⚛️ phenomenology

Resolving Lorentz-Violating New Physics at ESSnuSB Using High-Statistics Complementarity with T2HK

This paper demonstrates that combining the high-statistics measurements from the T2HK experiment with the second-oscillation-maximum configuration of ESSnuSB effectively resolves parameter degeneracies between Lorentz-violating effects and standard oscillation parameters, thereby enabling precise constraints on Planck-scale Lorentz Invariance Violation.

Original authors: Himanshu Bora, Debajyoti Dutta, Monjowara Khatun, Abinash Medhi

Published 2026-07-27

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CC BY 4.0

Original authors: Himanshu Bora, Debajyoti Dutta, Monjowara Khatun, Abinash Medhi

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

Imagine the universe as a giant, invisible ocean where tiny, ghostly particles called neutrinos are the swimmers. These particles are famous for being shy; they can zip through entire planets without bumping into anything. For decades, scientists have watched these neutrinos "dance" as they travel, changing their identity from one type to another—a phenomenon known as oscillation. This dance has already taught us that neutrinos have mass, which was a huge surprise because the original rulebook of physics (the Standard Model) said they should be weightless. But now, physicists are asking a deeper question: Is the stage itself perfectly smooth?

In the world of physics, there's a golden rule called "Lorentz Invariance." Think of it like the rules of a video game: no matter how fast you are moving or which direction you are facing, the laws of physics should feel exactly the same. If this rule is broken, it's like the game engine glitching, where the rules change depending on your speed or orientation. This is called "Lorentz Invariance Violation" (LIV). If such a glitch exists, it would be a sign of "New Physics" from the very edge of the universe, perhaps even from the Planck scale—the tiniest possible size in existence. Finding a glitch would be like discovering a secret cheat code for the universe, but it's incredibly hard to spot because the effects are usually tiny and get lost in the noise of normal neutrino behavior.

This paper is a detective story about how to find that glitch using two massive neutrino experiments: ESSnuSB and T2HK. The authors, a team of physicists, ran detailed computer simulations to see if combining data from these two different setups could untangle a tricky problem. When neutrinos travel, their "dance" can be confused by the presence of LIV, creating "fake solutions" where scientists might think they've measured a standard property of the neutrino (like its mixing angle or a phase related to time-reversal symmetry) when they've actually been fooled by a LIV glitch. The paper explores whether the unique strengths of ESSnuSB (which looks at the neutrinos' dance at a specific "second rhythm") and T2HK (which watches the "first rhythm" with huge numbers of particles) can work together to cancel out these confusions.

The team simulated the data from ESSnuSB, which has two proposed locations in Sweden: one 360 kilometers away and another 540 kilometers away from the neutrino source. They also simulated T2HK, a giant experiment in Japan with a 295-kilometer baseline. The simulations showed that looking at just one of these experiments often leads to a dead end. For instance, ESSnuSB's 540-kilometer setup is great at seeing the details of the neutrino's "second rhythm," but it struggles to tell if the neutrino is dancing in a "high" or "low" octant (a specific way of describing its mixing angle) because it doesn't have enough data from the antineutrinos to balance the books. Meanwhile, T2HK has a massive amount of data from both neutrinos and antineutrinos, but because it only watches the "first rhythm," it sometimes misses the subtle clues that ESSnuSB catches.

However, when the authors combined the data from ESSnuSB and T2HK in their simulations, the picture cleared up beautifully. By mixing the high-precision, high-statistics "first rhythm" data from T2HK with the "second rhythm" sensitivity of ESSnuSB, the fake solutions disappeared. The combination acted like a powerful filter, breaking the confusion caused by the LIV parameters. The results suggest that this partnership creates a "matter-independent" framework, meaning they can find these Planck-scale glitches without needing the heavy, complex effects of the Earth's core (which other experiments like DUNE rely on). While the 540-kilometer ESSnuSB setup alone couldn't fully solve the puzzle for all types of LIV parameters, pairing it with T2HK successfully resolved most of the degeneracies, allowing for a much clearer view of the neutrino's true properties. The paper concludes that while this duo isn't quite as powerful as a combination with the DUNE experiment (which uses the Earth's matter effects), it offers a highly effective, independent way to hunt for these fundamental cracks in the laws of physics.

Technical Summary: Resolving Lorentz-Violating New Physics at ESSnuSB Using High-Statistics Complementarity with T2HK

Problem Statement
Next-generation long-baseline (LBL) neutrino experiments aim to resolve critical unknowns in the three-flavor mixing framework, including the neutrino mass ordering, the Dirac CP-violating phase ( $\delta_{CP}$ ), and the octant of the atmospheric mixing angle ( $\theta_{23}$ ). A parallel objective is the search for Planck-scale Lorentz Invariance Violation (LIV). However, the presence of isotropic, CPT-violating LIV parameters ( $a_{\alpha\beta}$ ) introduces non-standard phase effects that distort standard oscillation probabilities. These distortions create severe parameter degeneracies, particularly between LIV effects and the standard parameters $\theta_{23}$ and $\delta_{CP}$ . If unaccounted for, these degeneracies can lead to incorrect determinations of standard oscillation parameters or "fake solutions" where new physics is misinterpreted as standard physics. Previous work indicated that while combining the two proposed baselines of the European Spallation Source neutrino Super Beam (ESSnuSB)—360 km and 540 km—improves LIV sensitivity, it fails to fully resolve these degeneracies, often leaving "wrong octant" fake solutions for $\theta_{23}$ .

