Prospects for measuring the neutrino mass hierarchy with KM3NeT/ORCA
XXXV European Cosmic Ray Symposium, Turin, Sept. 4-9 2016 Prospects for measuring the neutrino mass hierarchy with KM3NeT/ORCA
J. Hofest¨adt on behalf of the KM3NeT Collaboration
Erlangen Centre for Astroparticle Physics, Friedrich-Alexander University of Erlangen-N¨urnberg,Erwin-Rommel-Str. 1, 91058 Erlangen, Germany
ORCA (Oscillation Research with Cosmics in the Abyss) is the low-energy branch of KM3NeT,the next-generation research infrastructure hosting underwater Cherenkov detectors in the Mediter-ranean Sea. ORCA’s primary goal is the determination of the neutrino mass hierarchy by measuringthe matter-induced modifications on the oscillation probabilities of few-GeV atmospheric neutrinos.The ORCA detector design foresees a dense configuration of KM3NeT neutrino detection technol-ogy, optimised for measuring the interactions of neutrinos in the energy range of 3–20 GeV. To bedeployed at the French KM3NeT site, ORCA’s multi-PMT optical modules will exploit the excel-lent optical properties of deep-sea water to accurately reconstruct both shower-like (mostly electronneutrino) and track-like (mostly muon neutrino) events in order to collect a high-statistics sampleof few-GeV neutrino events.This contribution reviews the methods and technology of the ORCA detector, and discussesthe prospects for measuring the neutrino mass hierarchy as well as the potential to improve themeasurement precision on other oscillation parameters.
I. INTRODUCTION
A variety of experiments with solar, atmospheric,reactor and accelerator neutrinos, spanning energiesfrom MeV up to tens of GeV, demonstrated unam-biguously that neutrinos change from one flavour toanother during propagation. Neutrino oscillations im-ply non-zero neutrino masses, and that the massesof the three neutrino states are different. In thestandard 3-neutrino scheme, the mixing matrix re-lating the neutrino flavour eigenstates ( ν e , ν µ , ν µ )to the mass eigenstates ( ν , ν , ν ) is parameterisedin terms of three mixing angles θ , θ and θ ,and a CP-violating phase δ CP . Oscillation experi-ments are mostly sensitive to mass-squared differences∆ m ij = m i − m j ( i, j = 1 , , ν < ν < ν (normal hierarchy, NH) or ν < ν < ν (invertedhierarchy, IH). The ordering of the first two closelyspaced mass eigenstates, ν < ν , is known from so-lar neutrino physics. Further yet unknown neutrinoproperties are: the value of δ CP , the absolute masses,the Dirac/Majorana nature of neutrinos. Knowing theNMH is important for constraining the models thatseek to explain the origin of mass in the leptonic sectorand will allow to optimise the information obtainedfrom other neutrino experiments (targeting δ CP , ab-solute neutrino masses and neutrinoless double-betadecays). In addition, the NMH has a significant im-pact on the measurement precision of the oscillationparameters.The NMH can be determined by measuring the en-ergy and zenith angle dependent oscillation pattern of few-GeV atmospheric neutrinos that have traversedthe Earth towards the detector [2]. Due to matter-induced modifications on the oscillation probabilitiesin conjunction with different cross-sections and atmo-spheric neutrino fluxes for neutrinos and antineutri-nos, the expected event rates of neutrinos in the en-ergy regime of 3–20 GeV are different for NH and IH.Next-generation experiments, such asKM3NeT/ORCA [3], PINGU [4] and Hyper-Kamiokande [5], are planned to perform thismeasurement with megaton-scale water/ice-basedCherenkov detectors.In the following, the prospects for measuring theneutrino mass hierarchy with ORCA (Oscillation Re-search with Cosmics in the Abyss) are presented, andthe potential to improve the measurement precisionon θ and ∆ m is discussed.This contribution is mainly based on the ‘Letter ofIntent for KM3NeT 2.0’ [3]. II. THE KM3NET/ORCA DETECTOR
The KM3NeT detector design builds on the experi-ence of the successful deployment and operation of theANTARES detector [6], which has demonstrated thefeasibility of measuring neutrinos with a large-volumeCherenkov detector in the deep sea. The detectionprinciple is that of a 3-dimensional array of photo-sensors that register the Cherenkov light induced bycharged particles produced in a neutrino-induced in-teraction. From the arrival time of the Cherenkovphotons (nanosecond precision) and the position ofthe sensors ( ∼
10 cm precision), the energy and direc-tion of the incoming neutrino, as well as other parame-ters of the neutrino interaction, can be reconstructed.A key KM3NeT technology is the Digital Opti-cal Module (DOM), a pressure-resistant glass sphere eConf C16-09-04.3 a r X i v : . [ phy s i c s . i n s - d e t ] J a n XXV European Cosmic Ray Symposium, Turin, Sept. 4-9 2016
