Mem. S.A.It. Suppl. Vol. 9, 363c© SAIt 2006

Memorie della

Supplementi

Experimental high energy neutrino astronomy

E. Migneco1,2

1 Istituto Nazionale di Fisica Nucleare – Laboratori Nazionali del Sud, Via S. Sofia 62,I-95123 Catania, Italy

2 Department of Physics and Astronomy, Via S. Sofia 64, I-95123, Catania, Italy e-mail:migneco@lns.infn.it

Abstract. Neutrinos are considered promising probes for high energy astrophysics. Manyindications suggest, indeed, that powerful cosmic accelerators such as AGNs, GRBs andmicroQSOs can produce high energy neutrinos. Travelling in straight line, without beingabsorbed, these particles reach the Earth carrying direct information on the source. Theunderwater/ice Cherenkov technique is widely considered the most promising experimentalapproach to build high energy neutrino detectors. The first generation detectors, BAIKALand AMANDA, already set constraints on astrophysical source models for TeV neutrinoproduction. The construction of larger (km2) size detectors is started: in the South Pole,the ICECUBE neutrino telescope is going to be deployed. In the Northern Hemispherethree Collaborations (ANTARES, NEMO and NESTOR) are conducting R&D to install aneutrino telescope in the Mediterranean Sea.

Key words. Astronomy: high energy neutrino – Detectors: underwater/ice neutrino tele-scopes

1. Introduction

Several theoretical models predict that ultrahigh energy cosmic rays (UHECR), detectedby several experiments, are protons acceler-ated in cosmic objects, e.g. GRBs and AGNs,where Fermi acceleration mechanism takesplace. Such protons interacting with ambientgas or radiation can also produce high energyneutrinos from the decay of charged pions.Gaisser, Halzen & Stanev (1995); Waxman &Bahcall (1999). The flavour ratio at the sourceis νe : νµ : ντ = 1 : 2 : 0. Neutrino oscillationschange the ratio at the Earth into 1 : 1 : 1, sinceapproximately one half of the muon neutrinosconvert to tau neutrinos over cosmic baselines.

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Light and neutral neutrinos are optimalprobes for high energy astronomy, i.e. for theidentification of astrophysical sources of highenergy particles. Differently from charged par-ticles and gamma rays (Eγ >TeV) neutrinoscan indeed reach the Earth from far cosmicaccelerators in straight line, allowing sourcedetection. To fulfill this goal neutrino detec-tors must be design to optimise reconstruc-tion of particle direction and energy, thus theyare commonly referred as neutrino telescopes.High energy neutrinos are detected indirectlyfollowing weak Charged Current (CC) interac-tion with nucleons in matter and the productionof a charged lepton in the exit channel of the re-action. The low νN cross section (σνN ' 10−35

cm2 at '1 TeV) and the expected astrophysi-

364 Migneco: Experimental high energy neutrino astronomy

cal ν fluxes intensities require a ν interactiontarget greater than 1 GTon (1 km3 of water).In early ’60s the use of natural water (lake orseawater or polar ice) was proposed to detectneutrinos using the optical Cerenkov techniqueto track the charged lepton outgoing the νN in-teraction. Underwater neutrino telescopes arelarge arrays of optical sensors (typically photo-multipliers tubes of about 10” diameter) whichpermit to track charged leptons in water bytiming the Cerenkov wavefront emitted by theparticles. Search for neutrino point sources re-quires good pointing resolution, this can be ob-tained looking at the νµ channel. Muons areemitted co-linearly with the in-coming neu-trino and have long range in water (kilometresat TeV energy, tens of kilometres at EeV en-ergy). Muons emit, along their track, Cerenkovradiation that is detected by the lattice of PMTs(Photomultiplier Tubes). The Cerenkov wave-front, then the muon direction, is reconstructedusing the information of photon hits and PMTpositions. At PeV (1015 eV) muons generateshowers by bremsstrahlung and pair produc-tion increasing the number of Cerenkov pho-tons radiated in water. This is a good estima-tor for the muon energy. Due to the extremelyhigh cosmic muon background, originated bycosmic ray interaction in the atmosphere, neu-trino telescopes are optimised to observe up-going events: only neutrinos can, indeed, crossthe Earth and generate a muon event close orinside to the detector.

Apart from muon tracks, electron cascades(originated by νe) can also be detected. In theenergy range of interest cascade develops forfew tens of meters in water, thus can be consid-ered as quasi point-like compared to the spac-ing of OMs. While energy reconstruction isgood for these events, the total amount of lightradiated is proportional to electron energy, thereconstruction of the track direction is poor, ∼tens of degrees.

