New signal of atmospheric tau neutrino appearance: Sub-GeV neutral-current interactions in JUNO

We propose the first practical method to detect atmospheric tau neutrino appearance at sub-GeV energies, which would be an important test of ${\nu }_{\mu }\to {\nu }_{\tau }$ oscillations and of new-physics scenarios. In the Jiangmen Underground Neutrino Observatory (JUNO; starts in 2024), active-flavor neutrinos eject neutrons from carbon via neutral-current quasielastic scattering. This produces a two-part signal: the prompt part is caused by the scattering of the neutron in the scintillator, and the delayed part by its radiative capture. Such events have been observed in KamLAND, but only in small numbers and were treated as a background. With ${\nu }_{\mu }\to {\nu }_{\tau }$ oscillations, JUNO should measure a clean sample of 55 events/yr; with simple ${\nu }_{\mu }$ disappearance, this would instead be 41 events/yr, where the latter is determined from Super-Kamiokande charged-current measurements at similar neutrino energies. Implementing this method will require precise laboratory measurements of neutrino-nucleus cross sections or other developments. With those, JUNO will have $5\sigma$ sensitivity to tau-neutrino appearance in five years of exposure, and likely sooner.

Figure 1

Left panel: the measured ${\nu }_{\mu }$ spectrum at Super-K compared to our predictions without and with oscillations. For clarity, we do not show the ${\nu }_e$ spectrum, which is hardly affected by oscillations; it is $\simeq 0.5$ times as large as the ${\nu }_{\mu }$ spectrum without oscillations. Right panel: the predicted neutron spectrum (without detector response; that is addressed in Sec. 4) from NC events in JUNO under ${\nu }_{\mu }\to {\nu }_{\tau }$ versus ${\nu }_{\mu }$ disappearance. Further details of the figure are explained below.

Figure 2

Zenith-angle distributions for sub-GeV (top panel) and multi-GeV (bottom panel) muon-neutrino events in Super-K ($328\mathrm{kton}\text{−}\mathrm{yr}$ exposure), compared to our predictions, showing very good agreement. The statistical uncertainties are tiny, but there is an overall systematic uncertainty of $\sim 25\%$ (not shown).

Figure 3

Neutron energy distributions—initial and quenched total deposition—for two example neutrino energies.

Figure 4

Spectrum of KamLAND’s atmospheric NC events in 7.8–31.8 MeV ($6.72\mathrm{kton}\text{−}\mathrm{yr}$ exposure), compared to our predictions that take into account the full detector response. For the KamLAND data, we have subtracted backgrounds due to spallation, reactor, and atmospheric CC events (all larger in the gray regions), and rebinned the spectrum.

Figure 5

Distributions (separately normalized) of parent-neutrino energies for atmospheric NC events in KamLAND, without and with analysis cuts. The sharpness of the step at low energies is an artifact of the binning.

Figure 6

Predicted spectrum of atmospheric NC events in 11–29 MeV in JUNO ($183\mathrm{kton}\text{−}\mathrm{yr}$ exposure), including taking into account the full detector response. We do not show the statistical uncertainties because they are evident and because our focus is on the integrated counts.

Figure 7

Our predicted sensitivities (statistical uncertainties only; see text) to ${\nu }_{\tau }$ appearance in atmospheric neutrino NC interactions (pink bars) compared to present constraints based on atmospheric neutrino CC interactions (gray bars). we also show current constraints on ${\nu }_{\tau }$ appearance from beamline and astrophysical measurements (light gray). Our results are the first to target sub-GeV energies.

Figure 8

Key neutrino total cross sections as a function of neutrino energy, expressed as values per nucleon and per energy, as obtained from genie3 [56]. Top: the oxygen cross sections. Bottom: the carbon cross sections.