Exploring neutrino loss with diffuse astrophysical neutrino fluxes

arXiv:2605.13955 · hep-ph, astro-ph.CO, astro-ph.HE · Submitted 2026-05-13 · Read on arXiv

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Introduction to the show: ident: Astrophysics Radio. Generated commentary on the latest astrophysics papers.

Vera: Today's paper: "Exploring neutrino loss with diffuse astrophysical neutrino fluxes".

Jocelyn: The gist This work studies the sensitivity of IceCube's diffuse high-energy neutrino flux to new physics effects resulting in an exponential flux attenuation along the trajectory,

Vera: First, who's behind it and why it matters.

Title and authors: Vera: The paper starts by laying out this framework to see how sensitive our observations are to these types of attenuations along the path. The authors, Ivan Esteban, Alberto Gago, M. C. Gonzalez-Garcia, and Gabriel Zapata, set up a system to compute the flux arriving at Earth under these generic loss scenarios.

Jocelyn: It sounds like they're taking all that incoming data from IceCube and running it through a model where we introduce this energy-dependent neutrino loss term into the equation. That’s how they test if anything is wrong with our assumptions about propagation.

Subrahmanyan: The title itself tells you they are exploring neutrino loss using the diffuse astrophysical neutrino fluxes, which means they are not tethering their test to a single, bright event but looking at the overall background we see.

Vera: Exactly. It’s about moving beyond just looking at individual signals and seeing how this generic attenuation—the energy and distance dependent loss—affects the entire picture of what IceCube is detecting across its whole energy spectrum.

Jocelyn: And they immediately mention that even though the sources and their production redshifts are unknown, conservative energy conservation arguments allow them to severely constrain exponentially growing neutrino losses in most scenarios, beyond bounds derived from specific sources.

Subrahmanyan: That's a key point because it sets a high bar for any new physics we might be looking for; they’re using the basic rules of energy conservation as a very strong filter.

The paper's summary: Vera: So, the core of what they do in "Exploring neutrino loss with diffuse astrophysical neutrino fluxes" is building a formalism to compute that high-energy astrophysical neutrino flux arriving at Earth by including generic energy-dependent neutrino loss. This involves taking into account how the universe is expanding and how neutrinos lose energy over time.

Jocelyn: They set up a transport equation for the comoving density of neutrinos, which has to account for production, the energy redshift from cosmic expansion, and that neutrino loss we talked about earlier. It’s a whole chain of calculations just to get the flux at Earth per unit energy.

Subrahmanyan: The flux itself is an integral involving the comoving rate of neutrino production and an effective optical depth which represents the total neutrino loss rate integrated over the lookback time. This formalism lets them parameterize the diffuse astrophysical neutrino flux by a comoving density of sources, where that production rate is tied to an integral involving the source term.

Vera: They make some specific choices for how they model these functions; for instance, they assume neutrino loss is flavour-universal and depends on energy as a power law, i(E) = gamma zero E E zero n, where n is the index <ref:2605.13955#pg3,neutrino loss is flavour-universal and depends on energy as a power>.

Jocelyn: They test this by setting reference values for that index n, exploring cases from-two all the way up to two <ref:2605.13955#pg3>. This lets them see how changing that value shifts the resulting constraints on what kind of physics we might be dealing with.

The paper's improvements: Vera: One of the main things they suggest is improving our sensitivity by fitting this model against the actual High-Energy Starting Events from IceCube and studying how that flux varies when you change the energy dependence of the attenuation.

Jocelyn: They want to compare different source evolution scenarios against these observational data points, checking whether a loss mechanism affects neutrinos exclusively or if it interacts with other background noise. That gives us a much more robust way to compare theory with what we actually see in the sky.

Subrahmanyan: They also look at how different assumptions about the source distribution impact these attenuation calculations; they compare using star formation rate evolution against using BL-Lac object evolution.

Vera: And there's a specific improvement related to spectral index determination; the paper shows that including an energy-dependent attenuation at the level allowed in the fit can actually impact how we determine the spectral index of that diffuse flux.

Jocelyn: They specifically compare results when they set gamma = two against when they let it vary, which helps us figure out if a standard astrophysical spectrum is what we're seeing or if it’s being modified by neutrino loss <ref:2605.13955#pg1>.