At neutrino energies above 1 PeV, the Earthis opaque to neutrinos all neutrino flavours ex-cept tau, which undergo regeneration (ντ →τ + X → ντ + X). Tau events can be identifiedwhen both ντ interaction and τ decay happenwithin the detector originating two cascades(”double bang” event). Since this signature is

rare, the search for astrophysical neutrinos isconcentrated on events close to the horizon,where the atmospheric background is low.

2. The operating detectors: BaikalNT-200 and AMANDA

After the pioneering work carried out byDUMAND offshore Hawaii Island (Robertset al. 1992), Baikal was the first collabora-tion which installed an underwater neutrinotelescope and, after more than ten years ofoperation, it is still the only neutrino tele-scope located in the Northern Hemisphere. TheBAIKAL NT-200 is an array of 200 PMTs,moored between 1000 and 1100 m depthin lake Baikal (Russia) (Belolaptikov et al.1997). BAIKAL is a high granularity detec-tor with a threshold Eµ ' 10 GeV and an es-timated effective detection area ≤ 105 m2 forTeV muons. The limited depth and the pooroptical properties of water (light transmissionlength of 15÷20 m, high optical backgroundrate due to bioluminescence) limit the detec-tor performances as a neutrino telescope. TheCollaboration has successfully measured theatmospheric neutrino flux and set a 90% c.l.upper limit for diffuse astrophysical neutrinofluxes of E2

νΦν < 4 × 10−7 cm−2 s−1 sr−1 GeV(Wischnewski et al. 2005). The detector is go-ing to be improved with an external array offew tens PMTs that will enlarge the detectionvolume for high energy cascades.

AMANDA is currently the largest neutrinotelescope installed (Andres et al. 2001). Inthe present stage, named AMANDA II, the de-tector consists of 677 optical modules (OM)pressure resistant glass vessel hosting down-ward oriented PMTs and readout electronics.OMs are arranged in 19 vertical strings, de-ployed in holes drilled in ice between 1.3 and2.4 km depth. Vertical spacing between OMs is10÷20 m, horizontal spacing between stringsis 30÷50 m. The ice optical properties havebeen mapped as a function of depth: at detectorinstallation depth the average light (λ = 400nm) absorption length is La ' 100 m, the ef-fective light scattering length is Lb ' 20 m.This makes AMANDA a good calorimeter forastrophysical events, with a resolution 0.4 in

Migneco: Experimental high energy neutrino astronomy 365

log(E) for muons and 0.15 in log(E) for elec-tron cascades. The detector angular resolutionis between 1.5◦ and 3.5◦ for muons and ' 30◦for cascades. AMANDA data have permittedto measure for the first time the upgoing atmo-spheric neutrino spectrum in the energy rangefrom few TeV to 300 TeV (see figure 1).

Fig. 1. Atmospheric neutrino energy spectrum(preliminary) from AMANDA data, comparedto the Frejus spectrum (Daum et al. 1995) atlower energies. The two solid curves indicatemodel predictions Volkova (1980) for the hor-izontal (upper) and vertical (lower) flux.

The atmospheric neutrino spectrumrecorded by AMANDA-II was then used toset a 90% c.l. upper limit on a diffuse muonneutrino flux of E2

νµΦνµ < 2.6×10−7 GeV cm−2

s−1 sr−1 (100 < Eν < 300 TeV). AMANDAcan also detect cascades originated by theleptonic vertex of electron and tau neutrinocharged current interactions, and hadronicvertex cascades from all-flavor neutral currentinteractions. The measured 90% c.l. upperlimit with respect to the flux of all three flavorsis Φνe+νµ+ντE

2ν = 8.6 · 10−7cm−2s−1sr−1GeV

(50 TeV < Eν < 5 PeV) Ackermann etal. (2004). No excess above background is

observed for horizontal events, the measured90% c.l. upper limit on all flavors neutrino fluxis Φνe+νµ+ντE

2ν < 9.9 × 10−7cm−2s−1sr−1GeV

(1 PeV < Eν < 3 EeV) (Ackermann et al.2005a).

For point sources AMANDA reached asensitivity E2

νµΦνµ ' 7 × 10−8 GeV cm−2 s−1

calculated over 807 days live time (years 2000-2003) (Ackermann et al. 2005b). The searchfor 33 pre-selected sources yielded no evidencefor extraterrestrial point sources. The strongestexcess was observed from the direction of theCrab nebula, with 10 events where 5 were ex-pected (Spiering 2005). A much larger arraylike a km3 scale detector is therefore necessaryto detect astrophysical sources.

