor project

The Nestor project -the first in the Mediterranean sea- started in 1989. The immersion site is located next to Pylos, on the Greek Ionian coast, at a depth of about 4000 m. The structural part of the detector array is a tower rising 410 m from a sea-bed anchor. As shown in figure 2, the tower consists of twelve 6 arm titanium stars of 16 m radius [11]. A pair of OMs [12] housing 15” photomultipliers in a pressure resistant glass sphere is installed at the extremity of each arm. The full tower will hold 144 PMTs. The full sampled digitized wave-form is sent to shore for signal analysis through a 31 km long cable deployed in June 2000. In January 2002 an electro-optical junction box and other devices dedicated to environmental survey were added. All cable connections are made on the boat during deployment.
The first Nestor floor prototype -a reduced star (5 m radius) with 12 OMs- has been immersed in March 2003 [13]. More than 5 million triggers (4 fold coincidences) have been recorded. A reconstruction has been made to extract the zenithal dependency of atmospheric muons as well as a measurement of the muon flux [14]. These results validate the chosen techniques, even if the collaboration still has to demonstrate its ability to deploy a full scale rigid tower. This could be achieved with the recent construction of a dedicated vessel called “Delta-Bereniki”, made of triangular ballasted platform of 275 tons equipped with engines and thrusters for positioning.

3.3 The Nemo project

The Nemo collaboration was formed in 1998 and has close ties to the Antares experiment. The collaboration has located a suitable detector site, in Capo Passero, at a depth of 3500 m, 80 km off the eastern coast of Sicily, featuring low concentration of bioluminescent bacteria [15]. The Nemo conceptual design of a cubic kilometer NT calls for 64 semi-rigid towers (which minimizes the number of in situ connections) . Each of these towers would spread 16 storeys by 40 m vertical to a total length of 750 m. A sketch is also shown in figure 2. Each storey is composed of a 20 m long rigid arm that carries two OMs at each end and hosts instrumentation for positioning and environmental parameter monitoring. The structure is designed to be assembled and deployed in a compact configuration. Once unfurled the floors assume an orthogonal orientation with respect to their vertical neighbors. Communication to the shore is foreseen to be handled by one main and eight secondary junction boxes.
In order to validate the technology proposed for a km-scale detector, a Phase-1 project was launched in 2002 and achieved, in December 2006 with the operation, in a test site (2100 m depth, 25 km off Catania port) of a prototype mini-tower equipped with all the critical components of a NT. Unfortunately, after 4 months of immersion, an attenuation in the optical fibers transmission in the junction box was reported. A short-circuit finally occurred in May 2007 (forcing the recovery, reparation, and re-immersion in April 2008 of the junction box). In the mean time, a poor manufacturing process caused a loss of buoyancy to the mini tower which slowly sank down to the sea bed. Nevertheless, a small sample of down going muon events was analyzed (for a total livetime of 200 hours). Reconstructed tracks were compared to Monte Carlo simulations, revealing a good agreement in the shape of the zenith distributions [16].
The Nemo project has now entered the phase-2, that aims at operating a full scale tower in the Capo Passero site. The design of the tower has been modified according to the experience gained with the phase-1 project. A 100 km long Alcatel communication cable has been installed in July 2007. It links the 3500 m deep sea site to a brand new shore station inside the harbor area of Portopalo. The deep sea termination of the cable, which includes a 10 kW DC/DC converter system is under realization and should be installed early 2009, followed by the immersion and connection of the tower, in spring 2009.

3.4 First results from the Antares Nt

Antares is the first large scale NT constructed in the deep sea. It is located at depth of 2475 m,  km off La-Seyne-sur-Mer (Var, French Riviera) where the shore station is installed. The deployment stage ended in May 2008. The detector is currently running in the complete configuration. Here we give a short description of the detector and focus on the results obtained with the first 5/12 of the detector which took data from February to December 2007.

