Mass hierarchy determination via future atmospheric neutrino detectors

Abstract

We study the problem of determination of the sign of ∆m 2 31 , or the neutrino mass hierarchy, through observations of atmospheric neutrinos in future detectors. We consider two proposed detector types : (a) Megaton sized waterCerenkov detectors, which can measure the event rates of ν µ +ν µ and ν e +ν e and (b) 100 kton sized magnetized iron detectors, which can measure the event rates of ν µ andν µ . For energies and path-lengths relevant to atmospheric neutrinos, these rates obtain significant matter contributions from P µe , P µµ and P ee , leading to an appreciable sensitivity to the hierarchy. We do a binned χ 2 analysis of simulated data in these two types of detectors which includes the effect of smearing in neutrino energy and direction and incorporates detector efficiencies and relevant statistical, theoretical and systematic errors. We also marginalize the χ 2 over the allowed ranges of neutrino parameters in order to accurately account for their uncertainties. Finally, we compare the performance of both types of detectors vis a vis the hierarchy determination.

disappearance at the SPL, T2K-I, NOνA and the neutrino factory

“…8 we show the effect of a varying θ 13 (but fixed δ) on the previous fits at T2K-I for θ 23 Such a naive treatment of the experimental data is, however, not correct. The considered Super-Beam facilities have indeed been proposed to look for the appearance ν µ → ν e channel, the oscillation probability in vacuum expanded to the second order in the small parameters θ 13 and (∆ 12 L/E) [10,57] for which is:…”

“…The first term in the first parenthesis is the dominant one and is symmetric under θ 23 → π/2 − θ 23 . This is indeed the source of our present ignorance on the θ 23 -octant [13], parametrized by s oct = sign(tan 2θ 23 ). This symmetry is lifted by the other terms, that introduce a mild CP-conserving δ-dependence also, albeit through subleading effects very difficult to isolate.…”

“…The results of a four-parameters fit in (θ 23 The input value for δ isδ = 0 • . In each figure, dotted lines refer to the fit with θ 13 =θ 13 , δ =δ and s atm =s atm .…”

“…• Sensitivity to maximal θ 23 The potential to exclude maximal θ 23 has been computed in the following way: for a given ∆m 2 23 , we look for the largest value of θ 23 for which the two-parameter 3σ contours do not touch θ 23 = 45 • . We have considered both octants of θ 23 and we have found a behavior approximately symmetric of the sensitivity; we then display our results in the sin 2 θ 23 variable up to the maximal mixing value of 0.5.…”

“…It must be reminded, however, that this channel is afflicted by a four-fold degeneracy [20]: the sign of the atmospheric mass difference and the θ 23 -octant are poorly measured through this channel, indeed 2 . It has been proposed to face this problem using atmospheric neutrinos at T2K-II, [21,22], or at a magnetized iron calorimeter, [23,24].…”