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Medium-baseline reactor neutrino oscillation experiments (MBRO) have been proposed to determine the neutrino mass hierarchy (MH) and to make precise measurements of the neutrino oscillation parameters. With sufficient statistics, better than ~3%/sqrt{E} energy resolution and well understood energy non-linearity, MH can be determined by analyzing oscillation signals driven by the atmospheric mass-squared difference in the survival spectrum of reactor antineutrinos. With such high performance MBRO detectors, oscillation parameters, such as sin^22theta_{12}, Delta m^2_{21}, and Delta m^2_{32}, can be measured to sub-percent level, which enables a future test of the PMNS matrix unitarity to ~1% level and helps the forthcoming neutrinoless double beta decay experiments to constrain the allowed <m_{beta beta}> values. Combined with results from the next generation long-baseline beam neutrino and atmospheric neutrino oscillation experiments, the MH determination sensitivity can reach higher levels. In addition to the neutrino oscillation physics, MBRO detectors can also be utilized to study geoneutrinos, astrophysical neutrinos and proton decay. We propose to start a U.S. R&D program to identify, quantify and fulfill the key challenges essential for the success of MBRO experiments.
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In the past decade, the precise measurement of the lastly known neutrino mixing angle $theta_{13}$ has enabled the resolution of neutrino mass hierarchy (MH) at medium-baseline reactor neutrino oscillation (MBRO) experiments. On the other hand, recent calculations of the reactor neutrino flux predict percent-level sub-structures in the $bar u_e$ spectrum due to Coulomb effects in beta decay. Such fine structure in the reactor spectrum could be an important issue for the determination of neutrino MH for the MBRO approach since they could affect the sub-dominant oscillation pattern used to discriminate different hierarchies. Inconveniently, the energy resolutions of current reactor experiments are not sufficient to measure such fine structure, and therefore the size and location in energy of these predicted discontinuities has not been confirmed experimentally. There has been speculation that a near detector is required with sufficient energy resolution to resolve the fine structure such that it can be accounted for in any analysis which attempts to discriminate the MH. This article studies the impact of fine structure on the resolution of MH, based on the predicted reactor neutrino spectra, using the measured spectrum from Daya Bay as a reference. We also investigate whether a near detector could improve the sensitivity of neutrino MH resolution using various assumptions of near detector energy resolution.
The experimental bound on lifetime of nu_3, the neutrino mass eigenstate with the smallest nu_e component, is much weaker than those of nu_1 and nu_2 by many orders of magnitude to which the astrophysical constraints apply. We argue that the future reactor neutrino oscillation experiments with medium-baseline (~ 50 km), such as JUNO or RENO-50, has the best chance of placing the most stringent constraint on nu_3 lifetime among all neutrino experiments which utilize the artificial source neutrinos. Assuming decay into invisible states, we show by a detailed chi^2 analysis that the nu_3 lifetime divided by its mass, tau_3/m_3, can be constrained to be tau_3/m_3 > 7.5 (5.5) x 10^{-11} s/eV at 95% (99%) C.L. by 100 kt.years exposure by JUNO. It may be further improved to the level comparable to the atmospheric neutrino bound by its longer run. We also discuss to what extent nu_3 decay affects mass-ordering determination and precision measurements of the mixing parameters.
The Coulomb enhancement of low energy electrons in nuclear beta decay generates sharp cutoffs in the accompanying antineutrino spectrum at the beta decay endpoint energies. It has been conjectured that these features will interfere with measuring the effect of a neutrino mass hierarchy on an oscillated nuclear reactor antineutrino spectrum. These sawtooth-like features will appear in detailed reactor antineutrino spectra, with characteristic energy scales similar to the oscillation period critical to neutrino mass hierarchy determination near a 53 km baseline. However, these sawtooth-like distortions are found to contribute at a magnitude of only a few percent relative to the mass hierarchy-dependent oscillation pattern in Fourier space. In the Fourier cosine and sine transforms, the features that encode a neutrino mass hierarchy dominate by over sixteen (thirty-three) times in prominence to the maximal contribution of the sawtooth-like distortions from the detailed energy spectrum, given $3.2%/sqrt{E_mathrm{vis.}/mathrm{MeV}}$ (perfect) detector energy resolution. The effect of these distortions is shown to be negligible even when the uncertainties in the reactor spectrum, oscillation parameters, and counting statistics are considered. This result is shown to hold even when the opposite hierarchy oscillation patterns are nearly degenerate in energy space, if energy response nonlinearities are controlled to below 0.5%. Therefore with accurate knowledge of detector energy response, the sawtooth-like features in reactor antineutrino spectra will not significantly impede neutrino mass hierarchy measurements using reactor antineutrinos.
The sensitivity of the Deep Underground Neutrino Experiment (DUNE) to neutrino oscillation is determined, based on a full simulation, reconstruction, and event selection of the far detector and a full simulation and parameterized analysis of the near detector. Detailed uncertainties due to the flux prediction, neutrino interaction model, and detector effects are included. DUNE will resolve the neutrino mass hierarchy to a precision of 5$sigma$, for all $delta_{mathrm{CP}}$ values, after 2 years of running with the nominal detector design and beam configuration. It has the potential to observe charge-parity violation in the neutrino sector to a precision of 3$sigma$ (5$sigma$) after an exposure of 5 (10) years, for 50% of all $delta_{mathrm{CP}}$ values. It will also make precise measurements of other parameters governing long-baseline neutrino oscillation, and after an exposure of 15 years will achieve a similar sensitivity to $sin^{2} 2theta_{13}$ to current reactor experiments.
We examine the potential of the future medium-baseline reactor neutrino oscillation (MBRO) experiments in studying neutrino wave-packet impact. In our study, we treat neutrinos as wave packets and use the corresponding neutrino flavor transition probabilities. The delocalization, separation and spreading of the wave packets lead to decoherence and dispersion effects, which modify the plane-wave neutrino oscillation pattern, by amounts that depend on the energy uncertainties in the initial neutrino wave packets. We find that MBRO experiments could be sensitive to the wave-packet impact, since the baseline is long enough and also the capability of observing small corrections to the neutrino oscillations due to excellent detector energy resolution. Besides studying the constraints on the decoherence parameter, we also examine the potential wave-packet impacts on the precision of measuring $theta_{12}$ and other oscillation parameters in the future medium-baseline reactor neutrino oscillation experiments. Moreover, we also probe the potential benefits of an additional detector for studying such exotic neutrino physics.
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