The KM3NeT Collaboration has started the first phase of construction of a next generation high-energy neutrino telescope in the Mediterranean Sea. With several cubic kilometers instrumented and thousand of optical sensors,

KM3NeT will be the largest and most sensitive high-energy neutrino telescope. Thanks to its location in the Northern hemisphere and to its large instrumented volume KM3NeT will be the optimal instrument to search for neutrinos from the

Southern sky and in particular from the Galactic plane, thus making it complementary to IceCube. The full KM3NeT detector will be a distributed, networked infrastructure comprising several detector blocks. In Italy, off the coast of Capo Passero, and in France, off the coast of Toulon, the construction of the KM3NeT-It and KM3NeT-Fr infrastructures respectively is in progress. In this work the technologically innovative component of the detector, the status of construction and the first results from prototypes of the KM3NeT detector will be described and its

capability to discover neutrino sources is reported as well

Di Capua, F.

English

Scientific Open-access Literature Archive and Repository

DOI 10.1393/ncc/i2016-16241-xColloquia: IFAE 2015IL NUOVO CIMENTO 39 C (2016) 241The KM3Net project: A neutrino telescope in the depthsof the Mediterranean SeaF. Di Capua for the KM3NeT CollaborationUniversita` “Federico II” - Napoli, Italyreceived 7 January 2016Summary. — The KM3NeT Collaboration has started the first phase of con-struction of a next generation high-energy neutrino telescope in the MediterraneanSea. With several cubic kilometers instrumented and thousand of optical sensors,KM3NeT will be the largest and most sensitive high-energy neutrino telescope.Thanks to its location in the Northern hemisphere and to its large instrumentedvolume KM3NeT will be the optimal instrument to search for neutrinos from theSouthern sky and in particular from the Galactic plane, thus making it comple-mentary to IceCube. The full KM3NeT detector will be a distributed, networkedinfrastructure comprising several detector blocks. In Italy, off the coast of CapoPassero, and in France, off the coast of Toulon, the construction of the KM3NeT-Itand KM3NeT-Fr infrastructures respectively is in progress. In this work the tech-nologically innovative component of the detector, the status of construction andthe first results from prototypes of the KM3NeT detector will be described and itscapability to discover neutrino sources is reported as well.1. – IntroductionNeutrinos represent an unique probe for the exploration of the high-energy universe.Even more than 100 year later the discovery of cosmic rays, the main question regardingthe sources that can accelerate particles to energies far beyond what is achievable withman-made particle accelerators is still unanswered.The detection of very high energy gamma rays allows to trace back their sourcesand to define eventual candidate sites of cosmic rays acceleration. But conclusions areoften ambiguous because high-energy gamma ray photons, which can be produced in theinteraction of cosmic rays with matter and photon fields, indicating the acceleration sites,can also be generated through the Inverse Compton Scattering mechanism of low-energyphotons by accelerated electrons. From the observation of gamma ray spectra of twosupernova remnants (W44 and IC443), the photons seem to originate from pion decayand hence from acceleration of protons or heavy nuclei [1]. A clear and definitive picturecould be obtained with the detection of high-energy neutrinos.Creative Commons Attribution 4.0 License (http://creativecommons.org/licenses/by/4.0) 12 F. DI CAPUA for the KM3NET COLLABORATIONThanks to its low interaction probability, neutrinos can propagate between the astro-physical sources and the Earth preserving the initial direction and energy. For the samereason is very hard to detect them, and it has become clear that huge detectors of cubickilometer scale are necessary to exploit the physics potential of neutrino astronomy.2. – Neutrino telescopesThe detection principle of neutrino telescopes, first proposed in 1960 [2], is based onthe registration of Cherenkov light in an optically trasparent medium like ice or waterinduced by charged particles generated in neutrino interactions. The light has to berecorded by a large number of photomultipliers arranged in a three-dimensional array.The arrival time of the photons on the different photomultipliers allows for reconstruc-tion of the neutrino direction, the measured light intensity allow for the reconstructionof the neutrino energy.After the success with the demonstration of the detection principle with first underwa-ter (BAIKAL [3] and ANTARES [4]) and under ice (AMANDA [5]) detectors, the designand the construction of large neutrino telescopes started. This led to the construction ofthe IceCube telescope [6] at South Pole, which in its final configuration, reached in 