ν τ appearance with KM3NeT/ORCA T. Eberl, S. Hallmann*, J. Hofestädt for the KM3NeT Collaboration 35 th ICRC, Busan, South Korea, 15/07/2017 *speaker
KM3NeT/ORCA A detector to study oscillations of the atmospheric neutrino flux ( ν e , ν μ ) Deep-sea neutrino detector consisting of a 3D array of Digital Optical Modules (DOMs) with 31 PMTs per DOM Dense instrumentation: ~ 20m horizontal spacing ~ 9m vertical spacing between DOMs Total: 2078 DOMs Instrumented volume: ~6 Mm³ see the Letter of Intent for KM3NeT 2.0 [J.Phys.G43 (2016) no.8, 084001] and the talk by A. Kouchner in this session for more information Recent developments in detector layout, trigger and reconstruction performance will improve the sensitivity presented here. ICRC 2017 – 15/07/2017 – S. Hallmann (KM3NeT Collaboration) 2
Oscillation of atmospheric neutrinos Region of interest for determination of neutrino mass ordering → Broad oscillation maximum from ν μ → ν τ at ~25 GeV for upward- going neutrinos → No large modulation due to matter efgects [oscillation probabilities: https://github.com/joaoabcoelho/OscProb] ICRC 2017 – 15/07/2017 – S. Hallmann (KM3NeT Collaboration) 3
(Dis)appearance measurements Broad oscillation maximum at ~25 GeV can be used to study - dis-appearance of ν μ → performed and ongoing measurements by many experiments (NOvA, T2K, SuperK, IceCube, ANTARES, …) ∟ I. Salvadori, PoS(ICRC2017)1169, ∟A. Habig, PoS(ICRC2017)1168, poster board 152 poster board 151 - appearance of ν τ → challenging due to - energy threshold for τ production ( ~3.4 GeV ) - prompt τ decay with similar signatures as other interaction channels [Rev. Mod. Phys. 84, 1307 (2012)] ICRC 2017 – 15/07/2017 – S. Hallmann (KM3NeT Collaboration) 4
Why are we interested in ν τ `s? - ν τ channel currently least constrained [Phys. Rev. Lett. 115 (2015), 121802] → only non-appearance already excluded → normalisation not well determined - 3x3 neutrino mixing via the PMNS matrix is assumed unitary → true, if the current picture of 3 flavours is complete → deviations from unitarity are an indirect evidence for new physics (sterile neutrinos,…) ICRC 2017 – 15/07/2017 – S. Hallmann (KM3NeT Collaboration) 5
Normalisation determination Current measurements: - Able to exclude non-appearance - Measured central values are high - Large uncertainty on normalisation due to low statistics OPERA: event-by-event vertex resolution, >5σ with 5 events Super K: statistical basis, tens of events per year using multivariate analysis [Phys. Rev. Lett. 110 (2013), 181802] [Phys. Rev. Lett. 115 (2015), 121802] Bands: 90% confidence level ICRC 2017 – 15/07/2017 – S. Hallmann (KM3NeT Collaboration) 6
Normalisation determination IceCube DeepCore: - Sensitivity to exclude non-appearance on the 5σ-level Future experiments: Similar sensitivity - Optimised for mass hierarchy studies (3-10 GeV range) - Large effective volume at tau-appearance energies → large statistics - More stringent determination of normalisation possible this work [IceCube-Gen2 Coll., arXiv:1607.0267] [M. Larson (IceCube Coll.), Neutrino2016] [Phys. Rev. Lett. 110 (2013), 181802] [Phys. Rev. Lett. 115 (2015), 121802] Bands: 90% confidence level ICRC 2017 – 15/07/2017 – S. Hallmann (KM3NeT Collaboration) 7
Measurement on statistical basis - Small kink produced by the τ lepton impossible to see for large volume Cherenkov detectors → cannot measure ν τ appearance on event-by-event basis Particle ID in KM3NeT/ORCA: [J.Phys.G43 (2016) no.8, 084001] 70% (90%) of ν μ ( ν μ ) identifjed as track >80% of ν τ identifjed as shower → appearance measured on statistical basis by excess in the shower channel ICRC 2017 – 15/07/2017 – S. Hallmann (KM3NeT Collaboration) 8
Relevant energy and zenith range Statistical per-bin significance in the shower channel: N( ν τ CC) ν e ν μ ν τ ν selected up- C C C C C C N C going shower 17k 11k 2.5k 6k events year √ N(all neutrino interactions) Relevant range: up-going and 10 GeV – 30 GeV energy ICRC 2017 – 15/07/2017 – S. Hallmann (KM3NeT Collaboration) 9
Multi-parameter fitting Take into account: - uncertainties on oscillation parameters - uncertainties on atmospheric neutrino fluxes (nuisance parameters) Degradation due to nuisance parameters is moderate The fit result is robust against the octant of θ 23 and the assumed true neutrino mass ordering → measurement is possible before knowing the true mass ordering ICRC 2017 – 15/07/2017 – S. Hallmann (KM3NeT Collaboration) 10
KM3NeT/ORCA sensitivity Expectation from 3x3 unitary mixing No ν τ appearance → Non-appearance excluded (5 σ CL) after two months of operation → After 1 year of operation: ν τ normalisation determined better than ±15 % (90% CL) / ±4 0% (5 σ CL) ICRC 2017 – 15/07/2017 – S. Hallmann (KM3NeT Collaboration) 11
Summary Measurement of ν τ appearance possible with KM3NeT/ORCA: - Exclusion of non-appearance will be confjrmed within fjrst months of operation → one of the early physics results of KM3NeT/ORCA - In the longer run ν τ normalisation can be determined with high precision → possibility to test unitarity of 3x3 neutrino mixing Normalisation fjt is robust against - uncertainties in neutrino oscillation (mass ordering, octant of θ 23 ) - further uncertainties (nuisance parameters) ICRC 2017 – 15/07/2017 – S. Hallmann (KM3NeT Collaboration) 12
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