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XXV European Cosmic Ray Symposium, Turin, Sept. 4-9 2016 1 Prospects for measuring the neutrino mass hierarchy with KM3NeT/ORCA J. Hofestädt on behalf of the KM3NeT Collaboration Erlangen Centre for Astroparticle Physics, Friedrich-Alexander University of Erlangen-Nu¨rnberg, Erwin-Rommel-Str. 1, 91058 Erlangen, Germany ORCA (Oscillation Research with Cosmics in the Abyss) is the low-energy branch of KM3NeT, thenext-generationresearchinfrastructurehostingunderwaterCherenkovdetectorsintheMediter- raneanSea. ORCA’sprimarygoalisthedeterminationoftheneutrinomasshierarchybymeasuring thematter-inducedmodificationsontheoscillationprobabilitiesoffew-GeVatmosphericneutrinos. The ORCA detector design foresees a dense configuration of KM3NeT neutrino detection technology, optimised for measuring the interactions of neutrinos in the energy range of 3–20GeV. To be deployed at the French KM3NeT site, ORCA’s multi-PMT optical modules will exploit the excel- 7 lentopticalpropertiesofdeep-seawatertoaccuratelyreconstructbothshower-like(mostlyelectron 1 neutrino) and track-like (mostly muon neutrino) events in order to collect a high-statistics sample 0 of few-GeV neutrino events. 2 This contribution reviews the methods and technology of the ORCA detector, and discusses n the prospects for measuring the neutrino mass hierarchy as well as the potential to improve the a measurement precision on other oscillation parameters. J 6 2 I. INTRODUCTION few-GeV atmospheric neutrinos that have traversed the Earth towards the detector [2]. Due to matter- ] induced modifications on the oscillation probabilities t A variety of experiments with solar, atmospheric, e inconjunctionwithdifferentcross-sectionsandatmo- d reactor and accelerator neutrinos, spanning energies spheric neutrino fluxes for neutrinos and antineutri- - from MeV up to tens of GeV, demonstrated unam- s nos, the expected event rates of neutrinos in the en- biguously that neutrinos change from one flavour to n ergy regime of 3–20GeV are different for NH and IH. i anotherduringpropagation. Neutrinooscillationsim- . Next-generation experiments, such as s ply non-zero neutrino masses, and that the masses c of the three neutrino states are different. In the KM3NeT/ORCA [3], PINGU [4] and Hyper- i Kamiokande [5], are planned to perform this s standard 3-neutrino scheme, the mixing matrix re- y lating the neutrino flavour eigenstates (ν , ν , ν ) measurement with megaton-scale water/ice-based h e µ µ Cherenkov detectors. to the mass eigenstates (ν , ν , ν ) is parameterised p 1 2 3 In the following, the prospects for measuring the in terms of three mixing angles θ , θ and θ , [ 12 13 23 neutrino mass hierarchy with ORCA (Oscillation Re- and a CP-violating phase δ . Oscillation experi- CP 2 mentsaremostlysensitivetomass-squareddifferences search with Cosmics in the Abyss) are presented, and v ∆m2 = m2−m2 (i,j = 1,2,3). Global fits of avail- the potential to improve the measurement precision 8 ij i j on θ and ∆m2 is discussed. able data form a coherent picture and provide the 23 32 7 values of these oscillation parameters with reasonable This contribution is mainly based on the ‘Letter of 0 4 precision [1]. Intent for KM3NeT 2.0’ [3]. 0 An open question is the so-called neutrino mass hi- . 