2010,consists of 86 strings with 5160 optical modules instrumenting 1 km3 of clear glacial ice.In the 2006 the Mediterranean neutrino collaborations (ANTARES, NEMO,NESTOR) started the R&D for the construction of a several cubic-kilometer detector:KM3NeT [7].Thanks on its location in the Mediterranean Sea, the neutrino telescope, will belocated in an optimal position to investigate neutrino fluxes from the Southern sky,in particular from the Galactic Centre and from a large fraction of the Galactic planecompleting the field of view of the IceCube detector. The recent observation from IceCubeof 37 neutrino events [8] (of which 15 are compatible with background), represents theevidence of extraterrestrial neutrino events and the beginning of neutrino astronomy.The question where the detected neutrinos come from will be a crucial issue of the nextyears and of future investigations. The future KM3NeT telescope, with its higher angularresolution, larger field of view and effective area can put important constraints on theorigins of these neutrino event.3. – The KM3NeT detectorThe KM3NeT telescope consists of an array of Digital Optical Modules (DOMs) [9]attached to vertical structures, called detection units (DUs) [10]. The average distancebetween neighbouring DUs is 90m. Each DU carries 18 DOMs, starting 100m abovethe sea floor and with 36m distance between adjacent DOMs. An array of 115 DUs willconstitute a detector building block. Several building blocks will be installed at one ormore installation sites at depths ranging from about 2500m to 4500m.The DUs are supported by two prestretched Dyneema ropes and kept straight by asubmerged buoy at their top. A single vertical electro-optical cable (VEOC) is used toconnect the DOMs to the base of the DU. It consists of a flexible, oil-filled hose thatis in equi-pressure with the sea water and contains optical fibres for data transport andcopper wires for electrical power provision.For deployment, a detection unit will be wrapped on a spherical frame with diameterof about 2.2m (Launcher of Optical Modules, LOM [11]) which is deposited on the seabedTHE KM3NET PROJECT: A NEUTRINO TELESCOPE ETC. 3Fig. 1. – Schematic view of KM3Net Detection Units with Digital Optical Modules.and then unfurls in a rotating upwards movement. The LOM rises to the sea surface,where it is collected for reuse.The DOMs must withstand pressure up to about 500 bar, be resistant to corrosion andstress (vibration, shocks) during handling and deployment. The lifetime of the detectorshould exceed 10 years and the reliability of the DOMs should be high. Each DOM is apressure-resistant glass sphere of 17 inch diameter that carries 31 3-inch photomultipliertubes (PMTs) with their high-voltage bases as well as calibration devices and readoutelectronics. The lower hemisphere of each DOM contains 19 of the PMTs, which arethus downward-looking, whereas the other 12 PMTs look upwards. The novel design ofDOM offers significant improvements with respect to optical modules with a single largearea PMT: i) the total photocathode area is about three times larger; ii) a segmentedphotocathode allows for high-purity photon counting and directional sensitivity; iii) re-duced cost and risk; iv) almost 4π solid angle coverage by each DOM. All the necessaryelectronics for digitization and data transmission is contained within each DOM. Theposition calibration of each DOM is achieved at about 10 cm precision using acoustictriangulation. The acoustic system includes transponders at the seafloor and a receiverin each DOM. A schematic picture of a KM3Net structure is shown in fig. 1.Different types of PMTs, ETEL D783FL [12] and Hamamatsu R12199-02 [13], withcomparable performances and slightly varying dimensions will be used for DOM assem-bly. Typical performances are a transit time spread lower than 5 ns (FWHM), quantumefficiency of 22% at 470 nm and 27% at 404 nm.The PMTs are operated at a gain of 3 × 106. Dark count rates as measured in thelaboratory at room temperature vary typically between 200 and 1500Hz with a thresholdof 0.3 photoelectrons. The gain adjustment is performed for all PMTs by finding thehigh-voltage setting which gives a ToT of 26.4 ns for a single photoelectron.All PMTs are calibrated and characterized before being used in the production ofDOMs [14].4. – Results from a small detector unit prototypeA prototype Detector Unit has been installed at 3500m depth 80 km offshore theItalian coast. This prototype has a height of 160m (the full DU is 700 height) with threeDOMs and is taking data since its deployment in May 2014.The intra-DOM offsets of the PMTs inside every DOM is determined from coinci-dences from 40K decays. The radioactive decay of the 40K contained in sea water produces4 F. DI CAPUA for the KM3NET COLLABORATIONFig. 