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Heavy Sterile Fermions from the Atmosphere: LZ Limits on Neutrino Up-Scattering

This paper utilizes LUX-ZEPLIN (LZ) experimental data to establish new constraints on heavy sterile fermions produced via atmospheric neutrino up-scattering through various mediators, demonstrating that while dark matter detectors offer competitive limits for masses above 50 MeV, the observed 248 keV event is unlikely to be explained by this mechanism without invoking non-perturbative couplings.

Original authors: D. Aristizabal Sierra, V. De Romeri, D. K. Papoulias, I. M. Shoemaker

Published 2026-10-26

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CC BY 4.0

Original authors: D. Aristizabal Sierra, V. De Romeri, D. K. Papoulias, I. M. Shoemaker

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

Deep underground, shielded by miles of rock to block out the noise of the universe, sits a tank of liquid xenon waiting to catch a ghost. This is the LUX-ZEPLIN experiment, a detector designed to hunt for dark matter, the invisible substance that holds galaxies together. But in its quiet vigil, the tank recently caught something unexpected: a single, faint flash of light caused by a tiny particle bumping into an atom. This event happened at a specific energy level, far higher than the usual background noise, and it has sparked a wave of curiosity. Scientists are now asking if this flash was a sign of new physics, perhaps a particle we have never seen before, or simply a rare fluctuation in the natural background radiation that fills our world.

To understand what the researchers did, one must first understand the players involved. The universe is filled with neutrinos, tiny, nearly massless particles that stream through everything without stopping. Most of these come from the sun, but a different, more energetic group rains down from the sky, created when cosmic rays hit Earth's atmosphere. These atmospheric neutrinos are the key to this story. The researchers also considered the existence of "sterile" fermions, a hypothetical type of heavy particle that does not interact with light or ordinary matter, making it invisible to most detectors. The central question was whether a high-energy atmospheric neutrino could collide with an atom in the xenon tank and transform into one of these heavy, invisible particles, leaving behind a kick of energy that the detector could see.

A team of physicists decided to test this idea against the new data from the LUX-ZEPLIN experiment. They focused on the single event that occurred at an energy of 248 kiloelectronvolts, a unit used to measure the tiny amounts of energy released in these collisions. The team built a mathematical model to simulate what would happen if atmospheric neutrinos were indeed bumping into the xenon atoms and turning into heavy sterile particles. They tested four different ways this transformation could happen, imagining the interaction was mediated by different types of invisible forces, some acting like a push, others like a pull, and some involving magnetic-like properties. They ran their simulations to see if this process could explain the single flash of light without also creating a flood of other flashes at lower energy levels, which the detector did not see.

The results of this investigation were twofold. First, by looking at the vast majority of the data where no extra flashes were seen, the team established strict new limits on how heavy these sterile particles could be and how strongly they could interact with ordinary matter. They found that if these heavy particles exist, they cannot be interacting as strongly as some previous theories suggested, particularly for masses above 50 million electronvolts. This effectively rules out a wide range of possibilities for these hidden particles, narrowing the search for what lies beyond our current understanding of physics.

Second, the team turned their attention to that single, mysterious event at 248 kiloelectronvolts. Could it be the signature of a heavy sterile particle? The answer was a cautious no. For the model to produce a flash at that high energy without also producing many flashes at lower energies, the physics would require the particles to interact with a strength that pushes the laws of mathematics to their breaking point. In the language of physics, the required forces would be so strong that the theory itself stops working, suggesting the explanation is unlikely. Furthermore, the model involving magnetic-like interactions failed to fit the data entirely. While the idea of a heavy particle causing the event is not impossible in a strict sense, the conditions required to make it happen are so extreme that the researchers conclude it is not a compelling explanation.

Ultimately, the study suggests that the single event observed in the deep underground tank is probably not a sign of these heavy sterile particles transforming from atmospheric neutrinos. Instead, the data serves as a powerful tool to close the door on many theoretical possibilities, showing that dark matter detectors are sensitive enough to probe high-energy physics in ways previously untested. The researchers emphasize that while this specific explanation for the event does not hold up, the method they used is robust and will remain valid even if the event turns out to be something else entirely. The search continues, but the path forward is now clearer, having eliminated a significant set of guesses about what might be hiding in the dark.

Technical Summary: Heavy Sterile Fermions from the Atmosphere: LZ Limits on Neutrino Up-Scattering

Problem Statement
The LUX-ZEPLIN (LZ) collaboration recently reported a single nuclear-recoil event at $248 \pm 23(\text{stat}) \pm 23(\text{sys})$ keV within a low-background region, corresponding to an exposure of 2.84 tonne-years. While this event has spurred numerous dark matter interpretations, this paper investigates a specific neutrino-based explanation: the up-scattering of atmospheric neutrinos into heavy sterile fermions ( $\chi$ ) via interactions with xenon nuclei ( $\nu N \to \chi N$ ). The study addresses two primary questions: (1) What constraints do the existing LZ data (specifically the absence of excess events at lower energies) place on neutrino up-scattering scenarios involving heavy sterile fermions? (2) Can the up-scattering mechanism explain the single 248 keV event without violating the null results in the lower-energy bins?

