Nature Reviews Physics selected the 2023 discovery of high-energy neutrinos from the Galactic Plane by IceCube as one the highlights of the year. 

Here is my invited article overviewing the discovery for the journal’s 2023 Year in Review: rdcu.be/dtuNz . The link is open access, provided by Nature; feel free to share it widely online.

Neutrino telescopes, like IceCube, measure the directions with which high-energy neutrinos arrive at them. They also measure the flavor composition of the diffuse flux of high-energy astrophysical neutrinos, i.e., the the proportion of electron, muon, and tau neutrinos in the total flux. So far these capabilities have been kept largely disconnected from each other.

In a new paper led by NBI PhD student Bernanda Telalovic, for the first time, we bring them together to forge a new observable: the directional dependence of the high-energy neutrino flavor composition. From the distribution of arrival directions in a sample of detected events, we infer the directional dependence of the flux of electron, muon, and tau neutrinos responsible for it. To do that, we take a sample of IceCube HESE (High-Energy Starting Events) event sample and undo the detector response.

Our newly gained sensitivity to the directional dependence of the flavor composition allows to look for the contribution of different source populations to the diffuse flux, each producing neutrinos via a different mechanism and distributed differently in the sky. It also allows us to improve constraints on a poorly explored form of Lorentz-invariance violation, compass asymmetries, but about fifteen orders of magnitude vs. present-day limits.

Read more at

Flavor anisotropy in the high-energy astrophysical neutrino sky
Bernanda Telalovic & Mauricio Bustamante
2310.15224 astro-ph

In the next decade, new neutrino telescopes might discover ultra-high-energy (UHE) neutrinos, with energies above 100 PeV. There is vast potential to test astrophysics and fundamental physics via their flavor composition—the proportion of electron, muon, and tau neutrinos in their flux—but it is still unclear whether these new detectors will have flavor-identification capabilities (though progress is ongoing).

In a new paper led by MSc student Federico Testagrossa, from the University of Padova, and co-authored by NBI postdoc Damiano Fiorillo and myself, we show a new way to measure the UHE neutrino flavor composition that does not rely on the flavor-identification capabilities of single detectors. Instead, we combine the detection of neutrinos of all flavors, more or less indistinctly, by one detector, with the detection of predominantly tau neutrinos by another. We choose the radio array of IceCube-Gen2 for the former and GRAND for the latter.

We show that that combination grants us access to the UHE tau-neutrino content, even for conservative choices of the UHE neutrino flux and of the size of GRAND:

Our projections show that this measurement would allow us to extract important insight into the neutrino production mechanism at their sources and massively improve constraints on new physics that acts during neutrino propagation to Earth (we show examples for Lorentz-invariance violation).

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Two-detector flavor sensitivity to ultra-high-energy cosmic neutrinos
Federico Testagrossa, Damiano F. G. Fiorillo, Mauricio Bustamante
2310.12215 astro-ph

During August 21-26, I delivered four lectures on multi-messenger astrophysics at the 2023 Invisibles School in Bad Honnef, Germany.

The three sets of lecture slides are here:

Slides I (pdf): Introduction and ultra-high-energy cosmic rays

Slides II (pdf): High-energy cosmic neutrinos: introduction and astrophysical aspects

Slides III (pdf): High-energy cosmic neutrinos: particle-physics aspects

Soon (fingers crossed), new neutrino telescopes might be able to discover ultra-high-energy (UHE) neutrinos, with energies above 100 PeV. Two unknown quantities that will be targeted are the UHE neutrino flux and the UHE neutrino-nucleon cross section. Their measurement is co-dependent: the number of neutrino-initiated events detected will depend on their product.

Yet, typically, when forecasting one, important assumptions are made about the other, either on their size, their dependence on neutrino energy, or both (for recent work, see, e.g., 2007.10334, 2204.04237, 2205.09763, 2210.03756). This may have cast existing forecasts in too optimistic a light and downplayed the real capabilities of UHE neutrino telescopes to deliver measurements unhampered by the above assumptions.

In a new paper led by Víctor Valera and co-authored by Olga Mena and I, we have shown that upcoming UHE neutrinos telescopes should be able to jointly measure the UHE neutrino spectrum and cross section, including its energy dependence, without assuming prior knowledge of either:

We gear our forecasts to the radio-detection of UHE neutrinos in the radio array of IceCube-Gen2. We test different prescriptions, and recommend using flexible parametrizations of the neutrino spectrum—not a power law—that are able to capture its features. We find loose targets for the detector energy and angular resolution that are compatible with present-day design considerations, about 10% per decade in shower energy and about 2° in zenith angle.