1 erarchy (NMH). It refers to the ordering of the neu- 0 trino mass eigenstates, which is either ν < ν < ν II. THE KM3NET/ORCA DETECTOR 1 2 3 7 (normal hierarchy, NH) or ν < ν < ν (inverted 3 1 2 1 hierarchy, IH). The ordering of the first two closely TheKM3NeTdetectordesignbuildsontheexperi- : v spaced mass eigenstates, ν1 < ν2, is known from so- enceofthesuccessfuldeploymentandoperationofthe Xi lar neutrino physics. Further yet unknown neutrino ANTARES detector [6], which has demonstrated the properties are: the value of δCP, the absolute masses, feasibilityofmeasuringneutrinoswithalarge-volume r a theDirac/Majorananatureofneutrinos. Knowingthe Cherenkov detector in the deep sea. The detection NMH is important for constraining the models that principle is that of a 3-dimensional array of photo- seektoexplaintheoriginofmassintheleptonicsector sensors that register the Cherenkov light induced by and will allow to optimise the information obtained charged particles produced in a neutrino-induced in- from other neutrino experiments (targeting δCP, ab- teraction. From the arrival time of the Cherenkov solute neutrino masses and neutrinoless double-beta photons (nanosecond precision) and the position of decays). In addition, the NMH has a significant im- the sensors (∼10cm precision), the energy and direc- pact on the measurement precision of the oscillation tionoftheincomingneutrino,aswellasotherparame- parameters. ters of the neutrino interaction, can be reconstructed. The NMH can be determined by measuring the en- A key KM3NeT technology is the Digital Opti- ergyandzenithangledependentoscillationpatternof cal Module (DOM), a pressure-resistant glass sphere eConf C16-09-04.3 2 XXV European Cosmic Ray Symposium, Turin, Sept. 4-9 2016 Within KM3NeT, the same technology is employed also for the search for high-energy astrophysical neutrino sources with the ARCA detector [8], which will be deployed offshore from Sicily, Italy. The main dif- ferencebetweenbothdetectordesignsisthedensityof photosensors,whichisoptimisedforthestudyofneu- trinosinthefew-GeV(ORCA)andTeV-PeV(ARCA) energy ranges. III. EXPECTED DETECTOR PERFORMANCE ThekeyparametersfortheNMHdeterminationare theeffectivemassofthedetectorandtheexperimental resolutions for the energy E and zenith angle θ of ν ν the incoming neutrino. Detailed Monte Carlo simulations have been per- formed using GENIE[9] for simulating neutrino in- FIG.1: PhotographofaDOM(left)andschematicdraw- teractions and GEANT-based simulation packages ing of an detection string (right). [10, 11] for particle propagation and Cherenkov pho- ton generation. Optical background from 40K decays in the seawater as well as the background from down- housing 31 3-inch PMTs and their associated elec- going atmospheric muons is taken into account. Fur- tronics. This multi-PMT design offers several im- ther details are given in [3]. provements compared to traditional optical modules Twodistincteventtopologiesareconsidered: tracks hosting only a single large PMT (for example in and showers. Showers are initiated by energetic ANTARES),mostnotably: largerphotocathodearea, electrons and hadrons emerging from the neutrino wider field of view, directional information and dy- interaction, and develop over relatively short dis- namic range. The DOMs are arranged in strings held tances. Muons produce elongated tracks in the de- verticallybyabuoyandanchoredtotheseabed. Fig- tector. Therefore, track-like events are induced by ure 1 shows a DOM and a detection string. (cid:44) (cid:45) ν charged-current (CC) interactions, as well as µ In its current design, the ORCA detector will com- (cid:44)ν(cid:45) CC interactions with muonic tau decays. All τ prise 115 such detection strings. Each string com- other neutrino-induced events are called shower-like, prises 18 DOMs with a vertical spacing of about 9m. i.e. (cid:44)ν(cid:45) CC events, (cid:44)ν(cid:45) neutral-current events and e e,µ,τ The horizontal spacing between adjacent strings is (cid:44)ν(cid:45) CC events with non-muonic τ decays. τ roughly20m. Theinstrumentedmassisabout6Mton Dedicated reconstruction strategies for track-like of seawater. This detector configuration is the out- and shower-like events, as well as an event topology come of a optimisation study using the NMH sensi- classification algorithm, have been developed and are tivity as figure of merit. The proposed detector could described in [3]. The energy resolution is Gaussian- be