IceCube
IceCube Neutrino Observatory

PDD - Status of High Energy Neutrino Astronomy

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4 Status of High Energy Neutrino Astronomy
  • 4.1 Status of AMANDA
    • 4.1.1 Atmospheric Neutrinos
    • 4.1.2 Pointing Resolution
    • 4.1.3 Search for a diffuse high energy neutrino
    • 4.1.4 Point Sources
    • 4.1.5 Gamma-Ray Bursts
    • 4.1.6 WIMPs
    • 4.1.7 Supernovae
    • 4.1.8 Magnetic Monopoles
    • 4.1.9 Summary of AMANDA Status

4 Status of High Energy Neutrino Astronomy

The science of high energy neutrino astronomy is compelling. The main challenge is therefore to develop a reliable, expandable and affordable detector technology. The diagram in fig. 4 shows

Figure 4: Schematic representation [70] of the physics reach of various types of detectors in detector mass versus neutrino energy space.

schematically the range of various detectors in the space of volume vs. neutrino energy. IMB and Kamioka are so far the only detectors that have observed neutrinos from outside the solar system, with the detection of SN1987A. With its large volume and great sensitivity, Super-Kamiokande has pushed the frontiers of the study of atmospheric neutrinos in the sub-GeV and multi-GeV energy range, SNO and Super-Kamiokande have done the same with solar neutrinos, with SNO recently providing the first clear evidence of solar neutrino oscillations. There is significant activity with several underground detectors, including Borexino and KamLand, pushing toward lower energy and higher energy resolution on the solar neutrino frontier. Super-Kamiokande, along with Frejus, MACRO and Soudan, have also provided important limits on fluxes of high energy neutrinos.

With the termination of the pioneering DUMAND experiment, the efforts in water are, at present, spearheaded by the Baikal experiment [71]. The Baikal Neutrino Telescope is deployed in Lake Baikal, Siberia, 3.6 km from shore at a depth of 1.1 km. An umbrella-like frame holds 8 strings, each instrumented with 24 pairs of 37-cm diameter QUASAR photomultiplier tubes (PMT). Two PMTs in a pair are switched in coincidence in order to suppress background from natural radioactivity and bioluminescence. Operating with 144 optical modules since April 1997, the NT-200 detector has been completed in April 1998 with 192 optical modules (OM). The Baikal detector is well understood, and the first atmospheric neutrinos have been identified.

The Baikal site is competitive with deep oceans, although the smaller absorption length

Figure 5: Angular distribution of muon tracks in the Lake Baikal NT-200 experiment after all final cuts have been applied [73].

of Cerenkov light in lake water requires a somewhat denser spacing of the OMs. This does, however, result in a lower threshold which may be a definite advantage, for instance for oscillation measurements and WIMP searches. They have shown that their shallow depth of 1 km does not represent a serious drawback. By far the most significant advantage is the site with a seasonal ice cover which allows reliable and inexpensive deployment and repair of detector elements from a stable platform.

With data taken with the Baikal NT-200 detector, the Baikal collaboration has shown that atmospheric muons can be reconstructed with sufficient accuracy to identify atmospheric neutrinos, as illustrated in fig. 5. The neutrino events are isolated from the cosmic ray muon background by imposing a restriction on the chi-square of the Cerenkov fit, and by requiring consistency between the reconstructed trajectory and the spatial locations of the OMs reporting signals.

In the following years, NT-200 will be operated as a neutrino telescope with an effective area between 103∼ 5 × 103 m2, depending on energy. Presumably too small to detect neutrinos from extraterrestrial sources, NT-200 will serve as the prototype for a larger telescope. For instance, with 2000 OMs, a threshold of 10 ∼ 20 GeV and an effective area of 5 × 104 ∼ 105 m2, an expanded Baikal telescope would fill the gap between present underground detectors and planned high threshold detectors of km3 size. Its key advantage would be low threshold.

The Baikal experiment represents a proof of concept for deep ocean projects. These have the advantage of larger depth and optically superior water. Their challenge is to find reliable and affordable solutions to a variety of technological challenges for deploying a deep underwater detector. Several groups are confronting the problem; both NESTOR and ANTARES are developing rather different detector concepts in the Mediterranean. The NESTOR collaboration [74], as part of a series of ongoing technology tests, is testing the umbrella structure which will hold the OMs. They have already deployed two aluminum "floors," 34 m in diameter, to a depth of 2600 m. Mechanical robustness was demonstrated by towing the structure, submerged below 2000 m, from shore to the site and back. These tests should soon be repeated with fully instrumented floors. The actual detector will consist of a tower of 12 six-legged floors vertically separated by 30 m. Each floor contains 14 OMs with four times the photocathode area of the commercial 8 inch photomultipliers used by AMANDA and ANTARES.

The detector concept is patterned along the Baikal design. The symmetric up/down orientation of the OMs will result in uniform angular acceptance and the relatively close spacings in a low threshold. NESTOR does have the advantage of a superb site off the coast of Southern Greece, possibly the best in the Mediterranean. The detector can be deployed below 3.5 km relatively close to shore. With the attenuation length peaking at 55 m near 470 nm the site is optically superior to that of all other deep water sites investigated for neutrino astronomy.

The ANTARES collaboration [72] is investigating the suitability of a 2400 m-deep Mediterranean site off Toulon, France. The site is a trade-off between acceptable optical properties of the water and easy access to ocean technology. Their detector concept indeed requires remotely operated vehicles for making underwater connections. First results on water quality are very encouraging with an attenuation length of 40 m at 467 nm and a scattering length exceeding 100 m. Random noise exceeding 50 khz per OM is eliminated by requiring coincidences between neighboring OMs, as is done in the Lake Baikal design. Unlike other water experiments, they will point all photomultipliers sideways in order to avoid the effects of biofouling. The problem is significant at the Toulon site, but only affects the upper pole region of the OM. Relatively weak intensity and long duration bioluminescence results in an acceptable deadtime of the detector. They have demonstrated their capability to deploy and retrieve a string, and have reconstructed down-going muons with 8 OMs deployed on the test string.

With the study of atmospheric neutrino oscillations as a top priority, they had planned to deploy in 2001-2003 10 strings instrumented over 400 m with 100 OMs. After study of the underwater currents they decided that they can space the strings by 100 m, and possibly by 60 m. The ANTARES detector will consist of 13 strings, each equipped with 30 storeys and 3 PMTs per storey. The large photocathode density of the array will allow the study of atmospheric neutrino oscillations in the range 255 < L=E < 2550kmGeV??1 with neutrinos in the energy range 5 < Eν < 50 GeV. This detector will have an area of about 3 × 104m2 for 1 TeV muons–similar to AMANDA-II–and is planned to be fully deployed by the end of 2003.

A new R&D initiative based in Catania, Sicily has been mapping Mediterranean sites, studying mechanical structures and low power electronics. One must hope that with a successful pioneering neutrino detector of 10-3 km3 in Lake Baikal, a forthcoming 10-2km3 detector near Toulon, the Mediterranean efforts will converge on a 10-1km3 detector possibly at the NESTOR site.

As in many other fields, high energy neutrino astronomy would ideally have two or more independent experiments sensitive to the same energy regime. Such redundancy allows one to perform vital crosschecks and (hopefully) discovery verification. It is therefore in the best interests of the community that projects other than AMANDA and IceCube succeed. In addition, a detector in the northern hemisphere would provide the community with full TeV–PeV neutrino sky coverage, while at the same time having considerable coverage overlap regions.