Neutrino mass ordering in JUNO at risk from scalar NSI induced resonance

This paper demonstrates that the JUNO experiment's ability to determine the neutrino mass ordering could be severely compromised or completely lost due to a previously unrecognized resonant enhancement of the mixing angle $\theta_{12}$ induced by scalar non-standard neutrino interactions.

The Gist

Imagine the JUNO experiment as a giant, ultra-precise musical instrument designed to listen to the "song" of neutrinos—tiny, ghostly particles that zip through the Earth. The main goal of this instrument is to figure out the Neutrino Mass Ordering (NMO). Think of the three types of neutrinos as three siblings with different weights. Scientists know they have different weights, but they don't know the order: is the lightest sibling the first, second, or third? JUNO is built to solve this mystery with high confidence.

The paper by Sandhya Choubey and Andreas Lund warns that a hidden "tuning fork" from a new, unknown type of physics could break this instrument before it finishes its job.

Here is the breakdown of their discovery using simple analogies:

1. The Hidden Interference (Scalar NSI)

Usually, scientists assume neutrinos only interact in the ways we already know (the "Standard Model"). However, the authors ask: What if there is a new, invisible force acting on them? They call this a Scalar Non-Standard Interaction (SNSI).

Think of this new force as a ghostly hand that occasionally pushes the neutrinos as they travel. If this hand exists, it changes how the neutrinos "dance" (oscillate) between their different types. The scary part is that JUNO is so sensitive that even a tiny push from this ghostly hand could completely scramble the data.

2. The "Resonance" Trap

The paper's biggest discovery is a specific point where this ghostly hand causes a resonance.

Imagine you are trying to identify a song by its rhythm.

• Normal Scenario: The rhythm clearly tells you if the song is in a "Major" key (Normal Ordering) or a "Minor" key (Inverted Ordering). JUNO is designed to hear this difference perfectly.
• The Resonance: The authors found a specific strength of the "ghostly push" (a specific value of a parameter called $\eta_{ee}$) where the rhythm of the "Major" song and the "Minor" song become identical.

At this specific point (called the resonance), the neutrinos behave as if the "Major" and "Minor" keys are the same. It's like a magic trick where the two different songs merge into one indistinguishable sound. Because JUNO relies on hearing the difference between these two, it suddenly becomes blind. It can no longer tell which mass ordering is real.

3. The "Dark Side" Confusion

The paper explains that this happens because the new force changes the "mixing angle" (a setting that controls how neutrinos switch types).

Normally, this angle is like a dial set to a specific number (less than 45 degrees). But at the resonance, the dial gets pushed all the way to 45 degrees. If the force gets even stronger, the dial goes past 45 degrees into what the authors call the "Dark Side."

• The Problem: JUNO's computer analysis is programmed to assume the dial is not in the "Dark Side."
• The Result: If the real universe has the dial in the "Dark Side" (due to the new force), JUNO's computer tries to force the data to fit its old rules. It ends up picking the wrong answer (the wrong mass ordering) and confidently ruling out the correct answer.

4. The Bottom Line

The authors ran simulations to see how bad this could get:

• If the new force is weak, JUNO might still solve the mystery, though with less certainty.
• If the new force is strong enough to hit that "resonance" point, JUNO loses 100% of its ability to tell the difference between the two mass orderings. The statistical confidence drops to zero.

In summary: The JUNO experiment is a brilliant machine built to solve a specific puzzle. This paper warns that if a specific, previously unknown type of physics exists, it acts like a "chameleon" that makes the two possible answers look exactly the same. If this happens, JUNO will not just fail to find the answer; it might confidently declare the wrong answer as the truth, leaving the mystery of the neutrino mass ordering unsolved.

Technical Summary

Technical Summary: Neutrino Mass Ordering in JUNO at Risk from Scalar NSI Induced Resonance

Problem Statement
The Jiangmen Underground Neutrino Observatory (JUNO) is designed to definitively determine the neutrino mass ordering (NMO) with a projected $3\sigma$ statistical significance after 7.1 years of data-taking. This determination relies on the experiment's unprecedented precision in measuring the reactor antineutrino energy spectrum. However, the presence of physics beyond the Standard Model (BSM) could introduce sub-leading effects that compromise this primary goal. Specifically, this paper investigates the impact of Non-Standard Neutrino Interactions (NSI) mediated by a scalar field (SNSI). The central question addressed is whether the existence of SNSI, if present in nature but unaccounted for in the data analysis, could severely degrade or completely eliminate JUNO's sensitivity to the NMO.

Methodology
The authors employ a statistical analysis of simulated JUNO data over a 6.5-year active period, closely following the experimental details and simulation codes provided by the JUNO collaboration.