FIG. 1: Photograph of a DOM (left) and schematic draw-ing of an detection string (right). housing 31 3-inch PMTs and their associated elec-tronics. This multi-PMT design offers several im-provements compared to traditional optical moduleshosting only a single large PMT (for example inANTARES), most notably: larger photocathode area,wider field of view, directional information and dy-namic range. The DOMs are arranged in strings heldvertically by a buoy and anchored to the seabed. Fig-ure 1 shows a DOM and a detection string.In its current design, the ORCA detector will com-prise 115 such detection strings. Each string com-prises 18 DOMs with a vertical spacing of about 9 m.The horizontal spacing between adjacent strings isroughly 20 m. The instrumented mass is about 6 Mtonof seawater. This detector configuration is the out-come of a optimisation study using the NMH sensi-tivity as figure of merit. The proposed detector couldbe built in three years, with an investment budget ofabout 45 M e [7].The ORCA detector will be deployed at the FrenchKM3NeT site at a depth of 2450 m. The site is about40 km offshore from Toulon and about 10 km west ofthe operating ANTARES detector.The construction of the infrastructure has alreadystarted. The first main electro-optical cable andthe first junction box, needed to connect the detec-tion strings, have been successfully deployed and con-nected. The first detection string is foreseen to bedeployed in early 2017. An array comprising 7 detec-tion strings is funded and expected to be concludedand operational by the end of 2017. It will serve todemonstrate the feasibility of the measurement and tovalidate and optimise the detector design. The full-size ORCA detector comprising 115 detection stringscould be operational towards 2020. Within KM3NeT, the same technology is employedalso for the search for high-energy astrophysical neu-trino sources with the ARCA detector [8], which willbe deployed offshore from Sicily, Italy. The main dif-ference between both detector designs is the density ofphotosensors, which is optimised for the study of neu-trinos in the few-GeV (ORCA) and TeV-PeV (ARCA)energy ranges. III. EXPECTED DETECTORPERFORMANCE
The key parameters for the NMH determination arethe effective mass of the detector and the experimentalresolutions for the energy E ν and zenith angle θ ν ofthe incoming neutrino.Detailed Monte Carlo simulations have been per-formed using GENIE [9] for simulating neutrino in-teractions and GEANT-based simulation packages[10, 11] for particle propagation and Cherenkov pho-ton generation. Optical background from K decaysin the seawater as well as the background from down-going atmospheric muons is taken into account. Fur-ther details are given in [3].Two distinct event topologies are considered: tracksand showers. Showers are initiated by energeticelectrons and hadrons emerging from the neutrinointeraction, and develop over relatively short dis-tances. Muons produce elongated tracks in the de-tector. Therefore, track-like events are induced by (cid:44) (cid:45)ν µ charged-current (CC) interactions, as well as (cid:44) (cid:45)ν τ CC interactions with muonic tau decays. Allother neutrino-induced events are called shower-like,i.e. (cid:44) (cid:45)ν e CC events, (cid:44) (cid:45)ν e,µ,τ neutral-current events and (cid:44) (cid:45)ν τ CC events with non-muonic τ decays.Dedicated reconstruction strategies for track-likeand shower-like events, as well as an event topologyclassification algorithm, have been developed and aredescribed in [3]. The energy resolution is Gaussian-like with σ E ν /E ν ≈ ◦ for (cid:44) (cid:45)ν e CC and (cid:44) (cid:45)ν µ CC eventswith E ν = 10 GeV. Due to the long scattering lengthof light in deep-sea water, the reconstructions are ableto find the lepton ( e, µ ) in (cid:44) (cid:45)ν e,µ CC events and aretherefore able to gain access to the interaction inelas-ticity y . This allows a statistical separation of ν and ν interactions to further add to the NMH sensitivity[12]. This possibility has not yet been exploited inthe estimated NMH sensitivity presented below. Asshown in [13], the energy resolution is dominated byintrinsic light yield fluctuations in the hadronic showerand the direction resolution is limited by the kine-matic scattering angle between the outgoing leptonand the incoming neutrino.The purity of the event topology classification isabout 90% (70%) for (cid:44) (cid:45)ν e CC ( (cid:44) (cid:45)ν µ CC) events with E ν = 10 GeV. The same event classification algorithm eConf C16-09-04.3 XV European Cosmic Ray Symposium, Turin, Sept. 4-9 2016 Neutrino energy [GeV]5 10 15 20 25 30 S ca l e d e ff ec t i ve v o l u m e [ M m ^ ] e n and e n vertical spacing:6m9m12m15m KM3NeT
FIG. 2: Effective volume as a function of neutrino energy E ν for (cid:44) (cid:45)ν e CC events. Detector configurations with dif-ferent vertical spacings between the DOMs are shown asdifferent colours. also rejects downgoing atmospheric muons that aremis-reconstructed as upgoing. A contamination of lessthan a few percent of atmospheric muons in the finalsample of upgoing neutrino events is achieved.The effective mass of the detector is about 6 Mton,being reached for (cid:44) (cid:45)ν e CC and (cid:44) (cid:45)ν µ CC with energiesabove E ν = 10 GeV, while 50% efficient at 4 GeV. Fig-ure 2 shows the effective volume for (cid:44) (cid:45)ν e CC events fordetector configurations with different vertical spacingsbetween the DOMs, i.e. different photosensor densi-ties. In the E ν = 5 −
10 GeV range, which is most rel-evant for the NMH determination, the detector config-uration with 9 m vertical spacing provides the largesteffective mass and therefore largest available eventstatistics. This detector configuration was also foundto provide the best NMH sensitivity [3]. It will providedata samples of about 50,000 reconstructed upgoingneutrinos per year.