3. The future km3 neutrinotelescopes

AMANDA and BAIKAL have demonstratedthe possibility to use the Cerenkov techniqueto track muons induced by Eν >100 GeV neu-trinos. This success opened the way to the con-struction of km3 neutrino telescopes such asIceCube (Ahrens et al. 2003), that will extendthe AMANDA detector at South Pole.

3.1. IceCube

The IceCube telescope will be the natural ex-tension of AMANDA to the km3 size. Whencompleted (expected in 2010), it will consist of4800 downward looking PMTs arranged in 80strings. The distance between PMTs will be 16m with a spacing of 125 m between the strings.IceCube will be able to identify muon tracksfrom muon neutrinos with Eνµ > 100 GeV.Simulations run by the IceCube Collaborationshow that in three years of live time, the detec-tor will reach a sensitivity E2

νµΦνµ = 4.2× 10−9

GeV cm−2 s−1 sr−1 for a diffuse E−2ν neutrino

spectrum and a sensitivity E2νµ

Φνµ = 2.4× 10−9

GeV cm−2 s−1 for a point-like source (Ahrenset al. 2003). Besides, IceCube will be able todetect cascades from electron neutrinos withEνe > 10 TeV and from tau neutrinos withEντ > 1 PeV.

366 Migneco: Experimental high energy neutrino astronomy

It is also worthwhile to mention that theunder-ice detectors are not affected by ra-dioactive and biological optical noise whichis present in natural sea or lake waters. Thismakes them suitable for the search of low en-ergy neutrino fluxes from Galactic SuperNovaexplosions.

3.2. The Mediterranean km3

The contemporary observation of the full skywith at least two neutrino telescopes in oppo-site Earth Hemispheres is an important issue. Atelescope in the Northern Hemisphere, has alsothe possibility to observe the Galactic Centre(not seen by IceCube), already observed to bean intense TeV gamma source (Aharonian etal. 2004) and the sources. In the NorthernHemisphere a favourable region is offered bythe Mediterranean Sea, where several abyssalsites (> 3000 m) close to the coast are presentand where it is possible to install the detectornear scientific and industrial infrastructures.An underwater detector offers also the possibil-ity to be recovered, maintained and/or recon-figured. Detector installation at depth ≥3000 mwill reduce the atmospheric muon backgroundby a factor ≥5 with respect to 2000 m depth,where IceCube is located. The long light scat-tering length of the Mediterranean abyssal sea-water preserve the Cerenkov photons direc-tionality and will permit excellent pointing ac-curacy (order of 0.1◦ for 10 TeV muons). Onthe other hand the light absorption length inwater is shorter than in ice, then it reduces thephoton collection efficiency for a single PMT.In the following we discuss the project con-ducted by three collaborations operating in theMediterranean Sea (ANTARES, NEMO andNESTOR).

NESTOR (NESTOR web page 2000),the first Collaboration that operated in theMediterranean Sea, proposes to deploy a mod-ular detector at 3800 m depth in the IonianSea, near the Peloponnese coast (Greece). Eachmodule is a semi-rigid structure (the NESTORtower), 360 m high and 32 m in diameter,equipped with ' 170 PMTs looking both inupward and downward directions. After a longR&D period, in March 2003 NESTOR has suc-

cessfully deployed 1 tower floor, with a diam-eter of 12 m and equipped with 12 PMTs. Thismodule was deployed at a depth of 3800 m inorder to test the overall detector performanceand particularly that of the data acquisition sys-tems. It acquired, on-shore, underwater opti-cal noise and cosmic muon signals for about1 month. From data acquired during this time,745 atmospheric muon events have been recon-structed, making it possible to measure the cos-mic ray muon flux as a function of the zenithangle (Aggouras et al. 2005). In the next fu-ture the Collaboration aims at the deploymentof the first tower with 2× 104 m2 effective areafor Eµ >10 TeV muons.

ANTARES will be a demonstrator neutrinotelescope with an effective area of 0.1 km2 forastrophysical neutrinos (ANTARES web page2000). It will be located in a marine site near

Toulon (France), at 2400 m depth. ANTARESwill be a high granularity detector consistingof 12 strings, each equipped with 25 equidis-tant stories made of 3 PMTs (total 75), placedat an average distance of 60 m. The PMT are45◦ downward oriented, in order to avoid theirobscuration by sediments and bio-fouling. Thewhole detector installation is scheduled to becompleted in 2007 (Korolkova et al. 2004).After a first operation of two prototypal lines inspring 2003, a new improved version of a shortline instrumented with oceanographic sensorsand optical modules has been put in operationin April 2005 and it is taking data since then.Data recorded by optical modules show an un-expectedly high optical background rangingfrom 60 to several hundreds kHz, well abovethe one produced by 40K decay, then probablydue to bioluminescence.

