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):
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
5 years with
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
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:
atmospheric νe & νμ
MSW effect in Earth matter
(at different baselines)
M. Honda et al., Phys. Rev.D 75 (2007), 043006
Formaggio, Zeller, Rev. Mod. Phys. 84(3) 1307 (2012)
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).
Expected number of events
3 years with
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.
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
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