IV. SENSITIVITY TO NEUTRINO MASSHIERARCHY AND MORE
Building on the expected detector performance, asignificance analysis is performed by generating a largenumber of pseudo-experiments (PEs) with event dis-tributions in the reconstructed E ν – θ ν plane. For eachPE, a true hierarchy and a set of oscillation parame-ters is assumed. Each PE is analysed by performinga maximum likelihood fit with the oscillation parame-ters as free parameters and assuming either NH or IH.The likelihood ratio resulting from these fits is usedto quantify the separability between both hierarchies.Systematic uncertainties from the neutrinos fluxes,their cross sections as well as the detector response areparameterised as overall normalisation, energy scale, FIG. 3: Median NMH significance to exclude the otherhierarchy hypothesis assuming true NH (red) or IH (blue)as a function of true θ and assuming δ CP = 0 (solid) or δ CP = π (dashed). Three years of data taking with thefull-size ORCA detector are assumed. ν / ν skew, µ / e skew and NC/CC skew and are incor-porated as nuisance parameters. It is found that noneof these effects compromise substantially the ability ofORCA to determine the NMH [3].Figure 3 shows the median significance of ORCAto exclude the wrong hierarchy hypothesis after threeyears of data taking as a function of the true value of θ and assuming no CP-violation, i.e. δ CP equals 0or π . For the experimentally allowed range of θ andassuming δ CP = 0, the NMH can be measured withabout 3 σ after three years of operation. ORCA ismoderately sensitive to the CP-phase, the significancebeing reduced by at most 0 . σ if δ CP = π is realisedin nature. The significance increases dramatically incase of NH and θ > π/
4, reaching up to about 7 σ in three years of operation.Besides the NMH determination, ORCA can alsoimprove the uncertainties on ∆ m and θ . Bothparameters are determined without the need for con-strains from global data in conjunction with the NMH.Figure 4 shows the expected measurement precisionafter three years and compares it with current resultsof other experiments and their predicted performancesin 2020. The precision of ORCA is comparable orbetter, and is obtained with different systematic un-certainties. In particular, ORCA can determine theoctant of θ (above or below 45 ◦ ) for a wide range ofthe allowed parameter range.Additional science topics of ORCA include: testingthe unitary of the neutrino mixing matrix by studying (cid:44) (cid:45)ν τ appearance; indirect searches for sterile neutrinos,non-standard interactions and other exotic physics;indirect searches for dark matter; testing the chem-ical composition of the Earth’s core (Earth tomog-raphy); and low-energy neutrino astrophysics. Pre- eConf C16-09-04.3 XXV European Cosmic Ray Symposium, Turin, Sept. 4-9 2016
FIG. 4: One sigma contours of the measurement preci-sion in ∆ m and θ after three years of data taking withORCA for three assumed test cases (red). The currentresults from MINOS (back) and T2K (blue solid) are in-dicated, as well as the predicted performance of NO ν A(magenta) and T2K (blue dashed) in 2020. All contoursare at 1 σ for NH. liminary performance expectations are briefly sum-marised in [7]. The KM3NeT research infrastructurewill also house instrumentation for Earth and Sea sci-ences, such as marine biology, oceanography and geo-physics.Possible future options could be a long-baseline neu-trino beam targeted to ORCA [14], and a significantly denser detector instrumentation lowering the detec-tion threshold to measure the CP-phase δ CP with at-mospheric neutrinos [15]. V. CONCLUSIONS
With ORCA, a 6 Mton deep-sea Cherenkov detec-tor optimised for the detection of few-GeV neutrinos,the KM3NeT Collaboration aims to perform a high-statistics measurement of the zenith angle and energydependent event rates of atmospheric neutrinos thathave traversed the Earth. The oscillated neutrino fluxin the energy range 3–20 GeV holds the key to deter-mine the neutrino mass hierarchy. ORCA is expectedto achieve a 3–7 σ sensitivity to the neutrino mass hi-erarchy in three years of data taking. Simultaneously,ORCA will measure ∆ m and θ with competitiveprecision, and has a rich additional science program.In the first construction phase of ORCA, a 7-stringdemonstrator is expected to be concluded and oper-ational by the end of 2017. The full-size ORCA de-tector could be operational towards 2020, so that theneutrino mass hierarchy might be resolved as early as2023.Further details can be found in the recently pub-lished ‘Letter of Intent for KM3NeT 2.0’ [3]. [1] I. Esteban, et al., Updated fit to three neutrino mix-ing: exploring the accelerator-reactor complementar-ity (2016), arXiv:1611.01514 [hep-ph] .[2] E. K. Akhmedov, S. Razzaque and A. Y. Smirnov,
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