Read more at

Joint measurement of the ultra-high-energy neutrino spectrum and cross section
Victor B. Valera, Mauricio Bustamante, Olga Mena
2308.07709 astro-ph

One way to probe dark matter is to find whether it decays into neutrinos. Heavy dark matter, with a mass larger than 10^7 GeV, might decay into ultra-high-energy neutrinos, with energies larger than 10^7 GeV. In the next 10-20 years, a new generation of ultra-high-energy neutrino telescopes, like IceCube-Gen2, should be able to look for these neutrinos.

However, ultra-high-energy neutrinos not made in dark matter decay represent an unknown background to these dark-matter searches. These are the cosmogenic and astrophysical ultra-high-energy neutrinos, first predicted by Berezinsky in 1969, that we have been looking for for the the last 50+ years.

Essentially, while neutrinos from dark matter are concentrated in directions near the Galactic Center—where dark matter is more abundant—the flux of non-dark-matter neutrinos is isotropic. If that flux is high, it could obscure the signal of neutrinos from dark-matter decay.

In a recent paper led by Damiano Fiorillo, and co-authored by Víctor Valera and Walter Winter, we show that future searches for ultra-high-energy neutrinos should be able to survive the existence of even a medium-sized background of cosmogenic or astrophysical neutrinos, even if that background is not known a priori.

Discovery of heavy dark matter decay into ultra-high-energy neutrinos, or constraints on it, are relatively safe from backgrounds:

Read more at

Searches for dark matter decay with ultra-high-energy neutrinos endure backgrounds
Damiano F. G. Fiorillo, Victor Valera, Mauricio Bustamante, Walter Winter
2307.02538 astro-ph

A few years ago, Sanjib Agarwalla and I published a paper on looking for new, flavor dependent, long-range neutrino-electron interactions that could affect neutrino oscillations. The new interactions are introduced by gauging lepton-number symmetries Le-Lmu and Le-Ltau. They are mediated by new neutral vector bosons, Z’. Because we considered them to be ultra-light, with masses below 10^{-10} eV, the interaction range is ultra-long. This allowed us to use the large collections of electrons in the local and distant Universe — the Earth, Moon, Sun, Milky Way, and cosmological electrons — as sources of the new matter potential, and the measurement of the flavor composition of high-energy astrophysical neutrinos as IceCube as the observable:

Universe’s Worth of Electrons to Probe Long-Range Interactions of High-Energy Astrophysical Neutrinos
Mauricio Bustamante, Sanjib Kumar Agarwalla
Phys. Rev. Lett. 6, 061103 (2019) [1808.02042]

This week we uploaded to the arXiv two follow-up works, in collaboration with Sanjib’s students Masoom and Sudipta and postdoc Ashish:

Present and future constraints on flavor-dependent long-range interactions of high-energy astrophysical neutrinos
Sanjib Kumar Agarwalla, Mauricio Bustamante, Sudipta Das, Ashish Narang
arXiv:2305.03675

Flavor-dependent long-range neutrino interactions in DUNE & T2HK: alone they constrain, together they discover
Masoom Singh, Mauricio Bustamante, Sanjib Kumar Agarwalla
arXiv:2305.05184

In the first paper, we revisit, revamp, and extend the analysis that we did in 2018, using the flavor composition of high-energy astrophysical neutrinos. There two main improvements. First, we now use a Bayesian statistical analysis, which allows us to claim constraints at higher significance than in the original paper. It also allows us to consistently treat the uncertainties in neutrino mixing parameters and in the measurement of the flavor composition. Second, we now consider also new neutrino-neutron interactions, introduced by gauging Lmu-Ltau. The constraints that we get on the coupling strength are these:

From the abstract: We find that, already today, the IceCube neutrino telescope demonstrates potential to constrain flavor-dependent long-range interactions significantly better than existing constraints, motivating further analysis. 

In the second paper, we study the same problem, but now using next-generation long-baseline neutrino experiments DUNE and T2HK. The neutrino energies are much lower, MeV-GeV compared to the TeV-PeV of astrophysical neutrinos. However, the event rates are enormous, and that makes DUNE and T2HK sensitive to even subtle modifications of the oscillation probabilities. In this case, the constraints are these:

From the abstract: Alone, DUNE and T2HK may strongly constrain long-range interactions, setting new limits on their coupling strength for mediators lighter than 10^{−18} eV. However, if the new interactions are subdominant, then both DUNE and T2HK, together, will be needed to discover them, since their combination lifts parameter degeneracies that weaken their individual sensitivity.