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      On Possibility of Determining Neutrino Mass Hierarchy by the Charged-Current and Neutral-Current Events of Supernova Neutrinos in Scintillation Detectors

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          Abstract

          One of the unresolved mysteries in neutrino physics is the neutrino mass hierarchy. We present a new method to determine neutrino mass hierarchy by comparing the events of inverse beta decays (IBD), \(\bar{\nu}_e + p\rightarrow n + e^+\), and neutral current (NC) interactions, \(\nu(\overline{\nu}) + p\rightarrow\nu(\overline{\nu}) + p\), of supernova neutrinos from accretion and cooling phases in scintillation detectors. Supernova neutrino flavor conversions depend on the neutrino mass hierarchy. On account of Mikheyev-Smirnov-Wolfenstein effects, the full swapping of \(\bar{\nu}_e\) flux with the \(\bar{\nu}_x\) (\(x=\mu,~\tau\)) one occurs in the inverted hierarchy, while such a swapping does not occur in the normal hierarchy. In consequence, the ratio of high energy IBD events to NC events for the inverted hierarchy is higher than in the normal hierarchy. Since the luminosity of \(\bar{\nu}_e\) is larger than that of \(\nu_x\) in accretion phase while the luminosity of \(\bar{\nu}_e\) becomes smaller than that of \(\nu_x\) in cooling phase, we calculate this ratio for both accretion and cooling phases. By analyzing the change of this event ratio from accretion phase to cooling phase, one can determine the neutrino mass hierarchy.

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          Neutrino mass hierarchy and electron neutrino oscillation parameters with one hundred thousand reactor events

          Proposed medium-baseline reactor neutrino experiments offer unprecedented opportunities to probe, at the same time, the mass-mixing parameters which govern \(\nu_e\) oscillations both at short wavelength (delta m^2 and theta_{12}) and at long wavelength (Delta m^2 and theta_{13}), as well as their tiny interference effects related to the mass hierarchy (i.e., the relative sign of Delta m^2 and delta m^2). In order to take full advantage of these opportunities, precision calculations and refined statistical analyses of event spectra are required. In such a context, we revisit several input ingredients, including: nucleon recoil in inverse beta decay and its impact on energy reconstruction and resolution, hierarchy and matter effects in the oscillation probability, spread of reactor distances, irreducible backgrounds from geoneutrinos and from far reactors, and degeneracies between energy scale and spectrum shape uncertainties. We also introduce a continuous parameter alpha, which interpolates smoothly between normal hierarchy (alpha=+1) and inverted hierarchy (alpha=-1). The determination of the hierarchy is then transformed from a test of hypothesis to a parameter estimation, with a sensitivity given by the distance of the true case (either alpha=+1 or alpha=-1) from the undecidable case (alpha=0). Numerical experiments are performed for the specific set up envisaged for the JUNO project, assuming a realistic sample of O(10^5) reactor events. We find a typical sensitivity of ~2 sigma to the hierarchy in JUNO, which, however, can be challenged by energy scale and spectrum shape systematics, whose possible conspiracy effects are investigated. The prospective accuracy reachable for the other mass-mixing parameters is also discussed.
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            Author and article information

            Journal
            13 July 2018
            Article
            1807.05170
            82208b8d-0340-4ed1-9111-9cb9d53fde84

            http://arxiv.org/licenses/nonexclusive-distrib/1.0/

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            Custom metadata
            one column, 16 pages, 2 figures
            astro-ph.HE hep-ph

            High energy & Particle physics,High energy astrophysical phenomena
            High energy & Particle physics, High energy astrophysical phenomena

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