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Topical Research Meeting on Prospects in Neutrino Physics   London, December 19-20, 2013. ORCA ( O scillation R esearch with C osmics in the A byss):

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Topical Research Meeting on Prospects in Neutrino Physics   London, December 19-20, 2013

ORCA (Oscillation Research with Cosmics in the Abyss):

a feasibility study for the measurement of neutrino mass hierarchy

with atmospheric neutrinos in the Mediterranean

Véronique Van Elewyck (Laboratoire APC & Université Paris 7 Denis Diderot)

on behalf of the KM3NeT Collaboration





vertex position

5 years with

1.75 Mton

s(q23) (deg)

s(Dm2large) (eV2)


Measuring the neutrino mass hierarchy with atmospheric neutrinos

ORCA: an undersea neutrino Cherenkov detector within the KM3NeT framework



ORCA is building on the expertise of the KM3NeT Collaboration, whichaimsat

the deployment of a multi-km3 neutrino telescope in the Mediterranean to performhigh-energy (TeV-PeV) neutrino astronomy:

- samedetectionprinciple: Cherenkov light emitted by the secondaryparticles

produced by the neutrino interaction withmatter in and around the detector

- sametechnology and detector design (instrumentedlinesanchoredat the seabed and supportingmulti-PMT digital optical modules)

- denserarray to lower the energythreshold to ~GeVneutrino energies

- line spacing and lengthlimited by deploymentcontingencies

A reference detector isused for the first stages of the feasibilitystudy:

(full optimisation to beperformed on basis of an overdense detector withdifferent

optical module masking options)

The values of all neutrino oscillation parameters in the 3ν scheme can now be extracted from global fits of available data with a precision better than 15%, the largest remaining uncertainty being on θ23 and its octant. The relatively large value of θ13 (which drives the νe νμ oscillation) is an asset for the determination of the remaining unknown neutrino parameters: the value of the CP-violating phase δCP, and the ordering of the neutrino masses, aka the neutrino mass hierarchy (NMH: normal or inverted, see plot).

Standard strategy for the determination of NMH: probe νe νμ oscillation in presence of matter effects which allow to resolve the sign of Δm213 :




The oscillation enhancement is maximal at

resonant energy

Eres ≈ few GeV for Earth matter density:

good prospects for atmospheric neutrinos !

(anti-)neutrino cross-section: σνN ≈ 2σνN


Maximal effect at ≈ few GeV for Earth matter densities

(depending on the baseline)

BUT P(νμ  νμ , NH) = P(νμ  νμ , IH):

other ingredients are needed to create a difference in the number of events IH vs. NH:



Simulation chain

atmospheric νe & νμ

(anti-)neutrino fluxes

MSW effect in Earth matter

(at different baselines)

  • atmospheric νe + νμ (+anti) 1  500 GeV
  • (Bartol flux, Agrawal et al. PRD 53, (1996) 1314.)
  • neutrino interaction: QE+RES+DIS (GENHEN),
  • both charged and neutral current;
  • - For all particles crossing the can volume (size ~3 absorption lengths around the detector):
  • tracking + Cherenkov light emission
  • down-going atmospheric muon background based on MUPAGE parameterization (Carminati et al., Comput. Phys. Commun. 179 (2008) 915-923)
  • optical background (40K decay: flat noise, 5kHz/PMT + 500 Hz in-DOM time-correlated



M. Honda et al., Phys. Rev.D 75 (2007), 043006

Formaggio, Zeller, Rev. Mod. Phys. 84(3) 1307 (2012)

  • Distinctive patterns in (Eν, cos θν) oscillograms showing the difference in expected number of events for normal and inverted hierarchies
  • NH/IH difference intrinsically small
  • - further limited by kinematics and intrinsic
  • fluctuations
  • oscillograms blurred by limited energy and angular accuracy of the detector

Reconstruction strategy

  • muon track reconstruction: likelihood maximisation based on the hit time residuals (ANTARES-like)
  • + length estimation on basis of first/last emission points
  • Identification of hits belonging to the hadronic shower and re-estimation of the vertex position (assuming spherically expanding shower)
  • full shower reconstruction (for NC and e/τ CC)currently under study

E, θ smearing (kinematics + detector resolution)

E. Akhmedov et al., JHEP 02 (2013), 082

Current performances with the reference detector – νμ CC (track-like) channel

Peformance studies have been focusing so far on the νμ charged-current channel which provides a relatively clear experimental signature (muon track + hadronic shower).

