Atmospheric and Astrophysical Neutrinos

1. 1

   experiment

   kamiokande

   First hints of the atmospheric anomaly — muon/electron ratio below prediction — in data from the late 1980s. Limited statistics prevent definitive interpretation.

2. 2

   experiment

   imb

   Complementary large-volume water Cherenkov detector in Ohio. Joint IMB + Kamiokande + Baksan detection of 24 neutrinos from SN 1987A — first extragalactic neutrino observation.

3. 3

   concept

   supernova neutrinos

   The phenomenology of core-collapse neutrino emission. SN 1987A validates the thermal core-collapse picture.

4. 4

   concept

   atmospheric neutrinos

   The cosmic-ray-induced neutrino flux and its expected 2:1 muon-to-electron ratio — the unoscillated baseline that experiments measure against.

5. 5

   paper

   super kamiokande 1998

   The decisive atmospheric oscillation result. Zenith-angle-dependent deficit of νμ events establishes oscillation and, consequently, neutrino mass.

6. 6

   experiment

   super kamiokande

   Ongoing atmospheric program beyond the 1998 discovery — including the Gd-loading phase for diffuse supernova background sensitivity.

7. 7

   experiment

   icecube

   Cubic-kilometer ice detector at the South Pole. First detection of high-energy astrophysical neutrinos (2013), point-source identifications (TXS 0506+056 2017, NGC 1068 2022), galactic-plane emission (2023).

A thread that crosses three experimental eras

The atmospheric-and-astrophysical neutrino story spans three distinct detection technologies and four decades of data:

The water-Cherenkov 1980s — Kamiokande, IMB, and Baksan make the first extragalactic neutrino detections (SN 1987A) and begin to observe the atmospheric anomaly. The detectors are orders of magnitude larger than Reines-Cowan but still modest by modern standards.

The Super-Kamiokande 1990s–2010s — 50 kt of water resolves the atmospheric anomaly definitively as oscillation, establishes νμ → ντ as the dominant channel, and measures $\Delta m_{31}^2$ and ${\theta }_{23}$ with steadily improving precision. The 1998 result is the first direct evidence of neutrino mass.

The IceCube 2010s–present — a cubic-kilometer of instrumented ice at the South Pole opens neutrino astronomy. The astrophysical flux above 30 TeV is discovered in 2013; individual sources (blazars, active galactic nuclei, galactic-plane emission) are identified from 2017 onward.

Multi-messenger astronomy

The 2017 IceCube-170922A alert — a 290 TeV muon-neutrino event correlated in real time with a flaring blazar, TXS 0506+056, observed simultaneously in gamma rays — marks the founding event of high-energy multi-messenger astronomy with neutrinos. The identification relies on arcminute-scale pointing accuracy from long-track muon events, and on real-time alert infrastructure linking IceCube with optical, X-ray, and gamma-ray observatories worldwide.

Subsequent detections — NGC 1068 in 2022 (a nearby Seyfert galaxy), galactic-plane diffuse emission in 2023 — have extended the catalog of neutrino sources, though the overall power budget remains poorly accounted for.

Open questions

• What fraction of the astrophysical neutrino flux comes from identified sources vs. an unidentified diffuse background?
• What are the production mechanisms — hadronic (pp, pγ) versus more exotic channels?
• Can neutrino astronomy reach sub-PeV energies with useful angular resolution? KM3NeT and Baikal-GVD in the Mediterranean and Russian lakes will contribute in the coming decade.
• Can a pre-supernova warning system give optical astronomers a few-hour lead on the next galactic core collapse? Super-K-Gd, DUNE, and JUNO all target this capability.

The thread is one of the most active in contemporary neutrino science and likely to produce further discoveries through the 2030s.