Reactor neutrino spectral distortions play little role in mass hierarchy experiments

Abstract

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 (thirtythree) times in prominence to the maximal contribution of the sawtooth-like distortions from the detailed energy spectrum, given 3.2%/ Evis./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.

Mapping reactor neutrino spectra from TAO to JUNO

“…By varying all the known nuclear data inputs, we generate and analyze an ensemble of 10 5 spectra in JUNO, but find no reduction of the sensitivity to mass ordering, with or without TAO; we trace this unexpected result to subtle undersampling issues in the generated bundle of spectra. We repeat the analysis by constructing an equally numerous but "more densely sampled" bundle and find a small reduction of the JUNO sensitivity, consistent with [59] and improved with the help of TAO. These results, based on "known" nuclear inputs, suggest some cautionary comments on parametrizations of "unknown" substructure uncertainties, as those considered in [58].…”

“…We test how these variants affect the JUNO oscillation analysis, and how well they can be distinguished by TAO, by scanning appropriate χ 2 functions over the whole spectral set(s). We recover, through an independent χ 2 analysis, the results obtained in [59] through a Fourier analysis, namely, that known substructure uncertainties do not appear to pose a threat to precision oscillometry in JUNO. However, the quantification of this result is not trivial, and some subtle problems in the statistical analysis will be highlighted.…”

“…Observing or constraining (at least a few) prominent substructures in future, highresolution and high-statistics reactor neutrino experiments, would be beneficial both for nuclear spectroscopy (allowing to pin down the spectral contributions of specific fission products [57]), and for neutrino oscillometry (reducing small-scale fuzzy uncertainties that might affect the JUNO sensitivity to mass ordering [58]). Although the latter benefit may be marginal if one assumes "known" substructures from nuclear databases [59,60], the observation of unexpected spectral anomalies at large energy scales (normalization and bump issues) provides a warning about the possible emergence of "unknown" features also at small scales. For a recent and comprehensive overview of current issues and future prospects in understanding reactor neutrino spectra, see the contributions in [61].…”

“…V, we generate bundles of Oklo variants around the previous reference spectrum. We perform an χ 2 analysis of variant spectra in JUNO, alternative to the Fourier analysis in [59], and highlight several statistical issues arising from sampling the nuclear input uncertainties. By varying all the known nuclear data inputs, we generate and analyze an ensemble of 10 5 spectra in JUNO, but find no reduction of the sensitivity to mass ordering, with or without TAO; we trace this unexpected result to subtle undersampling issues in the generated bundle of spectra.…”

Experimental aspects of geoneutrino detection: Status and perspectives

“…In a more recent study by Danielson et al[229] it is argued that there is no need for the near detector from the point of view of the neutrino mass hierarchy measurement; nevertheless, the near detector will still be needed for the robust geoneutrino signal measurement.…”