Particle Astrophysics with High and Ultrahigh Energy Neutrinos

This paper summarizes recent observations of high and ultrahigh energy neutrinos, including the detection of a diffuse cosmic background, candidate sources, and Galactic plane emissions, which collectively establish neutrinos as a unique tool for probing inaccessible cosmic phenomena despite the ongoing uncertainty regarding the origins of most detected events.

The Gist

Imagine the universe as a giant, chaotic party where invisible particles are constantly crashing into each other, creating a storm of energy. For a long time, scientists could only see the "light" of this party (photons) and the "noise" (cosmic rays), but they couldn't see the "ghosts" (neutrinos) because ghosts are hard to catch.

This paper, written by Ke Fang and Kohta Murase, is a report card on how we are finally learning to catch these cosmic ghosts using a giant detector called IceCube, buried deep in the ice at the South Pole. Here is what they found, explained in simple terms:

1. The Ghost Hunters (Introduction)

Cosmic rays are like charged marbles that get bounced around by magnetic fields in space, so when they hit Earth, we can't tell where they came from. Neutrinos, however, are like invisible, ghostly messengers. They don't have an electric charge and barely interact with anything. Once they are born in a violent cosmic event, they fly in a perfectly straight line straight to Earth, pointing directly back to their birthplace.

For a decade, IceCube has been listening for these ghosts. In 2013, they confirmed that high-energy neutrinos are coming from outside our solar system. Since then, the big question has been: Who is throwing the party?

2. The All-Sky Background (The "Static" on the Radio)

The scientists found that there is a constant "hum" of neutrinos coming from all directions in the sky. It's like hearing static on a radio; you know there are stations out there, but you can't tune into a specific song yet.

• The Mystery: When they looked at the light (gamma rays) coming from the same places, they realized something strange. If these neutrinos were made in a place where light could escape easily, we should see a lot of gamma rays too. But we don't.
• The Conclusion: The neutrino factories must be hidden. Imagine a loudspeaker inside a thick, soundproof box. The sound (neutrinos) gets out, but the light (gamma rays) gets trapped inside. This suggests the neutrinos are coming from "hidden" cosmic accelerators, like the deep, dusty hearts of active galaxies.

3. Identifying the "Guests" (Extragalactic Sources)

The paper highlights two main types of cosmic "guests" that might be sending these neutrinos:

• The "Quiet" Giants (Jet-Quiet AGNs): Most active galaxies (like NGC 1068) don't have massive, flashy jets shooting out of them. They are like a busy kitchen with a closed door. IceCube found that NGC 1068 is spitting out a huge number of neutrinos but very little light. This confirms the "hidden kitchen" theory: the neutrinos are made in the dark, dusty core of the galaxy, where the light can't escape.
• The "Loud" Blazars (Jet-Loud AGNs): These are the flashy ones with massive jets pointing right at Earth (like TXS 0506+056). In 2017, IceCube caught a neutrino from one of these, and telescopes confirmed it was flaring. However, the paper notes that these flashy blazars probably aren't the main source of all the neutrinos we see; they are just the few we can clearly identify.

4. The Neighborhood Watch (The Galactic Plane)

For a long time, scientists thought our own galaxy, the Milky Way, was too quiet to be a major neutrino source. But in 2023, IceCube found a "smear" of neutrinos coming from the flat disk of our galaxy.

• The Surprise: It turns out our galaxy is a bit of a "neutrino desert" compared to what we expected. While we see a lot of light from our galaxy, the neutrino signal is surprisingly weak. This suggests that our galaxy lacks the super-powerful, hidden neutrino factories found in other galaxies (like the active black holes in NGC 1068). Our central black hole is currently "asleep."

5. The Ultra-High Energy Frontier (The "Super-Ghosts")

The paper also looks at the most energetic neutrinos imaginable (Ultrahigh Energy or UHE). These are the "super-ghosts" with energies millions of times higher than anything we can make in a lab.

• The Search: We haven't caught many of these yet. It's like trying to find a needle in a haystack the size of a continent.
• The Clue: There was one very strange event detected by a different telescope (KM3NeT) that might be a super-ghost. If it is, it's a huge deal, but we need more data to be sure.
• The Theory: These super-ghosts might be created when the most energetic particles in the universe crash into the leftover heat from the Big Bang (the Cosmic Microwave Background). If we find them, it will tell us exactly what kind of particles are being accelerated to these insane speeds.

Summary

This paper is a celebration of the first decade of Neutrino Astronomy.

• We know there is a background hum of neutrinos from hidden cosmic accelerators.
• We have found specific addresses for a few of them (like the hidden galaxy NGC 1068).
• We learned that our own galaxy is quieter than we thought.
• We are still hunting for the most energetic "super-ghosts" that could reveal the secrets of the universe's most powerful engines.

By catching these ghosts, we are finally able to see the "dark" side of the universe—the places where light is trapped, but where the most violent particle collisions in the cosmos are happening.

Technical Summary

Technical Summary: Particle Astrophysics with High and Ultrahigh Energy Neutrinos

Problem Statement
The origin of cosmic rays, particularly at the highest energies, remains a fundamental mystery in particle astrophysics. Because cosmic rays are charged particles, their trajectories are scrambled by Galactic and extragalactic magnetic fields, preventing direct tracing back to their accelerators. While photons and gravitational waves offer complementary probes, high-energy neutrinos provide a unique, unimpeded window into hadronic acceleration environments. The central challenge addressed by this review is the identification of the astrophysical sources responsible for the diffuse high-energy neutrino flux detected by the IceCube Neutrino Observatory and the characterization of neutrino emission across the energy spectrum, from TeV to ultrahigh energies (UHE, $\ge$100 PeV).

