Resolving the eightfold neutrino parameter degeneracy by two identical detectors with different baselines

Department of Physics, Tokyo Metropolitan University, Edo, Tōkyō, Japan
Physical review D: Particles and fields (Impact Factor: 4.86). 01/2007; 75(1). DOI: 10.1103/PhysRevD.75.013006
Source: arXiv


We have shown in a previous paper that two identical detectors with each fiducial mass of 0.27 megaton water, one in Kamioka and the other in Korea, which receive the (anti-) muon neutrino beam of 4 MW power from J-PARC facility have potential of determining the neutrino mass hierarchy and discovering CP violation by resolving the degeneracies associated with them. In this paper, we point out that the same setting has capability of resolving the θ23 octant degeneracy in region where sin⁡22θ23≲0.97 at 2 standard deviation confidence level even for very small values of θ13. Altogether, it is demonstrated that one can solve all the eightfold neutrino parameter degeneracies in situ by using the Tokai-to-Kamioka-Korea setting if θ13 is within reach by the next generation superbeam experiments. We also prove the property called “decoupling between the degeneracies”, which is valid to first-order in perturbation theory of the earth matter effect, that guarantees approximate independence between analyses to solve any one of the three different type of degeneracies.

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    • "The relatively large reactor angle opens up opportunities [7] for determining the mass hierarchy, the octant of the atmospheric mixing angle, and the CP phase. The first could be achieved with a medium baseline reactor experiment89101112131415161718192021 and long baseline accelerator experiments22232425262728293031323334353637 44] could measure all three of them. Atmospheric neutrino experiments could offer alternative ways to accomplish the same. "
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    ABSTRACT: We develop a general theoretical framework to analytically disentangle the contributions of the neutrino mass hierarchy, the atmospheric mixing angle, and the CP phase, in neutrino oscillations. To illustrate the usefulness of this framework, especially that it can serve as a complementary tool to neutrino oscillogram in the study of atmospheric neutrino oscillations, we take PINGU as an example and compute muon- and electron-like event rates with event cuts on neutrino energy and zenith angle. Under the assumption of exact momentum measurements of neutrinos with a perfect e-$\mu$ identification and no backgrounds, we find that the PINGU experiment has the potential of resolving the neutrino mass hierarchy and the octant degeneracies within 1-year run, while the measurement of the CP phase is significantly more challenging. Our observation merits a serious study of the detector capability of estimating the neutrino momentum for both muon- and electron-like events.
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    • "Adding the information from the 'silver' channel (ν e → ν τ ) to the 'golden' channel (ν e → ν µ ) in the proposed neutrino factory setup is demonstrated to be one of the elegant ways to tackle this degeneracy [46] [47]. The possibility of determining the deviation of θ 23 from maximal mixing and consequently the correct octant of θ 23 in very long-baseline neutrino oscillation experiments and as well as in future atmospheric neutrino experiments has been discussed in [48] [49] [50] [51] [52] [53] [54] [55] [56]. One clear message that has been conveyed by all these novel works is that one can achieve a very good sensitivity to the quantity |0.5 − sin 2 θ 23 | from the conventional beam experiments (MINOS, ICARUS and OPERA), the current generation superbeam experiments (presently running T2K and upcoming NOνA), and also from the current (Super-Kamiokande) and future atmospheric data (India-based Neutrino Observatory). "
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    ABSTRACT: Preliminary results of MINOS experiment indicate that theta23 is not maximal. Global fits to world neutrino data suggest two nearly degenerate solutions for theta23: one in the lower octant (LO: theta23 < 45 degree) and the other in the higher octant (HO: theta23 > 45 degree). numu to nue oscillations in superbeam experiments are sensitive to the octant and are capable of resolving this degeneracy. We study the prospects of this resolution by the current T2K and upcoming NOvA experiments. Because of the hierarchy-deltacp degeneracy and the octant-deltacp degeneracy, the impact of hierarchy on octant resolution has to be taken into account. As in the case of hierarchy determination, there exist favorable (unfavorable) values of deltacp for which octant resolution is easy (challenging). However, for octant resolution the unfavorable deltacp values of the neutrino data are favorable for the anti-neutrino data and vice-verse. This is in contrast to the case of hierarchy determination. In this paper, we compute the combined sensitivity of T2K and NOvA to resolve the octant ambiguity. If sin^2\theta23 =0.41, then NOvA can rule out all the values of theta23 in HO at 2 sigma C.L., irrespective of the hierarchy and deltacp. Addition of T2K data improves the octant sensitivity. If T2K were to have equal neutrino and anti-neutrino runs of 2.5 years each, a 2 sigma resolution of the octant becomes possible provided sin^2\theta23 \leq 0.43 or \geq 0.58 for any value of deltacp.
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