2. – (a) Mean photon rate measured by a PMT of DOM1. The Gaussian fit is superimposedand represent the optical background due to 40K decay. The tail on the right is due to transientbioluminescent phenomena. (b) Mean time offsets for DOM2 with respect to DOM1 evaluatedwith the usage of nanobeacon of DOM1 for different runs.a few hundred Cherenkov photons emitted along the track of the electron released in thedecay and constitutes the main source of signal detected by the DOMs.To calculate the inter-DOM time offsets (between DOMs) dedicated runs with a LEDnanobeacon activated are used. The LED nanobeacon installed in each DOM has awavelength of 470 nm and is positioned in the top half of the DOM pointing upwards.The LED nanobeacons are controlled with adjustable frequency and intensity in order toilluminate the neighbouring DOM on the line without saturating the PMTs. The inter-DOM time offsets depend on the electronics plus the cable lengths. The travel time oflight in sea water must be taken into account in this calibration procedure. To calculatethe travel time of the nanobeacon light, the distance between the nanobeacon and thehit PMT is used. A fixed detector position is assumed, as a real time positioning systemis not available for this prototype. The resulting mean time offsets for different runs areshown in fig. 2(b). The time accuracy achieved is of the order of ∼ 1 ns.The two main contributions to the single rates are the 40K decay and the biolumi-nescence activity. While the 40K decay is stable as a function of time and position, thebioluminescence activity can fluctuate significantly in time. The average singles rate pertime slice of 134 ms for one PMT of DOM1 is shown in fig. 2(a).A dedicated Monte Carlo simulates the expected atmospheric muon flux at a depthof 3457m, together with the optical background due to the 40K decay. The PMT char-acteristics and the optical water properties measured at the Capo KM3NeT-It site aretaken into account in the simulation. The optical background from 40K decays and biolu-minescence dominate the coincidence rates in a DOM up to 5 coincidences, while muonsare the dominant source at higher coincidences. Figure 3(a) shows the rate of events asa function of the coincidence level in a DOMs for data and Monte Carlo simulation. Thefull Monte Carlo histograms reported in fig. 3(a) refer to the sum of atmospheric muonevents and 40K only events. No normalization factor is applied to the Monte Carlo eventsthus showing an absolute excellent agreement between data an Monte Carlo simulations.A distribution of hits as a function of the PMT orientation is show in fig. 3(b) comparedto the distributions of the muon Monte Carlo simulation. A cut for coincidence level > 7is applied. Muon simulation is in a good agreement with data.A dedicated algorithm to reconstruct the zenith angle of the direction of atmosphericmuons has been developed. This algorithm uses the information about the position ofTHE KM3NET PROJECT: A NEUTRINO TELESCOPE ETC. 5Fig. 3. – (a) Rate of events as a function of the number of PMTs in coincidence compared toMonte Carlo simulation. (b) Rate of hits in each DOM as a function of PMT position for eventswith more than 7 coincidences on the respective DOM.the three DOMs and the time of the local coincidences detected. Only muon events thattrigger the three DOMs are reconstructed [15]). In fig. 4(a) the difference between thereconstructed and the true zenith angle is plotted using Monte Carlo muon events; aFWHM of 7.6 degrees is achieved. The distribution of cos θ for the selected events isshown for data and Monte Carlo in fig. 4(b) demonstrating a good agreement.5. – Performances and physics objectives of KM3NeTThe first physics goal of KM3NeT will be the investigation from a complementaryfield of view and with better angular resolution of the IceCube findings [8]. Evalu-ation of the telescope performance has been carried out using complete Monte Carlosimulation, including the neutrino interaction in the medium, the propagation of theresulting secondary particles, the Cherenkov light generation and propagation in wa-ter and the detector response. Depth and the optical water properties measured atthe KM3NeT-It site have been used [16]. Background due to the presence of 40Kin salt water was simulated adding an uncorrelated hit rate of 5 kHz per PMT plusFig. 4. – (a) Zenith angular resolution of tracking algorithm obtained with Monte Carlo simu-lation. (b) Reconstructed cos θ for data and Monte Carlo. The data sample used is equivalentto a livetime of ∼ 600 hours.6 F. DI CAPUA for the KM3NET COLLABORATIONFig. 