⚛️ phenomenology

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

This paper investigates the robustness of the JUNO-long-baseline synergy in determining the neutrino mass ordering under new physics scenarios, finding that while Scalar Non-Standard Interactions have negligible impact, couplings to ultralight scalar fields can lead to incorrect inferences, though these effects can ultimately be disentangled to also serve as a probe for new physics.

Original authors: Gustavo F. S. Alves, Hiroshi Nunokawa, Renata Zukanovich Funchal

Published 2026-03-19

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Gustavo F. S. Alves, Hiroshi Nunokawa, Renata Zukanovich Funchal

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Big Mystery: Which Neutrino is the Heaviest?

Imagine you have three siblings: Sibling 1, Sibling 2, and Sibling 3. You know for a fact that Sibling 2 is heavier than Sibling 1 (thanks to some old family records from a solar experiment). But you have no idea where Sibling 3 fits in.

There are only two possibilities:

Normal Order: Sibling 1 < Sibling 2 < Sibling 3 (The "Tall" sibling is the heaviest).
Inverted Order: Sibling 3 < Sibling 1 < Sibling 2 (The "Tall" sibling is actually the lightest).

Figuring out which one is true is one of the biggest goals in modern physics. The paper discusses a new, clever plan to solve this mystery using two different teams of scientists.

The "Dream Team" Strategy

To solve the mystery, the scientists are combining data from two very different types of experiments:

The Reactor Team (JUNO): Located in China, this is a massive detector sitting near a nuclear power plant. It watches neutrinos disappear. It's like a high-precision scale that can weigh things with incredible accuracy (to the "milli-gram" level).
The Accelerator Team (T2K & NOvA): These are experiments in Japan and the US that shoot beams of neutrinos through the Earth. They are like long-distance runners who have been collecting data for over a decade.

The Synergy (The "Sum Rule"):
Previously, neither team could solve the mystery alone. But recently, scientists realized that if you combine the "long-distance" data with the "high-precision" data, the two teams can cross-check each other.

Think of it like solving a puzzle where Team A has the edge pieces and Team B has the center pieces. When you put them together, the picture becomes clear. The paper says that by combining one year of JUNO's data with the existing data from the accelerator teams, we should be able to solve the "Sibling 3" mystery with 99.7% certainty (3-sigma).

The Plot Twist: What if Physics is "Fake"?

The authors of this paper asked a scary question: "What if the rules of the game are slightly different than we think?"

They wondered if "New Physics" (mysterious forces or particles we haven't discovered yet) could trick the scientists. Specifically, they looked at two scenarios where the "weight" of the neutrinos might change depending on where or when you measure them.

Scenario 1: The "Heavy Blanket" (Scalar Non-Standard Interactions)

Imagine the neutrinos are walking through a room.

The Accelerator Team walks through a room with a thin blanket.
The JUNO Team walks through a room with a slightly thicker blanket.

If a mysterious "heavy blanket" (a new particle interaction) exists, it might make the neutrinos feel heavier in one room than the other. This would make the two teams disagree on the weight, breaking the puzzle.

The Verdict: The authors checked the math and found that the "blanket" is too light to cause any trouble. The universe has already put strict limits on how heavy this blanket can be. So, this trick won't work. The synergy is safe from this specific problem.

Scenario 2: The "Shifting Clock" (Ultralight Scalar Fields)

This is the more interesting (and dangerous) scenario. Imagine the "weight" of the neutrinos isn't fixed; it's like a breathing balloon that expands and contracts very slowly over decades.

The Accelerator Team measured the balloon 10 years ago.
The JUNO Team is measuring it right now.

If the balloon is in a different phase of its "breathing" cycle when JUNO measures it compared to when the Accelerator Team measured it, the two teams will get different numbers.

The Danger: If JUNO measures the balloon while it's "inflated" and the Accelerator team measured it while it was "deflated," the math will get confused. The scientists might combine the data and conclude, "Oh, Sibling 3 is the lightest!" when in reality, "Sibling 3 is the heaviest." They would solve the puzzle, but solve it wrong.