Methodology and Scope
This paper serves as a comprehensive review of the current observational landscape and theoretical interpretations of high-energy (1 TeV–100 PeV) and ultrahigh-energy ($\ge$100 PeV) neutrinos. The methodology involves synthesizing recent results from the IceCube Neutrino Observatory, the Pierre Auger Observatory, KM3NeT, ANTARES, and Baikal-GVD. The review categorizes findings into:

1. Diffuse Flux Analysis: Characterizing the all-sky neutrino spectrum and flavor composition.
2. Source Identification: Evaluating specific extragalactic candidates (blazars, Seyfert galaxies) and Galactic sources (plane emission).
3. Multi-messenger Correlations: Comparing neutrino data with gamma-ray, cosmic-ray, and gravitational wave observations to constrain source models.
4. UHE Neutrino Searches: Reviewing limits and candidate events at energies exceeding 100 PeV.

Key Contributions and Results

• Diffuse All-Sky Flux:

  • IceCube has established the existence of a diffuse astrophysical neutrino background. The spectrum is generally consistent with a single power law ($E^{-\gamma}$) with an index $\gamma \approx 2.4 - 2.9$ and a per-flavor flux of $\sim (1-3) \times 10^{-18}$ GeV$^{-1}$ cm$^{-2}$ s$^{-1}$ sr$^{-1}$ at 100 TeV.
  • Flavor composition measurements are consistent with standard pion decay scenarios ($\nu_e:\nu_\mu:\nu_\tau \approx 1:2:0$ at the source evolving to $\approx 1:1:1$ at Earth) and standard oscillations, disfavoring pure beta-decay scenarios.
  • A tension exists between the observed neutrino flux and the diffuse gamma-ray background. If sources were transparent to gamma rays, the co-produced pionic gamma rays would exceed the observed isotropic gamma-ray background. This implies the existence of "hidden" neutrino sources where gamma rays are attenuated (e.g., by dense matter or radiation fields) within the source.

• Extragalactic Source Candidates:

  • Jet-Quiet AGN (Seyfert Galaxies): The detection of a significant neutrino excess from the nearby Type II Seyfert galaxy NGC 1068 (2022) is a major breakthrough. Unlike the blazar TXS 0506+056, NGC 1068 lacks powerful jets. Its neutrino flux exceeds its gamma-ray flux by an order of magnitude, supporting models where neutrinos are produced in the dense, gamma-ray-obscured corona of the active galactic nucleus (AGN). This suggests a population of hidden, jet-quiet AGNs may dominate the diffuse neutrino flux.
  • Jet-Loud AGN (Blazars): The 2017 detection of a neutrino (IceCube-170922A) coincident with the flaring blazar TXS 0506+056 provided the first multi-messenger link to a specific source. However, stacking analyses indicate that blazars contribute subdominantly to the total diffuse flux, particularly in the 10–100 TeV range, likely due to spectral hardness mismatches.
  • Other Candidates: The paper reviews constraints on other sources, including galaxy clusters (cosmic-ray reservoirs), starburst galaxies, gamma-ray bursts (GRBs), and gravitational wave sources. Notably, the non-detection of neutrinos from the brightest GRB (GRB 221009A) places stringent constraints on GRB models as the primary source of UHECRs.

• Galactic Plane Emission:

  • In 2023, IceCube reported the first evidence of high-energy neutrinos originating from the Galactic plane with a significance of $4.5\sigma$.
  • The emission is consistent with hadronic interactions of cosmic rays with interstellar medium (ISM) gas.
  • A critical finding is that the Milky Way is a "neutrino desert" compared to external galaxies of similar mass; its neutrino luminosity is less than 1% of what is expected from an external galaxy, suggesting a lack of powerful, persistent neutrino sources (like active AGN) in our Galaxy.

• Ultrahigh Energy (UHE) Neutrinos ($\ge$100 PeV):

  • No definitive detection of the diffuse UHE neutrino flux (cosmogenic neutrinos) has been made. IceCube's 12.6-year search yielded no events above 10 PeV, placing tight constraints on the proton fraction of UHECRs and disfavoring proton-dominated scenarios above $\sim$30 EeV.
  • Individual Events: The paper discusses intriguing individual candidates, including anomalous events from the ANITA balloon experiment (which remain unexplained and likely not of standard neutrino origin) and the KM3-230213A event detected by KM3NeT. This event, with an estimated neutrino energy of $\sim$220 PeV, is a horizontal muon track. If it belongs to a diffuse flux, it creates tension with IceCube limits, suggesting it may originate from a transient, individual source.

Significance and Conclusions
The paper concludes that the past decade has marked the birth of neutrino astrophysics, transitioning from the detection of a diffuse flux to the identification of specific source classes. The discovery of NGC 1068 as a hidden neutrino emitter and the detection of Galactic plane emission have fundamentally altered the understanding of cosmic-ray acceleration, pointing toward a diverse population of sources rather than a single dominant class.

The significance of these findings lies in their ability to probe environments inaccessible to photons. The "hidden" nature of sources like NGC 1068 and the "desert" status of the Milky Way highlight the diversity of astrophysical accelerators. While the bulk origin of the diffuse flux remains an open question, the field is moving toward a multi-messenger era where neutrinos, combined with gamma rays, cosmic rays, and gravitational waves, will be used to map the high-energy universe. Future progress relies on next-generation detectors (e.g., IceCube-Gen2, KM3NeT, radio arrays) with improved angular resolution and sensitivity to resolve individual sources and detect the elusive UHE neutrino population.