Impact of New Physics on the JUNO-Long-Baseline Synergy in Neutrino Mass Ordering Determination

Abstract

The determination of the neutrino mass ordering is one of the flagship goals in particle physics. A well-known and powerful synergy emerges when combining high-precision measurements of the effective atmospheric mass-squared splitting from electron antineutrino disappearance in reactor experiments with that from muon (anti)neutrino disappearance in accelerator-based long-baseline experiments. To fully exploit this synergy, percent-level precision in the atmospheric mass splitting is required-a target that JUNO is expected to achieve within a few months of data taking. This motivated the formulation of a mass ordering sum rule for neutrino disappearance channels, which shows that by combining data from T2K and NOvA with JUNO after one year of operation, the neutrino mass ordering can be determined at the $3σ$ confidence level. Since JUNO has recently started taking data, it is timely to ask whether this sum rule remains robust in the presence of new physics. We identify the necessary conditions for new physics to affect the sum rule and demonstrate that, in some cases, such effects could lead to an incorrect inference of the mass ordering. As concrete examples, we consider Scalar Non-Standard Interactions (SNSI) and neutrinos coupled to an ultralight scalar field. We find that, for SNSI, current constraints render any modification of the sum rule negligible, whereas in the latter case, the inference of the ordering requires caution. Nevertheless, these effects can be disentangled, illustrating how the sum rule can also be used to search for new physics.

Figures (4)

• Figure 1: Iso-contours of $\Delta \chi^2(\Delta m^2_{31}\vert^{\rm NO}_{\rm JU},\sigma_{\rm JU}$,$\delta m^2_{\rm LJ})$ representing the mass ordering preference when combining JUNO and LBL data. The blue region corresponds to values of $\Delta m^2_{31}\vert^{\rm NO}_{\rm JU}$, as measured by JUNO, for which the synergy favors NO, while the red region corresponds to values where IO is favored. The left plot reproduces the standard fan plot of Ref. Parke:2024xre, while the right plot shows a scenario where new physics introduces a displacement between the LBL and JUNO best-fit values. Notably, this displacement is sufficient to incorrectly infer the neutrino mass ordering.
• Figure 2: Expected time evolution of the atmospheric mass-splitting best-fit values for LBL experiments and JUNO, shown for two different neutrino mass values: $m_3 = 0.065$ eV (blue) and $m_3 = 0.1$ eV (orange). The scalar field parameters are fixed at $\eta_\phi = 0.01$, $\theta = 0$, and $m_\phi = 2\times10^{-24}$ eV. Time $t=0$ corresponds to the start of LBL data taking, with JUNO beginning operations 12 years later. The vertical arrow indicates the critical shift $\delta m^2_{\rm LJ}$ that could lead to an incorrect mass-ordering inference. LBL uncertainty bars are back-propagated from current measurements, and the points are slightly displaced for visibility. JUNO uncertainty bars follow the expected precision improvement with accumulated statistics. Normal ordering is assumed.
• Figure 3: Expected time evolution of JUNO's best-fit atmospheric mass splitting when analyzing data in consecutive one-year slices where it is assumed that JUNO started taking data at $t=12$ years (see text for details), shown for two neutrino mass values. The scalar parameters are fixed as in Fig. \ref{['fig:best_fit_time_dep']}. The uncertainty bars are fixed at the one-year precision level. The colored bands represent $1\sigma$ (innermost), $2\sigma$ (middle), and $3\sigma$ (outermost) confidence regions around the initial best-fit value. The systematic drift of the central values demonstrates how time-sliced analysis can reveal the presence of ultralight scalar background. A nearly $3\sigma$ deviation emerges after five years for $m_3 = 0.065$ eV and after only three years for $m_3 = 0.1$ eV.
• Figure 4: Same as Fig. \ref{['fig:best_fit_time_dep']} but for different scalar phases. The neutrino mass is kept fixed at $m_3 = 0.1$ eV and the other scalar parameters are fixed as in Fig. \ref{['fig:best_fit_time_dep']}. The phases $\theta = \pi/2$ and $\theta = 5\pi/4$ were selected to maximize the deviation from the $\theta = 0$ case.