The NEMO Collaboration was formed in1998 with the aim to carry out the neces-sary R&D towards the km3 neutrino telescope.The activity has been mainly focused on thesearch and characterization of an optimal sitefor the installation and on the development ofa feasibility study of the detector (Migneco etal. 2004). NEMO has intensively studied theoceanographic and optical properties in sev-eral deep sea (depth ≥ 3000 m) sites close theItalian coast. Results indicate that a large re-gion located 80 km SE of Capo Passero (Sicily)

Migneco: Experimental high energy neutrino astronomy 367

is optimal for the installation of the km3 detec-tor. The bathymetric profile of the region is ex-tremely flat over hundreds km2, with an aver-age depth of ' 3500 m. Deep sea currents are,in the average, as low as 3 cm s−1, and neverstronger than 12 cm s−1. The average value ofblue (λ =440 nm) light absorption length isLa = 66 ± 5 m. The same device measuredLa(440nm)' 48 m in Toulon site (Riccobeneet al. 2003) and La(488nm)= 27.9 ± 0.9 m inBaikal lake (Balkanov et al. 2003). The op-tical background noise was also measured at3000 m depth in Capo Passero. Data show thatoptical background induces on 10” PMTs (0.5s.p.e.) a constant rate of 20 ÷ 30 kHz (com-patible with the one expected from 40K de-cay), with negligible contribution of biolumi-nescence bursts. These results were confirmedby biological analysis that show, at depth>2500, extremely small concentration of dis-solved bioluminescent organisms (Riccobeneet al. 2003).

Concerning the detector design, NEMOproposes an innovative structure to host OMs:the NEMO tower. It is designed to deploy,during a single operation, a large number ofPMTs (≥ 60) arranged in a 3-dimensionalshape in order to locally permit event trig-ger and track reconstruction (Musumeci et al.2003). The structure is mechanically flexible,

being composed by a sequence of 16 ÷ 20 sto-ries hosting OMs, interlinked by a net of syn-tectic fiber ropes. Each storey is 15 ÷ 20 mlong and hosts two optical modules (one down-ward looking and one looking horizontally) ateach storey end. The vertical inter-spacing be-tween stories is ' 40 m. One of the detectorgeometries proposed by the Collaboration isthe NEMO140dh. It consists of a squared of9 × 9 NEMO towers equipped with 5832 op-tical modules (10” diameter PMTs). The dis-tance between the towers is 140 m. This con-figuration reaches an effective area of 1 km2 atmuon energy of about 10 TeV, with an angu-lar resolution lower than 0.1 degrees (Sapienzaet al. 2005). Figure 2 shows a preliminary re-sult of the expected sensitivity to a E−2

ν muonneutrino spectrum coming from a point-likesource, obtained for a search bin of 0.3 degree(Distefano et al. 2005). The same figure re-

ports, as a comparison, the sensitivity of theIceCube detector, obtained for a search bin of 1degree (Ahrens et al. 2003). The NEMO140dhdetector has, therefore, a better sensitivity andsmaller search bin, allowing a better identifica-tion of point-like sources.

Fig. 2. Preliminary result of the NEMO140dhsensitivity to a E−2

ν muon neutrino spectrumcoming from a point-like source (Distefano etal. 2005), as a function of years of data tak-ing, and comparison with the IceCube detector(Ahrens et al. 2003).

As an intermediate step towards the km3,the Collaboration has decided to realize a tech-nological demonstrator including most of thecritical elements of the proposed detector. Thisproject, called NEMO Phase 1 (Migneco et al.2004), is under realization at the Underwater

Test Site of the Laboratori Nazionali del Sudin Catania. The facility consists of a shore in-frastructure, a 28 km long electro-optical ca-ble, connecting the shore to the abyssal testsite, and the underwater laboratory. The shorebuilding hosts the land termination of thecable, mechanics and electronics workshops,the data acquisition room and power suppliesfor underwater instrumentation. The 28 kmlong electro-optical (e.o.) cable that connectsthe DAQ room to deep sea is a telecom ca-ble, containing 10 optical fibers and 6 elec-trical conductors (3 mm2) protected with ex-ternal steel armor, specifically designed forhigh bandwidth data transmission and power