Methodology
The authors utilize the General Long Baseline Experiment Simulator (GLoBES) to perform a phenomenological analysis comparing individual and combined capabilities of the proposed ESSnuSB and T2HK (Tokai to Hyper-Kamiokande) experiments.

Theoretical Framework: The study employs the Standard-Model Extension (SME) framework, focusing on isotropic, CPT-odd coefficients ( $a_{\alpha\beta}$ ). The analysis considers the modified effective Hamiltonian for neutrino propagation, where $a_{\alpha\beta}$ acts as a static, energy-independent perturbation. Antineutrino propagation is treated with a sign inversion and complex conjugation of the LIV contribution to probe CPT violation.
Experimental Configurations:
- ESSnuSB: Simulated with two baselines: 360 km (Zinkgruvan) and 540 km (Garpenberg), both using 538 kton Water Cherenkov detectors. The 540 km baseline targets the second oscillation maximum ( $\sim$ 0.35 GeV), while the 360 km baseline samples both the first and second maxima. The runtime is split equally (5 years) between neutrino and antineutrino modes.
- T2HK: Simulated with a 295 km baseline and a 187 kton Water Cherenkov detector, targeting the first oscillation maximum ( $\sim$ 0.6 GeV). The runtime is split 1:3 (2.5 years neutrino, 7.5 years antineutrino) to balance event statistics.
Analysis Strategy: The study employs a $\chi^2$ analysis with marginalization over standard oscillation parameters (using NuFit-6.0 and JUNO constraints) and LIV phases. The analysis tests the "true" hypothesis of standard oscillations (zero LIV) against test hypotheses including non-zero LIV parameters. The authors examine bi-event plots (neutrino vs. antineutrino event rates) and 95% Confidence Level (C.L.) contours in the $(\theta_{23}, a_{\alpha\beta})$ and $(\delta_{CP}, a_{\alpha\beta})$ planes.

Key Contributions and Results

Intrinsic Limitations of Single Experiments:
- ESSnuSB: The 540 km configuration, while excellent for constraining the phase of $\delta_{CP}$ due to amplified CP asymmetry at the second oscillation maximum, suffers from a significant statistical imbalance between neutrino and antineutrino channels. This leads to persistent "wrong octant" fake solutions for $\theta_{23}$ when LIV is present. The 360 km configuration offers better statistical constraints but still fails to fully resolve degeneracies for diagonal parameters like $a_{ee}$ and off-diagonal parameters like $|a_{e\tau}|$ .
- T2HK: Despite operating at the first oscillation maximum with smaller matter effects, T2HK's high-statistics, balanced neutrino/antineutrino data allows for better resolution of octant degeneracies for certain parameters (e.g., $|a_{\mu\tau}|$ ) compared to ESSnuSB alone. However, T2HK alone admits multiple distinct LIV values for the same mixing angle and struggles to resolve degeneracies for $a_{ee}$ and $|a_{e\tau}|$ .
Synergistic Resolution of Degeneracies:
- The primary finding is that combining ESSnuSB with T2HK creates a powerful, matter-independent framework to break LIV-induced degeneracies.
- $\theta_{23}$ Resolution: The combination of T2HK's high-statistics first maximum data with ESSnuSB's second maximum sensitivity effectively eliminates "wrong octant" fake solutions for most LIV parameters.
  - The ESSnuSB (540 km) + T2HK combination successfully resolves the octant degeneracy for $a_{ee}$ , a case where the 360 km + T2HK combination failed.
  - For $|a_{e\tau}|$ , both combinations significantly reduce the allowed parameter space, though a small residual "island" in the lower octant remains, indicating this parameter retains the most persistent degeneracy.
- $\delta_{CP}$ Resolution: The combination provides tight constraints on $\delta_{CP}$ , keeping it localized near the true maximal CP-violating value ( $-90^\circ$ ). The second oscillation maximum sensitivity of ESSnuSB is crucial here, as it is more robust against LIV mimicry of CP phases compared to the first maximum.
Comparison with DUNE:
- The authors compare these results to their previous study on ESSnuSB + DUNE. While the DUNE combination (leveraging strong matter effects) provides strong constraints on LIV magnitudes, the ESSnuSB + T2HK combination is shown to be sufficient for resolving $\theta_{23}$ degeneracies without relying on large matter effects.

Significance and Claims
The paper claims that the synergy between ESSnuSB and T2HK provides an effective mechanism to constrain Planck-scale LIV physics, specifically for isotropic, CPT-odd parameters. The authors emphasize that resolving these degeneracies does not strictly require the profound matter effects found in long-baseline experiments like DUNE (1300 km). Instead, the complementarity between the high-statistics, first-oscillation-maximum measurements of T2HK and the CP-amplified, second-oscillation-maximum sensitivity of ESSnuSB is sufficient to break parameter degeneracies for most LIV parameters.

The study concludes that while the ESSnuSB + T2HK combination is slightly less effective than the ESSnuSB + DUNE combination in constraining the full LIV parameter space, it offers a distinct, matter-independent pathway to establish bounds on LIV physics and ensure the accurate determination of standard oscillation parameters. The work highlights the necessity of multi-baseline, multi-energy approaches to disentangle new physics from standard oscillation ambiguities.

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