• Data Generation: An Asimov data set ($N^{true}_i$) is generated assuming the Inverted Ordering (IO) is the true mass ordering, with a lightest neutrino mass $m_l = 0.01$ eV. The data includes various true values of the SNSI parameter $\eta_{ee}$.
• Hypothesis Testing: The simulated data is fitted against two hypotheses: Normal Ordering (NO) and Inverted Ordering (IO). Crucially, the fits assume standard oscillations (i.e., $\eta^{fit}_{ee} = 0$), meaning the analysis does not account for the SNSI present in the generated data.
• Statistical Metric: The sensitivity is quantified using the $\chi^2$ test statistic, defined as the difference $\Delta\chi^2 = \chi^2_{NO} - \chi^2_{IO}$. Systematic uncertainties (reactor correlated/uncorrelated, background, and shape uncertainties) are incorporated via pull terms.
• Analytical Approach: To explain the numerical results, the authors derive an analytical formulation using a Jacobi method to diagonalize the effective Hamiltonian modified by SNSI. This allows for the calculation of effective oscillation parameters ($\theta^{eff}_{12}$, $\Delta m^{2, eff}_{21}$, etc.) as functions of $\eta_{ee}$.

Key Contributions and Results
The paper demonstrates that the presence of scalar NSI can induce a resonant enhancement of the solar mixing angle $\theta_{12}$, leading to a degeneracy between the survival probabilities of NO and IO.

1. Degradation of Sensitivity: The study finds that the NMO sensitivity ($\Delta\chi^2$) deteriorates significantly for specific ranges of $\eta_{ee}$.

   • For $\eta_{ee} < -7.1 \times 10^{-3}$ and $\eta_{ee} > 3.3 \times 10^{-3}$, the sensitivity falls below the $2\sigma$ level.
   • At a critical resonance value $\eta^{res}_{ee} \approx 5.7 \times 10^{-3}$, the sensitivity is completely lost ($\Delta\chi^2 = 0$). At this point, the $\chi^2$ for both NO and IO hypotheses is zero, rendering the experiment unable to distinguish between the two orderings.

2. The Resonance Mechanism: The loss of sensitivity is attributed to a resonance in the effective solar mixing angle, $\theta^{eff}_{12}$.

   • Similar to the MSW resonance in solar neutrinos, the SNSI term creates a condition where the denominator of the effective mixing angle equation vanishes: $\Delta m^2_{21} \cos 2\theta_{12} = \eta^{res}_{ee} B$.
   • At this resonance, $\theta^{eff}_{12}$ reaches $\pi/4$. For $\eta_{ee} > \eta^{res}_{ee}$, $\theta^{eff}_{12}$ enters the "dark side" ($\theta > \pi/4$).
   • This resonance causes the $\bar{\nu}_e$ survival probability for IO (with SNSI) to become nearly identical to the probability for NO (without SNSI) across the entire energy spectrum relevant to JUNO.

3. Fit Behavior and "Dark Side" Solutions:

   • When fitting the data (generated with IO and $\eta_{ee} > \eta^{res}_{ee}$) assuming NO, the fit can achieve $\chi^2_{NO} \approx 0$ because the effective parameters align with the NO hypothesis.
   • When fitting the same data assuming IO, the outcome depends on constraints on the solar mixing angle. If the fit allows $\theta^{fit}_{12}$ to vary freely (including the "dark side" $\theta > \pi/4$), $\chi^2_{IO} \approx 0$, resulting in $\Delta\chi^2 = 0$. If the fit is restricted to $\theta^{fit}_{12} \leq \pi/4$ (excluding dark side solutions), $\chi^2_{IO}$ becomes large, potentially leading the analysis to incorrectly favor the wrong NMO (NO) or rule out the correct one (IO).

4. Dependence on Lightest Mass: The resonance value $\eta^{res}_{ee}$ is found to be largely independent of the lightest neutrino mass ($m_l$) for $m_l \lesssim 10^{-2}$ eV, but shows some dependence for larger masses.

Significance and Claims
The authors claim that this work identifies a previously unrecognized risk to the JUNO experiment's primary physics goal. The paper asserts that if scalar NSI mediated by a BSM scalar field exist with parameters near the resonance condition ($\eta_{ee} \approx 5.7 \times 10^{-3}$), JUNO will lose its ability to determine the neutrino mass ordering.

The significance lies in the mechanism: unlike standard matter effects (MSW) mediated by gauge bosons, this resonance is driven by a scalar interaction. This leads to a specific degeneracy where the survival probabilities for different mass orderings become indistinguishable. The authors note that while current constraints from Borexino data exist, they are at the 90% confidence level, leaving the parameter space relevant to this resonance open. The results suggest that the interpretation of JUNO data must account for the possibility of such scalar NSI to avoid false conclusions regarding the mass ordering. The paper concludes that similar losses of sensitivity occur for other SNSI parameters and if the true ordering were Normal, indicating a general vulnerability of the NMO determination to scalar-mediated interactions.