368 Migneco: Experimental high energy neutrino astronomy

Fig. 3. The titanium frame installed on Test Site.The ROV operable electro-optical connectors arevisible on the front panel. The hydrophones andelectronics housing of the acoustic station are alsoshown (see text). The frame height is about 3.5 m)

feeding of deep sea installation. A sea cam-paign was conducted on January 2005 to in-stall, on the cable, submarine terminationswith electro-optical connectors mounted on ti-tanium frames. The frames were deployed onthe seabed at ∼ 2050 m depth (see figure 3).The installed connectors permit to plug and un-plug underwater experimental instruments bymeans of Remotely Operated Vehicles (ROV)avoiding further operations on the main e.o.cable. During the sea campaign two experi-mental devices were deployed and connected.The INGV (Istituto Nazionale di Geofisica eVulcanologia, Italy) attached the seismic mon-itoring station Submarine Network 1, whichis the first node of the ESONET project. TheNEMO Collaboration operated an array of hy-drophones for the measurement of the deep seaacoustic background . The installation is fullyoperational and transmits data to shore. Thecompletion of Phase 1 is foreseen by the endof 2006.

4. Conclusions

The first generation of underwater/iceCerenkov neutrino telescopes, BAIKAL andAMANDA, are successfully running. Theatmospheric neutrino spectrum was measured

up to 100 TeV demonstrating the feasibilityof the high energy neutrino detection. Thesedetectors have also set first constraints onastrophysical models which foresee TeVneutrino production in cosmic sources. Theforthcoming km3 neutrino telescopes willbe discovery detectors that could widen theknowledge of the Universe. In the SouthPole the km3 detector IceCube detector isunder construction, extending the AMANDAdetection area. The detector will be completedwithin year 2010, being probably the firstkm3 telescope running. In the MediterraneanSea, ANTARES is going to install a 0.1 km2

neutrino telescope demonstrator, NESTORhas deployed 12 PMTs in deep water, aimingat a 2 × 104 m2 tower and NEMO is installinga technological demonstrator for a neutrinotelescope and proposed Capo Passero as anoptimal installation site for the Mediterraneankm3. The research and development activitiesconducted by the three Mediterranean collab-orations can represent a valuable experiencein the construction of the underwater km3

detector, which will be the result of theirefforts.

References

Ackermann M. et al. 2004, Astrop. Phys., 22,127

Ackermann M. et al. 2005, Astrop. Phys., 22,339

Ackermann M. et al. 2005,Phys. ReV.,D077102

Aggouras G. et al. 2005, Astrop. Phys., 23, 377Aharonian F. et al. 2004, A&A, 425, L13Ahrens J. et al. 2004, Astrop. Phys., 20, 507Andres E. et al. 2001, Nature, 40, 441

AMANDA web page http://amanda.uci.edu.ANTARES: A Deep Sea Telescoper for

High Energy Neutrinos, astro-ph/9907432,http://antares.in2p3.fr

Balkanov V.et al. 2003, NIM, A498, 37231Belolaptikov I. et al. 1997, Astrop. Phys., 7,

263Daum K. et al.1995, Z. Phys., C66, 417Distefano C. et al. 2005, LNS Activity Report,

http://www.lns.infn.it/info/Report2004

Migneco: Experimental high energy neutrino astronomy 369

Gaisser, T.K., et al., Phys. Rep. 1995, D258,173

Korolkova E.V. 2004, for the ANTARESCollaboration. Proc. of the CRIS 2004,Catania, Italy

Migneco E. et al. 2004, Proc. of the CRIS2004, Catania, Italy

Musumeci M. 2003 for the NEMOCollaboration. Proc. of VLVνT Workshop,Amsterdam

NESTOR web page, http://icecube.wisc.edu.Riccobene G. 2003 for the NEMO

Collaboration. Proc. of VLVνT Workshop,Amsterdam

Riccobene G. 2005 for the NEMOCollaboration. Proc. of 29th ICRC, Pune,

IndiaRoberts A. et al. 1992, Rev. of Mod. Phys., 64,

259Sapienza P. et al. 2005, LNS Activity Report,

http://www.lns.infn.it/info/Report2004Spiering C. 2004, Proc. of the Nobel

Symposium on Neutrino Physics, HagaSlott, Sweden

Volkova L.V. 1980, Sov. J.Nucl.Phys., 31, 784Waxman, E. & Bahcall, J. 1999, Phys. Rev.,

D64, 023001Wischnewski R. 2005 for the BAIKAL

Collaboration, Proc. of 29th ICRC, Pune,India