Methodology
The authors analyze the LZ dataset by dividing the nuclear recoil energy range $[5.4, 270]$ keV into two bins:

Bin 1 ( $B_1$ ): $[5.4, 225]$ keV, where no excess over the background expectation is observed.
Bin 2 ( $B_2$ ): $[225, 270]$ keV, containing the single anomalous event.

The theoretical framework considers four simplified models for the neutrino-sterile-fermion interaction mediated by new states or operators:

Vector ( $V$ ) and Axial-Vector ( $A$ ) interactions mediated by new bosons.
Scalar ( $S$ ) interaction mediated by a new scalar.
Dipole interaction via a transition magnetic moment coupling the active neutrino to the sterile fermion and the photon.

Key technical components of the analysis include:

Cross-Section Calculations: Differential cross-sections ( $d\sigma/dT_{nr}$ ) are derived for all four scenarios. The calculations incorporate nuclear form factors, comparing the Klein-Nystrand (KN) phenomenological parametrization with shell-model calculations to account for the diffraction minima near the event energy ( $T_{nr} \approx 248$ keV).
Kinematics: The analysis enforces strict kinematic thresholds. Unlike elastic scattering, up-scattering to a massive state requires a non-zero minimum recoil energy ( $T_{nr}^{\min}$ ) that scales with the sterile fermion mass ( $m_\chi$ ). This kinematic feature is crucial for suppressing low-energy events while allowing high-energy recoils.
Flux and Rate: The atmospheric neutrino flux is integrated over the relevant energy range (up to $\sim 10^5$ GeV) to calculate the expected event rates. The analysis utilizes a binned Poisson likelihood ( $\chi^2$ ) to compare theoretical predictions against observed counts ( $R_{obs}^1 = 1709$ , $R_{obs}^2 = 1$ ) and background expectations ( $B_1 = 1712$ , $B_2 = 0.0106$ ).
Perturbativity: Constraints are applied to ensure couplings remain within the perturbative regime ( $g_X \leq \sqrt{4\pi}$ ) and mediator masses are sufficiently large ( $m_X \geq 5$ GeV) to justify the effective field theory limit.

Key Contributions and Results

New Exclusion Limits:
Using the null result in Bin 1, the authors derive new exclusion limits on the coupling strength versus sterile fermion mass ( $m_\chi$ ) and mediator mass ( $m_X$ ).
- For scalar and vector interactions, LZ data rule out previously allowed regions for $m_\chi \gtrsim 60$ MeV. These limits are competitive with, and in some cases extend beyond, the projected sensitivity of the DUNE experiment.
- For the axial-vector case, similar constraints are established for $m_\chi \gtrsim 50$ MeV.
- For the dipole portal, the LZ bounds are competitive with LEP limits for $m_\chi \gtrsim 100$ MeV (assuming photon-only exchange), surpassing constraints from COHERENT, CONUS+, and XENONnT in the high-mass regime where those experiments lose sensitivity due to kinematic thresholds.
Analysis of the 248 keV Event:
The authors test whether up-scattering can explain the single event in Bin 2 while respecting the Bin 1 constraints.
- Kinematic Necessity: To produce a 248 keV recoil without populating lower energy bins, the sterile fermion must be heavy ( $m_\chi \sim \text{GeV}$ scale), pushing the interaction to the high-energy tail of the atmospheric flux.
- Fit Quality:
  - Scalar, Vector, and Axial Models: The best-fit points for these scenarios lie at the edge of the perturbative regime ( $\Lambda_X \equiv m_X/g_X \approx 1.41$ GeV). While a fit is technically possible, it requires couplings that are nearly non-perturbative.
  - Dipole Model: This scenario provides a poor fit. The dipole cross-section is dominated by a $1/T_{nr}$ term, which predicts a significant number of events in Bin 1, contradicting the observed null result.

Significance and Claims
The paper concludes that while dark matter detectors like LZ can provide competitive limits on neutrino up-scattering for sterile fermion masses above $\sim 50$ MeV, up-scattering is not a compelling explanation for the 248 keV event.

Modest Interpretation: The authors emphasize that explaining the event requires couplings at the very edge of perturbativity for scalar, vector, and axial mediators, and fails entirely for the dipole portal. Consequently, a neutrino-related explanation for the event is deemed unlikely within the simplified models considered.
Complementarity: The work highlights the unique role of high-recoil-energy data from direct detection experiments. These data probe a "gap" in parameter space between low-energy coherent elastic neutrino-nucleus scattering (CE $\nu$ NS) experiments (sensitive to light states) and high-energy collider/beam experiments.
Robustness: The exclusion limits derived from Bin 1 are robust and independent of the nature of the 248 keV event; they would remain valid even if the event were a statistical fluctuation or background.
Future Outlook: The authors note that the sensitivity of these detectors to atmospheric neutrino backgrounds is approaching the "neutrino fog" (expected $\sim 0.1$ events in Bin 1 from SM CE $\nu$ NS). They suggest that a dedicated analysis using full S1c–S2c information by experimental collaborations could further sharpen these bounds, but the current work establishes that well-understood background measurements are already a valuable tool for constraining Beyond Standard Model (BSM) scenarios in the intermediate energy regime.

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