SNO: where did the Sun’s missing neutrinos go?

The Sun was sending fewer signals than expected

Nuclear reactions inside the Sun produce electron neutrinos. Calculations of those reactions, developed through work including John Bahcall’s, predicted how many should reach Earth. But measurements, notably Raymond Davis Jr.’s radiochemical experiment, consistently found fewer than expected.

That shortfall left a difficult choice. Were the calculations of the Sun wrong? Had the experiments missed something? Or were the neutrinos doing something unexpected on the way here?

A detector can only report the interactions it is able to see. If electron neutrinos became another flavor—another neutrino type—during their journey, an experiment mainly sensitive to the original flavor could count too few even though the particles were still arriving.

Sources: [1] · [2]

Heavy water made a new comparison possible

The Sudbury Neutrino Observatory, or SNO, used heavy water in Canada. In heavy water, the hydrogen nuclei contain a neutron as well as a proton. These nuclei, called deuterons, can be broken apart by neutrino interactions.

Two ways of breaking them apart were especially useful. At solar-neutrino energies, one reaction was sensitive to electron neutrinos. The other was sensitive to all three active flavors: electron, muon and tau. Comparing the two let SNO ask both “how many arrive as electron neutrinos?” and “how many arrive altogether?”

The technical names are charged-current and neutral-current reactions. SNO also measured neutrinos scattering from electrons. That third measurement responds to all three flavors, but more strongly to electron neutrinos, providing another way to compare the results.

Sources: [3]

One number was smaller; the total was still there

In the 2002 neutral-current analysis, SNO inferred about 1.76 million electron neutrinos per square centimetre per second, compared with about 5.09 million for all active flavors together. Physicists call this arrival rate per unit area a “flux”.

The electron component was roughly a third of the total. The rest required a contribution from other flavors. Meanwhile, the total was compatible with what solar calculations predicted. The experiment had found a way to account for the missing electron neutrinos.

These were inferred rates, obtained using the detector response, reaction probabilities and an assumed boron-8 neutrino spectrum. The paper reported the non-electron component at 5.3 standard deviations above zero under those assumptions. The numbers are rounded here; the original paper gives the statistical and systematic uncertainties.

Sources: [3]

Why this was more convincing than another low count

Imagine trying to explain the early shortfall only by reducing the number of neutrinos the Sun produces. That would lower the total arriving flux too. SNO’s comparison showed a different pattern: fewer arrived as electron neutrinos, but the total active flux remained consistent with the solar prediction.

Flavor change explains how both observations can be true. The Sun produces electron neutrinos, and some arrive in forms that the earlier electron-sensitive measurements largely missed. The comparison gave direct evidence for that transformation.

No instrument followed a single neutrino all the way from the Sun. The evidence came from comparing many interactions at the destination. Neutrino oscillations provide the physical explanation for how the detected flavor can change during propagation.

Sources: [3] · [2]

Two experiments, two ways to test the journey

SNO becomes especially interesting beside Super-Kamiokande’s atmospheric-neutrino result. SNO compared different sensitivities to the flavors arriving from one source. Super-K compared arrivals associated with short and long journeys through or above Earth.

Those are different measurements, and together they helped establish the picture of neutrino flavor change recognized by the 2015 Nobel Prize.

The Cherenkov Lab in this collection explains an earlier step: how a fast charged particle produces light that a water detector can collect. SNO then had to identify and compare reaction channels, with their own efficiencies and backgrounds. Understanding the light is a beginning; the heavy-water comparison is what answers the question about the Sun.

Sources: [4] · [5] · [2]

Try it in the Lab

Primary sources & revision

1. John Bahcall · Solving the mystery of the missing neutrinos
2. Nobel Committee · Neutrino oscillations, scientific background (2015)
3. SNO Collaboration · Direct Evidence for Neutrino Flavor Transformation (2002)
4. Kamioka Observatory · Super-Kamiokande detector
5. Super-Kamiokande Collaboration · Evidence for Oscillation of Atmospheric Neutrinos (1998)

First published 2026-10-09; last revised 2026-10-11. 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.