How does the atmosphere make neutrinos all around us?

The incoming cosmic ray is only the beginning

High-energy protons and other nuclei arrive from space and collide with nuclei in the air. The collision produces secondary particles, which can interact again or decay. This developing collection is called an atmospheric particle shower.

Charged pions and kaons in the shower are important parents of atmospheric neutrinos. Their decay products include muons, which can also decay and make more neutrinos. The atmosphere is therefore a particle source spread over the whole Earth.

These neutrinos are called atmospheric because they are produced in air. The original cosmic ray may have travelled from a distant astrophysical accelerator, but that does not turn the secondary neutrino into one produced at that distant source. This distinction matters when searching for cosmic-neutrino astronomy signals.

Sources: [1] · [2] · [3]

Follow one pion through two decays

A positive pion can decay into a positive muon and a muon neutrino. If that muon also decays, it produces a positron, an electron neutrino and a muon antineutrino. The corresponding negative-pion chain reverses the particle and antiparticle assignments.

Counting just this complete chain gives two muon-flavor neutrinos or antineutrinos for every one electron-flavor particle. That is the origin of the familiar approximate two-to-one argument at relatively low energies, before oscillations.

It is a count of products from a simplified chain. Their energies are not equal, and a chosen detector energy bin does not simply collect one full chain at a time. The argument is a guide to the source mixture, rather than a prediction of exactly twice as many muon events.

Sources: [1] · [2] · [4]

At higher energies, the parents do not all get time to decay

A faster muon has a longer lifetime in the Earth’s frame because of relativistic time dilation. It may reach the ground before decaying. Its missing decay removes the electron-flavor neutrino and one of the muon-flavor contributions from the atmospheric chain.

Pions and kaons can also interact before they decay. The competition depends on energy and atmospheric density. A nearly horizontal path through thinner air offers a different decay opportunity from a downward path through denser layers.

Detailed flux calculations therefore use the incoming cosmic-ray spectrum, particle-production models, atmospheric structure and the Earth’s magnetic field. Measurements of atmospheric muons help check the same cascade physics. The source ratio has useful cancellations, but it is not independent of all these inputs.

Sources: [1] · [5] · [2]

A neutrino from below began in the other side’s atmosphere

For the GeV energies important to the original atmospheric-oscillation studies, most neutrinos can cross the Earth without being absorbed. A detector can consequently receive neutrinos made overhead and neutrinos made in air on the opposite side of the planet.

The first group has a relatively short journey. The second can travel thousands of kilometres before interacting near the detector. Arrival direction therefore provides a way to compare propagation distances without moving either the detector or a man-made source.

This does not imply a uniform sky. Geomagnetic effects, cascade geometry, energy and detector acceptance all influence the expected angular distribution. At much higher energies, absorption inside the Earth also matters, as IceCube measurements later demonstrated.

Sources: [5] · [6] · [7]

The natural source became a propagation experiment

A detector observes charged particles made by neutrino interactions, rather than a source-flux table. Comparing flavors requires interaction cross sections, particle identification and efficiency. Comparing directions also requires accounting for the relationship between a neutrino’s direction and its visible products.

Super-Kamiokande’s 1998 analysis used the different flavor and direction patterns to test oscillations. The important deficit was connected to the long travel distances of upward-going muon-flavor events, while the electron-like sample supplied a complementary comparison.

Atmospheric neutrinos later served two roles: a signal for studying neutrino properties and a background to high-energy cosmic-neutrino searches. Both uses begin with the same question: what did the atmosphere produce, and how did propagation and the instrument transform it into this record?

Sources: [6] · [1] · [3] · [4]

Try it in the Lab

Primary sources & revision

1. Honda et al. · Atmospheric flux with a model calibrated to muon data (2007)
2. Gaisser & Honda · Flux of Atmospheric Neutrinos (2002)
3. IceCube Collaboration · Evidence for High-Energy Extraterrestrial Neutrinos (2013)
4. Particle Data Group · Neutrino Masses, Mixing, and Oscillations (2024)
5. Honda et al. · A three-dimensional atmospheric-neutrino flux calculation (2004)
6. Super-Kamiokande Collaboration · Evidence for Oscillation of Atmospheric Neutrinos (1998)
7. IceCube Collaboration · Multi-TeV neutrino cross section using Earth absorption (2017)

First published and source-checked on 9 October 2026. Original explanatory text and diagrams by Sandbox Physics. Illustrations are schematic; no experimental event records are reproduced here. This is an independent educational publication, not an official Nobel or experiment collaboration publication.

Continue the story