The Solution: Check the Clock

The paper offers a brilliant solution to this "Shifting Clock" problem.

Since JUNO is so incredibly precise, it doesn't just need to measure the weight once. It can measure the weight every year.

If the weight stays the same every year, the "breathing balloon" theory is wrong, and the standard solution is correct.
If the weight slowly drifts up or down over the years, JUNO will catch the "breathing" pattern.

The authors suggest that JUNO should analyze its data in one-year slices. If they see the numbers drifting, they will know that "New Physics" is at play. They can then adjust their math to account for the breathing balloon and still find the correct answer.

The Bottom Line

The Plan: Combining JUNO (China) with T2K/NOvA (Japan/US) is a powerful way to solve the neutrino mass mystery.
The Risk: A mysterious, invisible "breathing field" in the universe could trick the math, leading scientists to the wrong conclusion about which neutrino is heaviest.
The Safety Net: Because JUNO is so precise, it can watch for this trick in real-time. By checking the data year-by-year, JUNO can spot the "breathing" pattern, correct the math, and ensure we get the right answer.

In short: The plan is solid, but we need to keep our eyes open for invisible cosmic tricks.

1. Problem Statement

The determination of the neutrino mass ordering (whether the neutrino mass eigenstates follow a Normal Ordering [NO: $m_1 < m_2 < m_3$ ] or Inverted Ordering [IO: $m_3 < m_1 < m_2$ ]) is a primary goal of modern particle physics.

The Synergy: A powerful method to resolve this involves combining data from:
1. Long-Baseline (LBL) experiments (T2K, NOvA): Measuring the effective atmospheric mass-squared splitting ( $|\Delta m^2_{\mu\mu}|$ ) via $\nu_\mu$ disappearance.
2. Reactor experiments (JUNO): Measuring the effective atmospheric mass-squared splitting ( $|\Delta m^2_{ee}|$ ) via $\bar{\nu}_e$ disappearance with unprecedented precision (sub-percent level).
The Sum Rule: Previous work established a "mass ordering sum rule" showing that combining these datasets allows for a $3\sigma$ determination of the mass ordering within one year of JUNO operation.
The Risk: This paper investigates whether New Physics (NP) could distort the relationship between the measurements from LBL and JUNO, potentially leading to an incorrect inference of the mass ordering (e.g., concluding IO when nature is NO, or vice versa).

2. Methodology

The authors analyze the robustness of the mass ordering sum rule under the influence of New Physics by modeling how NP modifies the effective mass-squared differences measured in different experimental environments.

General Framework:
- They define a $\chi^2$ function combining LBL and JUNO data.
- They introduce a mass shift parameter, $\delta m^2_{LJ}$ , representing the discrepancy in the measured atmospheric mass splitting between LBL and JUNO caused by New Physics.
- The modified sum rule becomes:
  $(\Delta m^2_{31}|^{NO}_{LBL} - \Delta m^2_{31}|^{NO}_{JU} - \delta m^2_{LJ}) + (|\Delta m^2_{32}|^{IO}_{JU} - |\Delta m^2_{32}|^{IO}_{LBL} - \delta m^2_{LJ}) \approx \text{Standard Terms}$
- They calculate the condition under which $\Delta \chi^2$ (the statistical preference for NO vs. IO) flips sign, leading to a wrong conclusion. They find that a shift of $\delta m^2_{LJ} \approx 4 \times 10^{-5} \text{ eV}^2$ is sufficient to swap the mass ordering preference.
Specific Scenarios Investigated:
1. Scalar Non-Standard Interactions (SNSI): Neutrinos coupled to a light scalar mediator $\phi$ that modifies neutrino masses in matter.
2. Time-Dependent Neutrino Masses: Neutrinos coupled to an ultralight scalar field acting as a classical stochastic background, causing the neutrino mass to oscillate over time.