The Antares detector

The final detector consists of an array of 12 flexible individual mooring lines separated from each other on the sea bed by 60-80 m. The lines are weighted to the sea bed and held nearly vertical by syntactic-foam buoys. Each line is equipped with 75 optical modules [18]. The OMs are inclined by with respect to the vertical axis to ensure maximum sensitivity to upward moving Cherenkov light fronts. A fifth of the line is dedicated for the study of the acoustic background noise for further possible acoustic neutrino detection. This part is known as the AMADEUS system [19].
The sea water properties have been extensively studied revealing low light scattering, mainly forward and an average optical background (induced by bacteria and decays) of 70 kHz per detection channel. Though the optical activity is highly variable, long time measurements have now confirmed these numbers [17]. A mini-instrumented line equipped with 3 OMs and mainly dedicated to study environmental parameters has been in operation since spring 2005. The results are presented into details in [20].

DAQ and calibration

The default readout mode [21] of the detector is the transmission of the time and amplitude of any light signal above a threshold corresponding to 1/3 of a photo-electron for each OM. All signals are sent to shore and treated in a computer farm to find hit patterns corresponding to muon tracks or other physics events producing light in the water. Time measurements are relative to a master reference clock signal distributed from shore. Timing calibration is ensured by a dedicated network of laser and LED beacons. The line motions are monitored by acoustic devices and by inclinometers regularly spread along the line, allowing redundancy. The system provides a location of each OM with a precision close to 10 cm. Together with the data from the time calibration devices, this guarantees an angular resolution within design expectations () and allow to look for point source with high sensitivity, as reported in the next section.

First deep-sea neutrinos

Figure 4: The elevation angle distribution of data taken with the 5 line detector, compared to simulations. The colored band accounts for systematic uncertainties mainly due to Monte Carlo model uncertaintis, OM acceptance and attenuation length.

Several algorithms are available within the collaboration for the reconstruction of a muon trajectory. Some are specialized for reconstruction with a single line and can therefore be applied for search of vertical muons (figure 1), others can be used for search of through-going muons. A recent algorithm has been developed which can be used on the data before the full alignment of the detector.The algorithm can therefore run quasi-online. The result from this algorithm is in good agreement with Monte Carlo simulations, as shown in figure 4. The sample comprises 168 neutrino candidates recorded over 167 active days.

A sub-set of these data set has been used to search for steady point sources by looking for clusterization of events on top of the atmospheric background. This required the full alignment of the detector and a more sophisticated reconstruction strategy so to match the expected pointing accuracy. In order to prevent from bias, the analysis was first performed by scrambling the time of the events in declination bands, thus providing realistic background samples. An unbinned method [23] was then applied to look for clusters in the direction of known sources. In absence of excess, limits could be achieved as indicated on figure 5. Though these limits have not been obtained with the final configuration of the detector, they already supersede the ones obtained after many years of operation by first generation experiments.

Figure 5: Result of the search for high energy neutrinos from known point sources with the Antares 5-line detector. The plain line is the one year sensitivity of the full detector.

3.5 KM3NeT: toward a km-scale NT in the Mediterranean.

The three existing collaborations Antares, Nemo and Nestor have formed the KM3NeT consortium with the goal to design and locate the future Mediterranean km-scale detector. Funding was granted by the EU in 2 different stages.
First, a 3-year design phase started in February 2006, at the end of which the consortium will deliver a Technical Design Report defining the technological solutions for the construction. Minimum requirements are an instrumented volume , sensitive to all neutrino flavors and with an angular resolution for muon neutrinos of about in the TeV range. These requirements are listed into details in the “conceptual design report” [25] published early 2008. Different possible detector configurations have been investigated in detailed simulation studies [24].
As a second stage, a preparatory phase has started in March 2008 meant to address political, governance and financial issues related to the KM3NeT detector, including the site selection. The preparatory phase will also include prototyping work. The construction of the detector should start in 2012. The overall cost is estimated to be 220-250 Meuros. KM3NeT is part of the ESFRI (European Strategic Forum on Research Infrastructures) road map for future large scale infrastructures. The project enlarges indeed the scope to an international and multidisciplinary endeavor. It is foreseen to instrument the detector with specialized equipment for seismology, gravimetry, radioactivity, geomagnetism, oceanography and geochemistry, making KM3Net a complex laboratory for a large science community.

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