Particle Astrophysics with High and Ultrahigh Energy Neutrinos

TL;DR

This paper surveys the emergence of neutrino astrophysics as a probe of high-energy cosmic accelerators, detailing diffuse high-energy neutrino flux measurements, first astrophysical source candidates, and the Galactic-plane contribution, while outlining theories and searches for ultrahigh energy neutrinos. It emphasizes the IceCube era as a turning point, with a diffuse flux described by a power-law $\Phi_\nu \propto E_\nu^{-\gamma_{\rm astro}}$ ($\gamma_{\rm astro} \sim 2.4$–$2.9$) and notable source associations such as TXS 0506+056 and NGC 1068, as well as a measurable Galactic component around 30 TeV. The paper discusses a broad array of extragalactic candidate sources ( jet-quiet/loud AGN, starbursts, GRBs, TDEs) and potential hadronic sites (cosmic-ray reservoirs, shocks, coronae), along with the current lack of firm ultrahigh-energy detections and the stringent limits on cosmogenic and GRB-related neutrinos. Collectively, these results establish neutrinos as a crucial tool for multimessenger astronomy and set the stage for next-generation detectors (IceCube-Gen2, KM3NeT, Baikal-GVD, P-ONE) that will sharpen source identifications and probe new physics.

Abstract

We summarize recent results of the observations of high (1 TeV-100 PeV) and ultrahigh ($\geq 100$ PeV) energy neutrinos, including the detection of a diffuse cosmic high-energy neutrino background, the identification of the first neutrino source candidates, and the observation of high-energy neutrinos from the Galactic plane. These findings open a new window to the universe by enabling the use of neutrinos to probe the cosmos that are otherwise inaccessible via photons. Although the origins of most detected neutrinos remain uncertain, we highlight several distinctive features of their sources that have emerged from current observations.

Figures (4)

• Figure 1: Current observational landscape of high and ultrahigh energy neutrinos. Blue data points and shaded regions represent measurements of the diffuse astrophysical neutrino flux from various IceCube datasets IceCube:2020wumAbbasi:2021qfzIceCube:2021rpzIceCube:2024fxo. Also shown are detections of individual sources by IceCube (scaled by $4\pi$), including the Galactic plane IceCube:2023ame (yellow band) and the Seyfert galaxy NGC 1068 IceCube:2022der (red band). Green curves indicate upper limits on the diffuse UHE neutrino flux from IceCube IceCube:2025ezc and Auger AbdulHalim:2023SN. A green marker denotes the UHE neutrino candidate event reported by KM3NeT KM3NeTEvent.
• Figure 2: Left: Best-fit single power-law parameters, including the per-flavor normalization and spectral index (solid markers) and 68% contours obtained from analyses using various data samples. Plot is from Ref. IceCube:2025ary. Right: Measured flavor composition of IceCube data comparing to various neutrino production models after propagation (colored markers). Contours show the $1\sigma$ and $2\sigma$ confidence intervals of the measurements. Plot is from Ref. IceCube:2020fpi.
• Figure 3: Left: All-flavor neutrino (thick blue lines) and isotropic diffuse gamma-ray background (thin red lines) fluxes for $pp$ and $p\gamma$ scenarios, compared to IceCube and Fermi-LAT observations of the diffuse emission backgrounds Murase:2015xka. Right: Model spectra of MeV–TeV gamma-ray emission from NGC 1068 compared to Fermi-LAT data (black points) Ajello:2023hkh. AGN corona and starburst models are shown as red and blue shaded bands, respectively. The all-flavor coronal neutrino spectrum that explains the IceCube data (gray band) is shown by the black solid curve. Projected sensitivities of AMEGO-X and e-ASTROGAM are also shown.
• Figure 4: Left (from Ref. IceCubeGP): The plane of the Milky Way galaxy in photons and neutrinos. Each panel from top to bottom shows the flux of the Milky Way in latitude and longitude in the following wavelengths or messengers: (A) optical, (B) gamma-ray based on the Fermi-LAT 12-year survey at energies greater than 1 GeV, (C) neutrino, as a template derived from the $\pi^0$ template that matches the Fermi-LAT observations of the diffuse gamma-ray emission, (D) neutrino, as an emission template from panel (C) but including the IceCube sensitivity to cascade-like events. The dotted white circle indicates the angular uncertainty of a typical event. Contours indicate the central regions that contain 20% and 50% of the predicted diffuse neutrino emission signal. (E) neutrino, as the pre-trial significance of the IceCube neutrino observations, calculated from the all-sky scan for point-like sources using the cascade neutrino event sample. Contours are the same as in panel (D). Right (from Ref. Fang:2023azx): All-sky-averaged intensities, scaled by $E^{2.5}$, of the Galactic diffuse emission (GDE) and extragalactic background (EB) in gamma-ray and neutrino. In this plot, blue and grey colors indicate neutrino measurements and red color corresponds to gamma-ray observations. Non-hatched bands indicate observations of the Galactic diffuse emission and hatched bands correspond to observations of the extragalactic diffuse emission. Galactic components include the diffuse neutrino emission (per-flavour flux) from the Galactic plane measured by IceCube using the $\pi^0$ template IceCubeGP and the neutrino flux derived from the gamma-ray GDE measured by Tibet AS$\gamma$ (grey-shaded region) Tibet21 and the Galactic interstellar emission model of Fermi-LAT (red-shaded region)4FGL. Extragalactic components include the isotropic diffuse neutrino background measured by IceCube (blue-hatched region)Aartsen:2020aqd and the EGB measured by Fermi-LAT (red-hatched region)FermiIGRB.