built in three years, with an investment budget of like with σ /E ≈ 25%. The median zenith angle about 45Me [7]. resolution iEsνaboνut 5◦ for (cid:44)ν(cid:45) CC and (cid:44)ν(cid:45) CC events e µ The ORCA detector will be deployed at the French with E =10GeV. Due to the long scattering length ν KM3NeT site at a depth of 2450m. The site is about oflightindeep-seawater,thereconstructionsareable 40km offshore from Toulon and about 10km west of to find the lepton (e,µ) in (cid:44)ν(cid:45) CC events and are e,µ the operating ANTARES detector. therefore able to gain access to the interaction inelas- The construction of the infrastructure has already ticity y. This allows a statistical separation of ν and started. The first main electro-optical cable and ν interactions to further add to the NMH sensitivity the first junction box, needed to connect the detec- [12]. This possibility has not yet been exploited in tionstrings, have been successfullydeployed andcon- the estimated NMH sensitivity presented below. As nected. The first detection string is foreseen to be shown in [13], the energy resolution is dominated by deployed in early 2017. An array comprising 7 detec- intrinsiclightyieldfluctuationsinthehadronicshower tion strings is funded and expected to be concluded and the direction resolution is limited by the kine- and operational by the end of 2017. It will serve to matic scattering angle between the outgoing lepton demonstratethefeasibilityofthemeasurementandto and the incoming neutrino. validate and optimise the detector design. The full- The purity of the event topology classification is (cid:44) (cid:45) (cid:44) (cid:45) size ORCA detector comprising 115 detection strings about 90% (70%) for ν CC (ν CC) events with e µ could be operational towards 2020. E =10GeV.Thesameeventclassificationalgorithm ν eConf C16-09-04.3 XXV European Cosmic Ray Symposium, Turin, Sept. 4-9 2016 3 ^3] 10 vertical spacing: KM3Ne T m M 9 6m me [ 8 9m e volu 7 1125mm ectiv 6 n e and n e CC eff 5 d ale 4 c S 3 2 1 0 5 10 15 20 25 30 Neutrino energy [GeV] FIG. 2: Effective volume as a function of neutrino energy FIG. 3: Median NMH significance to exclude the other Eν for (cid:44)ν(cid:45)e CC events. Detector configurations with dif- hierarchyhypothesisassumingtrueNH(red)orIH(blue) ferent vertical spacings between the DOMs are shown as as a function of true θ and assuming δ =0 (solid) or 23 CP different colours. δ = π (dashed). Three years of data taking with the CP full-size ORCA detector are assumed. also rejects downgoing atmospheric muons that are mis-reconstructedasupgoing. Acontaminationofless ν/ν skew, µ/e skew and NC/CC skew and are incor- than a few percent of atmospheric muons in the final poratedasnuisanceparameters. Itisfoundthatnone sample of upgoing neutrino events is achieved. oftheseeffectscompromisesubstantiallytheabilityof The effective mass of the detector is about 6Mton, ORCA to determine the NMH [3]. (cid:44) (cid:45) (cid:44) (cid:45) being reached for ν CC and ν CC with energies e µ Figure 3 shows the median significance of ORCA aboveE =10GeV,while50%efficientat4GeV.Fig- ν to exclude the wrong hierarchy hypothesis after three (cid:44) (cid:45) ure 2 shows the effective volume for ν CC events for e years of data taking as a function of the true value of detectorconfigurationswithdifferentverticalspacings θ and assuming no CP-violation, i.e. δ equals 0 23 CP between the DOMs, i.e. different photosensor densi- or π. For the experimentally allowed range of θ and 23 ties. IntheE =5−10GeVrange,whichismostrel- ν assuming δ = 0, the NMH can be measured with CP evantfortheNMHdetermination,thedetectorconfig- about 3σ after three years of operation. ORCA is uration with 9m vertical spacing provides the largest moderatelysensitivetotheCP-phase,thesignificance effective mass and therefore largest available event being reduced by at most 