5. – Significance as a function of the observation time for the detection of a diffuse fluxof neutrinos corresponding to the signal reported by IceCube, in the up-going muon channel(black), the cascade channel (red) and the combined analysis (blue star).higher-fold coincidence rates as determined by GEANT simulations and in agreementwith the results from the prototype optical modules. KM3NeT is sensitive to all neu-trino flavors, since events of different topology can be detected and identified. Tracklike events, generated mainly by νμ Charged Current (CC) interactions, constitute the“classic” detection channel. These events can be generated far from the detector, ashigh-energy muons can travel several kilometers in water. The identification of theseevents is however limited to up-going tracks due to the presence of the atmosphericmuon background. Shower-like events, such as those generated in νe Charged Current(CC) and in all flavors Neutral Current (NC) interactions can also be detected, providedthat the interaction occurs inside or close to the detector volume. An isotropic one-flavour flux Φ = 1.2 × 10−8(E/GeV)−2e−(Eν/3PeV) GeV−1 cm−2 s−1 sr−1, as reportedin [8], is assumed.Results referred to two building blocks are represented in fig. 5. For the track channela significance of 5σ is reached after less than 2 years of observation. The major un-certainty for this channel (shown as a shaded area in the figure) is represented by theuncertainty on the conventional atmospheric neutrino flux. The shower channel givesa much better sensitivity, reaching a 5σ significance after less than one year. In thisanalysis, background due to atmospheric muons can be rejected applying geometricalcuts on the reconstructed interaction vertex (containment) and by an energy cut appliedusing the time over threshold information of the PMTs, which is correlated to the chargecollected. The final step to select signal events is performed by a machine learning algo-rithm. The major contribution to systematic uncertainty for this channel is due to theuncertainty on the prompt neutrino flux. A combined analysis, incorporating the resultsfrom both search strategies has also been developed, giving as a final result a significanceof 4.8σ in 0.5 years of observation.Identifying Galactic neutrino sources is a priority physics goal with the full KM3NeTdetector. The sensitivity for the detection of galactic sources has been determined usingas a test case the very intense SuperNova Remnant gamma source RXJ1713.7-3946 [17]and the Pulsar Wind Nebula Vela-X [18]. The neutrino energy spectrum was estimatedfrom the gamma-ray spectrum assuming a 100% hadronic mechanism and a source trans-parent to gamma-ray emission according to [19] for RXJ1713.7-3946 and [20] for Vela-X.THE KM3NET PROJECT: A NEUTRINO TELESCOPE ETC. 7Fig. 6. – Significance of RXJ1713.7-3946 and Vela-X observations as a function of the years ofdata taking for KM3NeT.Under these assumptions, the significance of the source observation, with a 50% proba-bility, as a function of the observation time for KM3NeT has been calculated [21] andis shown in fig. 6. An observation with 3σ significance is expected after about 2.5 and4 years for Vela-X and RXJ1713.7-3946, respectively. The extension of the KM3NeT tele-scope to its final configuration of six detector blocks will allow to reach a 5σ significanceafter about 2.5 years for Vela-X and 4 years for RXJ1713.7-3946.REFERENCES[1] Ackermann M. et al., Science, 339 (2013) 807.[2] Markov M. A. and Zheleznykh I. M., Nucl. Phys., 27 (1961) 385.[3] Aynutdinov V. et al., The Baikal Collaboration, Nucl. Instrum. Methods A, 567(2006) 433.[4] Ageron M. et al., The Antares Collaboration, Nucl. Instrum. Methods A, 656 (2011)11.[5] Andres E. et al., Astropart. Phys., 13 (2000) 1.[6] Achterberg A. et al., IceCube Collaboration, Astropart. Phys., 26 (2006) 155.[7] Bagley P. et al., KM3Net Collaboration, Technical Design Report (2010) availableon http://www.km3net.org. ISBN 978-90-6488.[8] Aartsen M. G. et al., IceCube Collaboration, Phys. Rev. Lett., 113 (2014) 101101.[9] Bruijin R. et al., KM3NeT Collaboration, ICRC Conf. Proc. (2015) ID1157.[10] Kooijman P. et al., KM3NeT Collaboration, ICRC Conf. Proc. (2015) ID1173.[11] De Wolf E. et al., KM3NeT Collaboration, Nucl. Instrum. Methods A, 725 (2013)241.[12] Bormuth R. et al., KM3NeT Collaboration, AIP Conf. Proc., 1630 (2014) 114.[13] Aiello A. et al., KM3NeT Collaboration, AIP Conf. Proc., 1630 (2014).[14] Mollo C. et al., KM3NeT Collaboration, ICRC Conf. Proc. (2015).[15] Biagi S. et al., KM3NeT Collaboration, ICRC Conf. Proc. (2015) ID1164.[16] Riccobene G. et al., KM3NeT Collaboration, Astropart. Phys., 27 (2006) 1.[17] Aharonian F. et al., H.E.S.S. Collaboration, Astron. Astrophys., 464 (2007) 235.[18] Abramowski A. et al., H.E.S.S. Collaboration, Astron. Astrophys., 548 (2012) A38.[19] Kelner S. R. et al., Phys. Rev., D74 (2006) 034018.[20] Villante F. L. and Vissani F., Phys. Rev., D78 (2008) 103007; Villante F. L. andVissani F., Nucl. Instrum. Methods A, 588 (2008) 123; Vissani F., Astropart. Phys., 26(2006) 310.[21] Trovato A. et al., KM3NeT Collaboration, ICRC Conf. Proc. (2015) ID1113.

The KM3Net project: A neutrino telescope in the depths of the Mediterranean Sea

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The KM3Net project: A neutrino telescope in the depths of the Mediterranean Sea

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