Conclusion: Vera: So to wrap up the "Exploring neutrino loss with diffuse astrophysical neutrino fluxes" paper, the authors show that even in extreme scenarios, diffuse neutrino fluxes can provide a bound on these kinds of exponential losses. They found an interesting interplay between the allowed attenuation factor and the best-fit spectral index in their All Particle Attenuation scenario.

Jocelyn: They noted that this behavior gets cut off by the minimum redshift they set for those sources, which is conservatively z = two times ten-four. But even with that minimum redshift, the study concludes that diffuse neutrino fluxes can still provide a bound on what these losses could be.

Subrahmanyan: From my side, the energy conservation arguments are really strong; they showed that for the All Particle Attenuation scenario with z = two times ten-four you need n to be less than a few times ten-thirty-eight GeV.

Vera: It’s interesting how conservative those bounds are, but it shows that the observational constraints from IceCube are still powerful even when we don't know the exact source details.

Jocelyn: So, this work really sets up a clear path for how we can use these diffuse signals to constrain new physics models affecting neutrino propagation across cosmic distances.

Subrahmanyan: It’s a solid piece of work because it connects the high-energy astrophysical neutrino flux directly to the cosmological evolution of sources, which is where you need to be for this kind of investigation.

Department of Physics, University of the Basque Country UPV/EHU · EHU Quantum Center, University of the Basque Country UPV/EHU · Secci´on F´ısica, Departamento de Ciencias, Pontificia Universidad Cat´olica del Per´u · C.N. Yang Institute for Theoretical Physics, Stony Brook University · Departament de Fisica Quantica i Astrofisica and Institut de Ciencies del Cosmos, Universitat de Barcelona · Instituci´o Catalana de Recerca i Estudis Avancats (ICREA)

hep-ph, astro-ph.CO, astro-ph.HE

Submitted: 2026-05-13

Updated: 2026-10-07

Comments: 16 pages, 4 figures, 2 table. Minor corrections, results unchanged, version accepted for publication in JCAP

Journal ref: JCAP10(2026)020

DOI: 10.1088/1475-7516/2026/10/020

License: http://arxiv.org/licenses/nonexclusive-distrib/1.0/

Importance score: 79/100

The gist: The gist This work studies the sensitivity of IceCube's diffuse high-energy neutrino flux to new physics effects resulting in an exponential flux attenuation along the trajectory, such as invisible

Key concepts

Exponential Flux Attenuation: This describes a scenario where the number of neutrinos reaching Earth decreases exponentially with distance traveled. This attenuation can be caused by new physics processes, such as hypothetical neutrino decay or interactions with the cosmic background radiation encountered during propagation.
Conservative Energy-Conservation Arguments: These are physical arguments used to severely restrict how large neutrino losses can be. They compare the total energy carried by diffuse neutrinos before loss against the total energy available in all galaxies in the Universe, providing strict upper bounds on neutrino loss rates.
All Particle Attenuation Scenario: This specific constraint requires that the energy density of diffuse high-energy neutrinos, even after accounting for propagation effects, must be less than the total energy density of all matter in the Universe. This sets a limit on the overall present-day diffuse high-energy neutrino energy density.
Comoving Density and Optical Depth: The formalism uses comoving density to track how neutrino production evolves while accounting for cosmic expansion and energy redshift. The effective optical depth represents the total integrated rate of neutrino loss over the entire lookback time, determining the final flux observed at Earth.

Terminology

All Particle Attenuation Scenario — 14× in this paper · explained in 1 episode
Dark Matter — 8× in this paper
High-Energy Starting Events — 5× in this paper
Degeneracy — 3× in this paper · explained in 3 episodes
Core — 2× in this paper · explained in 2 episodes
Monte Carlo — 2× in this paper
AGN — 1× in this paper · explained in 1 episode
Benchmark — 1× in this paper · explained in 2 episodes
In the Standard Model of Particle Physics — 1× in this paper
Spectral Distortions — 1× in this paper · explained in 2 episodes

Summary

The gist

This work studies the sensitivity of IceCube's diffuse high-energy neutrino flux to new physics effects resulting in an exponential flux attenuation along the trajectory, such as invisible neutrino decay or new interactions with the background encountered during propagation.