0.5σ if δ = π is realised CP statistics. This detector configuration was also found in nature. The significance increases dramatically in toprovidethebestNMHsensitivity[3]. Itwillprovide case of NH and θ > π/4, reaching up to about 7σ 23 data samples of about 50,000 reconstructed upgoing in three years of operation. neutrinos per year. Besides the NMH determination, ORCA can also improve the uncertainties on ∆m2 and θ . Both 32 23 parameters are determined without the need for con- IV. SENSITIVITY TO NEUTRINO MASS strainsfromglobaldatainconjunctionwiththeNMH. HIERARCHY AND MORE Figure 4 shows the expected measurement precision after three years and compares it with current results Building on the expected detector performance, a ofotherexperimentsandtheirpredictedperformances significanceanalysisisperformedbygeneratingalarge in 2020. The precision of ORCA is comparable or number of pseudo-experiments (PEs) with event dis- better, and is obtained with different systematic un- tributionsinthereconstructedE –θ plane. Foreach certainties. In particular, ORCA can determine the ν ν PE, a true hierarchy and a set of oscillation parame- octant of θ23 (above or below 45◦) for a wide range of ters is assumed. Each PE is analysed by performing the allowed parameter range. amaximumlikelihoodfitwiththeoscillationparame- Additional science topics of ORCA include: testing tersasfreeparametersandassumingeitherNHorIH. theunitaryoftheneutrinomixingmatrixbystudying The likelihood ratio resulting from these fits is used (cid:44)ν(cid:45) appearance;indirectsearchesforsterileneutrinos, τ to quantify the separability between both hierarchies. non-standard interactions and other exotic physics; Systematic uncertainties from the neutrinos fluxes, indirect searches for dark matter; testing the chem- theircrosssectionsaswellasthedetectorresponseare ical composition of the Earth’s core (Earth tomog- parameterised as overall normalisation, energy scale, raphy); and low-energy neutrino astrophysics. Pre- eConf C16-09-04.3 4 XXV European Cosmic Ray Symposium, Turin, Sept. 4-9 2016 denser detector instrumentation lowering the detection threshold to measure the CP-phase δ with at- CP mospheric neutrinos [15]. V. CONCLUSIONS With ORCA, a 6Mton deep-sea Cherenkov detector optimised for the detection of few-GeV neutrinos, the KM3NeT Collaboration aims to perform a high- statisticsmeasurementofthezenithangleandenergy dependent event rates of atmospheric neutrinos that havetraversedtheEarth. Theoscillatedneutrinoflux in the energy range 3–20GeV holds the key to deter- FIG. 4: One sigma contours of the measurement preci- mine the neutrino mass hierarchy. ORCA is expected sionin∆m232 andθ23 afterthreeyearsofdatatakingwith to achieve a 3–7σ sensitivity to the neutrino mass hi- ORCA for three assumed test cases (red). The current erarchyinthreeyearsofdatataking. Simultaneously, results from MINOS (back) and T2K (blue solid) are in- ORCA will measure ∆m2 and θ with competitive dicated, as well as the predicted performance of NOνA 32 23 precision, and has a rich additional science program. (magenta) and T2K (blue dashed) in 2020. All contours are at 1σ for NH. In the first construction phase of ORCA, a 7-string demonstrator is expected to be concluded and operational by the end of 2017. The full-size ORCA de- liminary performance expectations are briefly sum- tector could be operational towards 2020, so that the marised in [7]. The KM3NeT research infrastructure neutrino mass hierarchy might be resolved as early as willalsohouseinstrumentationforEarthandSeasci- 2023. ences, such as marine biology, oceanography and geo- Further details can be found in the recently pub- physics. lished ‘Letter of Intent for KM3NeT 2.0’ [3]. Possiblefutureoptionscouldbealong-baselineneu- trinobeamtargetedtoORCA[14],andasignificantly [1] I. 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