December 13, 2004 11:51 FILE AMANDA-DPF-2004International Journal of Modern Physics Ac World Scientific Publishing CompanyTHE AMANDA NEUTRINO TELESCOPEANDREA SILVESTRI for the AMANDA CollaborationDepartment of Physics and Astronomy, University of California, Irvine, CA 92697, USAReceived (Day Month Year)Revised (Day Month Year)We present recent results from the Antarctic Muon And Neutrino Detector Array(AMANDA), located at the South Pole in Antarctica. AMANDA-II, commissioned in2000, is a multipurpose high energy neutrino telescope with a broad physics and as-trophysics scope. We summarize the results from searches for a variety of sources ofultra-high energy neutrinos: TeV-PeV diffuse sources by measuring either muon tracksor cascades, neutrinos in excess of PeV by searching for muons traveling in the down-going direction and point sources.Keywords: Neutrino Detector; Neutrino Telescopes; Neutrino Astronomy; Antarctic Ice;Ultra High Energy Neutrinos; AMANDA.IntroductionAMANDA is the first neutrino telescope constructed in transparent ice, and de-ployed between 1500 m and 2000 m beneath the surface of the ice at the geographicSouth Pole in Antarctica. It is designed to search for neutrinos that originate in themost violent phenomena in the observable universe. Galactic objects like SupernovaRemnants and extragalactic objects such as Active Galactic Nuclei and gammaray bursts (GRB) are expected to be the powerful engines accelerating protonsand nuclei to the highest energies, which eventually interact to generate neutri-nos. Although not explicitly discussed here, the detector has measured atmosphericneutrinos and searched for neutrinos correlated with GRB1.1. The DetectorThe AMANDA-II neutrino telescope2is an array of 677 Optical Modules (OM)arranged in 19 strings. The best sensitivity of the detector is achieved for neutrinoswith Eνbetween 103-106GeV. Down-going atmospheric-µ’s and up-going ν’s arethe major contribution to the background. The muon tracks are reconstructed witha maximum likelihood approach which models the arrival times and amplitudes ofCherenkov photons recorded by the photomultiplier tubes, achieving an angularresolution of 1.5◦-2.5◦. Above a few PeV, the Earth becomes opaque to neutrinos,and only those moving down or horizontally can reach the detector. Such events1December 13, 2004 11:51 FILE AMANDA-DPF-20042 Andrea Silvestrineed to be separated from large muon bundles from down-going atmospheric airshower events.2. Search for Diffuse Neutrino FluxThis section presents three different methods to search for the diffuse flux of neu-trinos. Fig.1 (left) summarizes the results of the two search methods sensitive toall ν-flavors. The experimental limits assume a 1:1:1 ratio of neutrino flavors atthe Earth due to oscillation. The dotted and dashed lines represent a sample ofmodel predictions, adjusted for oscillation if appropriate, which have been excludedby these analyses (see3,4for details). The leftmost (rightmost) solid line is theAMANDA limit obtained from the cascade (UHE) analysis. The length of the ex-perimental lines corresponds to the energy interval that contains 90% of the signalfor a spectrum proportional to E−2.log10(Eν) (GeV)E2Φ(νe+νµ+ντ) (GeV cm-2s-1sr-1)10-610-54 5 6 7 8 9 10)δsin(0 0.2 0.4 0.6 0.8 1-2 cm-1 Gev s-6 dN/dE / 102E0.050.10.150.20.250.3)δsin(0 0.2 0.4 0.6 0.8 1-2 cm-1 Gev s-6 dN/dE / 102E0.050.10.150.20.250.3Time period20002001200220032000-2003Fig. 1. Left: AMANDA-II limits for the diffuse flux of all ν -flavors calculated for an E−2spec-trum. Right: AMANDA-II sensitivities from a point source as a function of declination angle forE−2spectra above Eν= 10 GeV at 90% C.L. Individual curves are presented for the indicatedyears of operation. Bottom curve includes full data set.The cascade analysis3delivers excellent energy resolution of 0.1-0.2 in log(∆E/E)and improved sensitivity to all ν-flavors. AMANDA is sensitive to ν-induced cas-cades from any direction, but with relatively modest angular resolution of 30◦-40◦.The diffuse flux limit from the cascade analysis is E2Φνall(E) ≤ 8.6 × 10−7GeV ·cm−2· s−1· sr−1derived for an E−2spectrum at 90% confidence level (C.L.). Asindicated in Fig.1 (left), 90% of the signal lies within the energy interval between50 TeV < Eν< 5 PeV. Strategies for the UHE analysis4were developed to sep-arate signal from background based on the large amount of Cherenkov light gen-erated by interactions of neutrinos with Eν> 1015eV. Five events were observed,while 4.6 events were expected. This corresponds to a flux limit for an E−2spec-trum of E2Φνall(E) ≤ 0.99 × 10−6GeV · cm−2· s−1· sr−1at 90% C.L., in theenergy range of 1 PeV < Eν< 3 EeV (see Fig.1 (left)). The energy spectrum ofDecember 13, 2004 11:51 FILE AMANDA-DPF-2004The AMANDA Neutrino Telescope 3atmospheric-ν’s provides a third analysis tool5to search for a diffuse source ofhigh energy neutrinos. The measured atmospheric Eνspectrum extends to 300TeV, and agrees with theoretical parameterizations. Since the spectrum of ob-served events is consistent with atmospheric-ν’s, it was used to set a preliminarylimit on a potential extraterrestrial νµ-flux with spectral dependence of E−2toE2Φνµ(E) ≤ 2.6 × 10−7GeV · cm−2· s−1· sr−1at 90% C.L., in the energy range of100 TeV < Eν< 300 TeV. The combined systematic uncertainty is typically 30%,although the value varies slightly with the analysis method.3. Search for Point SourcesAMANDA-II has surveyed the entire northern sky since 2000 for non-statisticalexcesses in small regions of the sky6. None were found so far. Fig. 1 (right) sum-marizes the preliminary results of a 4-year combined analysis showing the expectedsensitivity, based on the assumption of no signal as a function of declination forE−2spectra above Eν= 10 GeV. A final sample of 3369 events was observed,which is in good agreement with the expected number of 3438 from atmospheric-ν’s. An unbinned sky search was performed for statistical fluctuation. No fluctuationexceeded 3.4σ, which is compatible with random fluctuation in the spatial distribu-tion of atmospheric-ν’s. Due to the large number of search bins a fluctuation of atleast this magnitude would occur in 40% of similar data samples.4. ConclusionAMANDA has yet to observe an extraterrestrial neutrino source, but “she” hasdemonstrated the cost-effectiveness and robustness of the