Introduction and Motivation

Ultra-high-energy cosmic rays are most likely produced and accelerated in extreme astrophysical environments, which are expected to interact with radiation and matter surrounding the sources, as well as with background radiation on their way to Earth. The Standard Model of Particle Physics neutrinos interact very weakly with matter and radiation, implying that the observation of HE neutrinos has potential sensitivity to new-physics effects affecting their propagation, particularly those that would attenuate the neutrino flux arriving at Earth. In general, all these effects result in the exponential attenuation of the neutrino flux arriving at Earth with a (generically energy-dependent) exponent that grows with the distance traveled by the neutrino. This distance dependence makes known astrophysical sources natural testbeds for these effects.

Formalism for Neutrino Flux Evolution

The formalism computes the astrophysical neutrino flux arriving at Earth including generic energy-dependent neutrino loss by taking into account the expansion of the Universe. The evolution of the comoving density of neutrinos is governed by a transport equation that includes production, energy redshift due to cosmic expansion, and neutrino loss. The flux at Earth per unit energy is given by an integral involving the comoving rate of neutrino production and an effective optical depth that represents the integral of the neutrino loss rate over lookback time. The diffuse astrophysical neutrino flux is parametrized by a comoving density of sources, where the production rate is given by an integral involving the source term.

Constraints from Energy Conservation

Conservative energy-conservation arguments allow to severely constrain exponentially large neutrino losses in most scenarios beyond bounds derived from specific sources. Two limiting scenarios are considered:

Only ν Attenuation, which uses the Waxman-Bahcall (WB) bound, constraining the neutrino production rate based on the expected high-energy cosmic-ray flux.
All Particle Attenuation, where one requires that the energy density contained in the diffuse neutrino flux before loss is smaller than the total energy density contained in all galaxies in the Universe. This sets a limit on the present-day diffuse high-energy neutrino energy density.

Analysis Choices and Results

The analysis explores various assumptions regarding source evolution, neutrino production spectrum, and flavor composition. Key choices include:

**: Redshift distribution of high-energy diffuse astrophysical neutrino sources is parametrized by the star formation rate or the distribution of BL-Lac objects. The smallest redshift of neutrino sources, zmin, is conservatively set to 2 × 10−4 in the All Particle Attenuation scenario. The neutrino production spectrum is assumed to be a power law with spectral index γastro as a free parameter. In the Only ν attenuation scenario, for energy-independent or energy-growing attenuation (i.e., n ≥ 0), the bounds derived from this analysis are stronger than the strongest bound in the literature derived from SN1987A neutrinos. In the All Particle Attenuation scenario, for our choice zmin = 2 × 10−4, this requires Γn ≲ few × 10−38 GeV for either source evolution. The study shows an interesting interplay between the allowed attenuation factor and the best-fit spectral index, leading to a quasi-degeneracy in the All Particle Attenuation scenario which is cut off by the minimum redshift. This behavior is cut off by the fact that there is a minimum redshift for the sources of the neutrinos, which we conservatively set to 2×10−4. The study concludes that even in this extreme scenario, diffuse neutrino fluxes can provide a bound.">BARE">BARE">BARE">BARE">BARE">BARE">BARE">BARE">BARE">BARE">BARE">BARE">BARE">BARE">BARE">BARE">BARE">BARE">BARE">BARE">BARE">BARE">BARE">BARE">BARE">BARE">BARE">BARE">BARE">BARE">BARE</ref:2605.

Improvements for AI systems

textbfIncreased constraint sensitivity for new physics models: The improved system can quantify bounds on neutrino loss by fitting the High-Energy Starting Events from IceCube and studying their variation with the energy dependence of the attenuation, the assumed redshift distribution of the neutrino sources, and whether the attenuation affects neutrinos exclusively or no. This allows for a more robust comparison between theoretical models (like different source evolution scenarios) and observational data.
textbfImproved spectral index determination: The system can now show that including an energy-dependent attenuation at the level allowed in the fit may impact the determination of the spectral index of the diffuse flux, which is crucial for distinguishing between a standard astrophysical spectrum and one modified by neutrino loss, as shown by comparing results when gamma = 2 versus when it varies.
textbfEnhanced scenario comparison: The improved AI can perform detailed comparisons between theoretical scenarios like Only ν Attenuation (using the Waxman-Bahcall bound) and All Particle Attenuation, quantifying the relative strength of constraints, noting that in the latter case, the suppression factor is only bounded to be ≲ 5 × 10−9–5 × 10−10.
textbfHandling source evolution uncertainty: The system can explicitly compare how different source distributions impact attenuation, noting that attenuation for SFR source evolution is stronger than that for BL-Lac evolution because the median redshift zmed differs significantly between the two scenarios.