3. Key Contributions & Results

A. General Conditions for Failure

The paper establishes that the synergy fails if New Physics induces a differential shift in the best-fit values of the mass splitting between the two experimental setups.

Critical Threshold: If the difference in the measured $\Delta m^2_{atm}$ between LBL and JUNO exceeds $\sim 4 \times 10^{-5} \text{ eV}^2$ , the combined analysis can incorrectly favor the wrong mass ordering.
Mechanism: This occurs because the sum rule relies on the specific difference between the NO and IO best-fit values in JUNO being a fixed constant. If NP shifts the LBL and JUNO values differently, this balance is broken.

B. Case Study 1: Scalar Non-Standard Interactions (SNSI)

Model: A light scalar mediator $\phi$ couples to neutrinos and matter, inducing a mass shift $\delta M \propto \eta F(R_E, m_\phi)$ .
Analysis: The shift depends on the matter density ( $\rho$ ). Since JUNO (medium baseline, $\rho \approx 2.45 \text{ g/cm}^3$ ) and LBL (long baseline, $\rho \approx 2.8 \text{ g/cm}^3$ ) traverse different average densities, they would measure different effective mass splittings.
Result: To produce the critical shift of $4 \times 10^{-5} \text{ eV}^2$ , the coupling parameter $\eta$ would need to be large. However, current constraints from fifth-force searches, equivalence principle tests, and cosmology strictly limit $\eta$ .
Conclusion: The required shift is negligible under current experimental bounds. SNSI is unlikely to compromise the mass ordering determination.

C. Case Study 2: Ultralight Scalar Field (Time-Dependent Masses)

Model: The neutrino mass $m_3(t)$ oscillates due to a coupling with an ultralight scalar field ( $m_\phi \sim 10^{-24}$ eV).
Analysis:
- LBL experiments (T2K/NOvA) have collected data over $\sim 10$ years.
- JUNO starts data taking later (offset by $t_{off} \sim 10\text{--}15$ years).
- Because the scalar field oscillates, the time-averaged mass splitting measured by LBL and JUNO will differ if the oscillation period is comparable to the experimental timescales.
Result:
- For specific parameters (e.g., $m_3 = 0.065$ eV or $0.1$ eV, $\eta_\phi = 0.01$ ), the time-averaged shift $\delta m^2_{LJ}$ can easily reach the critical $4 \times 10^{-5} \text{ eV}^2$ .
- Consequence: A combined analysis could infer Inverted Ordering even if the true ordering is Normal Ordering.
- Detection: While LBL experiments lack the precision to detect this drift, JUNO's sub-percent precision allows it to observe the systematic drift in the best-fit value over time.
- Time-sliced Analysis: The authors show that analyzing JUNO data in 1-year slices can reveal this drift. A $3\sigma$ deviation from a constant mass splitting could be observed in 2–5 years depending on the absolute neutrino mass.

4. Significance and Conclusions

Robustness of the Synergy: The JUNO-LBL synergy is robust against Scalar Non-Standard Interactions (SNSI) due to tight experimental constraints.
Vulnerability to Time-Varying Masses: The synergy is vulnerable to ultralight scalar fields that induce time-dependent neutrino masses. If unaccounted for, this could lead to a catastrophic misidentification of the neutrino mass ordering.
JUNO as a New Physics Probe: The paper highlights a dual role for JUNO:
- It can determine the mass ordering.
- It can simultaneously search for ultralight scalar dark matter by monitoring the stability of the mass splitting over time.
Recommendation: The authors strongly recommend that JUNO perform time-sliced analyses of its data. By checking for systematic drifts in the best-fit $\Delta m^2$ $Δ m^{2}$ values, JUNO can:
- Validate the mass ordering determination.
- Disentangle New Physics effects from standard oscillation parameters.
- Probe parameter space for ultralight scalars that is currently unexplored.

In summary, while the standard synergy is powerful, the presence of specific New Physics (ultralight scalars) requires a vigilant, time-resolved analysis strategy to ensure the correct determination of the neutrino mass hierarchy.