Semicontained events (i.e. with the reconstructed vertex inside the instrumented volume) selected with a quality criterium on the track fit provide reasonably good angular accuracy (median Δθ better than 10° for Eν ≥ 5 GeV). The corresponding effective mass of the detector reaches its plateau value of ≈ 1.8 Mton at Eν ≈ 8 GeV.

The muon energy estimation is most reliable when an additional containment condition is imposed on the full muon track, ensuring that most of the Cherenkov light is deposited inside the detector. Using the median reconstructed energy as an estimator of the neutrino energy, a resolution of 35% can be reached at 10 GeV. Studies are ongoing to evaluate the energy deposited in the hadronic shower in order to improve the determination of the neutrino energy.

Further studies are also conducted to evaluate the possible contribution from the cascade-like channels ( νe and ντ CC, all flavours NC); preliminary results suggest a significant enhancement of the mass hierarchy discrimination power (see e.g. Ge & Hagiwara, arXiv1312.0457).

Sensitivity study

Expected number of events




Statistical method

  • The performance of ORCA for the determination of the neutrino mass hierarchy is assessed by means of a likelihood ratio test with nuisance parameters:
  • For each toy dataset generated with a given
  • hierarchy, the mixing parameters are fit with
  • either IH or NH and the log-likelihood ratio
  • is computed. The separation of the two
  • resulting distributions is used to compute
  • the mass hierarchy significance.

3 years with

5 Mton

  • The sensitivity curves shown hereabove have been obtained from a preliminary study with simplified assumptions:
  • - perfect muon zenith angle reconstruction
  • - true neutrino vertex contained
  • - at least 15 hits (from GEANT4 simulation)
  • - no reconstruction assumed
  • 2.5  4σ in 5 years with the reference detector
  • 3  5σ in 3 years with 5 Mton
  • with a significant dependence in the reconstruction performance
  • More detailed studies ongoing…


A neutrino beam to ORCA ?

Impact of systematics


Muon background rejection capabilities


Counting events related to a neutrino beam of given flavour can help increase the sensitivity of ORCA to the mass hierarchy even with a moderate capability of flavour determination in the detector.

In the appearance channel νμ  νe and for a typical νμ beam in the energy range 2-8 GeV, an optimal separation between NH and IH is found for cosθ = 0.2, i.e. a baseline of 2600 km. The oscillation probabilities are only marginally affected by the value of δCP.

  • The main physical background in the detector comes from the abundant flux of down-going atmospheric muons which can be misreconstructed as upgoing, mimicking a νμ CC event. Preliminary studies show that the muon contamination (Cμ) can be efficiently reduced by cuts on the reconstructed muon track parameters, while maintaining a high neutrino signal efficiency:
  • angular error estimate:
      • β < 2°
  • track fit quality parameter:
  • Λ > (-4)-(-4.5)
  • reconstructed vertex position:
  • Rν < 75-80 m
  • (from detector centre)

Intrinsic detector capabilities are not the only factor that can spoil the measurement of the mass hierarchy. Other sources of uncertainties can lead to biases in the statistical distributions that may result in a degraded discrimination power or in wrong hierarchy determination (false positives). Several independent studies have addressed the impact of systematic errors on the sensitivity and identified the most critical among them.



D. Franco et al., JHEP 04 (2013) 008

  • Earth density profile (PREM model): negligible impact
  • Atmospheric neutrino fluxes:
    • moderate impact of the shape uncertainties
    • large overall normalization uncertainty can be mitigated by anchoring the fit at Eν > 20 GeV
  • Oscillation parameters: significant impact of uncertainties in atmospheric sector
  • (Δm2large, θ23, θ13), in particular θ23 octant ; solar sector and δCP have negligible impact
  • ORCA can improve on the precision of Δm2large, θ23 through global fit procedure !




Rν < 75 m (solid)

A beam with a suitable baseline could be built from the IHEP in Protvino (near Moscow, Rus-sia) towards one of the potential sites for ORCA, with a baseline of 2588 km and a moderate (~11.7°) inclination. Assuming Npot≈1.5*1021 within a few years, a 7σ measurement of the hierarchy can be achieved (3σ with 3-4% systematic uncertainty on the event rate).

J. Brunner, arXiv:1304.6230

W. Winter, Phys. Rev. D88 (2013) 113007