What Are Neutrinos? The Ghost Particles That Pass Through You by the Trillion

Right now, as you read this, “thousands of billions of neutrinos are flowing through your body every second” (NobelPrize.org^[1]). You cannot see them or feel them, and almost none of them will ever touch a single atom of you. Neutrinos are the ghosts of particle physics, born in the weak nuclear force that our Decay Dancer tee personifies. This post builds on our explainer How Did the First Elements Form?, because neutrinos were made in enormous numbers in the early universe and in every star since.

What are neutrinos?

Neutrinos are electrically neutral particles with a tiny mass that interact only through the weak force (and gravity). That is why they are so hard to catch: they carry no charge for electromagnetism to grab and feel nothing of the strong force. The Nobel Foundation notes that they “rush through space almost at the speed of light and hardly ever interact with matter” and that, second only to photons, they are the most numerous particles in the universe (NobelPrize.org^[1]).

They come from the Sun, from supernovae, from nuclear power plants and from natural radioactivity. Even your own body releases “an average of 5,000 neutrinos per second” when an isotope of potassium decays (NobelPrize.org^[1]).

Why did Pauli invent the neutrino?

In beta decay, a nucleus spits out an electron. If only two bodies were involved, the electron would always leave with the same energy. Instead it comes out with a spread of energies, as our chart below shows, and some energy seems to vanish. In December 1930 Wolfgang Pauli wrote to colleagues he addressed as “Dear Radioactive Ladies and Gentlemen,” suggesting that the missing energy is carried away by “an electrically neutral, weakly interacting and very light particle” (NobelPrize.org^[1]). The popular account adds that he proposed it “in a desperate attempt” to keep energy conservation, and hardly believed in it himself. Our post Beta Decay Unveiled covers the decay in detail.

How were neutrinos first detected?

A particle that barely interacts needs a huge source. In the 1950s, nuclear reactors provided one. In the early 1950s, Frederick Reines “passed radiation from a nuclear reactor through a water tank and discovered reactions that proved the neutrino’s existence” (NobelPrize.org^[2]). In June 1956 Reines and Clyde Cowan sent Pauli a telegram: the neutrinos had left traces in their detector (NobelPrize.org^[1]).

Reines received half of the 1995 Nobel Prize in Physics “for the detection of the neutrino”; the other half went to Martin Perl “for the discovery of the tau lepton” (NobelPrize.org^[3]).

How many kinds of neutrino are there?

Three, each paired with a charged partner: the electron neutrino, the muon neutrino and the tau neutrino (NobelPrize.org^[1]).

• Muon neutrino, 1962. At Brookhaven, Danby, Gaillard, Goulianos, Lederman, Mistry, Schwartz and Steinberger showed that the neutrinos made alongside muons are different from those made alongside electrons; their 1962 paper is titled “Observation of High-Energy Neutrino Reactions and the Existence of Two Kinds of Neutrinos” (Danby et al. 1962^[4]). Lederman, Schwartz and Steinberger shared the 1988 Nobel Prize “for the neutrino beam method and the demonstration of the doublet structure of the leptons through the discovery of the muon neutrino” (NobelPrize.org^[5]).
• Tau neutrino, 2000. The DONUT experiment reported the “Observation of Tau Neutrino Interactions” (DONUT Collaboration 2001^[6]).

Do neutrinos have mass?

Yes, and proving it won the 2015 Nobel Prize in Physics, awarded to Takaaki Kajita and Arthur McDonald “for the discovery of neutrino oscillations, which shows that neutrinos have mass” (NobelPrize.org^[7]).

In 1998 the Super-Kamiokande detector in Japan reported a “zenith angle dependent deficit of muon neutrinos” made by cosmic rays in the atmosphere: muon neutrinos coming straight down were more numerous than those that had first passed through the globe (Super-Kamiokande 1998^[8]; NobelPrize.org^[1]). In 2001–2002 the Sudbury Neutrino Observatory in Canada measured neutrinos from the Sun two ways. The Sun only makes electron neutrinos, yet SNO found fewer electron neutrinos than expected while the total of all three types matched predictions (NobelPrize.org^[1]; SNO 2002^[9]). The neutrinos had changed flavour on the way. Out of the more than 60 billion solar neutrinos crossing each square centimetre of Earth every second, SNO caught “only three per day” in its first two years (NobelPrize.org^[1]).

How do neutrino oscillations work?

Each flavour is a quantum mixture of three mass states. If the masses differ, the mixture shifts as the neutrino travels. In the simplest two-flavour case, the Particle Data Group gives the probability of a neutrino changing flavour as (PDG 2024 review^[10])

P = sin²(2θ) sin²(1.267 Δm² L/E)

θ is the mixing angle, Δm² is the difference of the squared masses in eV², L is the distance travelled in kilometres and E is the neutrino energy in GeV (equivalently L in metres and E in MeV). If all neutrinos had the same mass, Δm² would be zero and nothing would oscillate. That is why oscillation proves mass.

Using the PDG 2024 value Δm²₃₂ = 2.455 × 10⁻³ eV² for normal ordering (PDG 2024^[11]), the first maximum of flavour change happens when 1.267 Δm² L/E = π/2, at L/E ≈ 505 km/GeV. A 1 GeV atmospheric neutrino needs about 500 km, far less than the Earth’s diameter of about 12 700 km, which helps explain why muon neutrinos that had crossed the Earth went missing while those from directly overhead, having travelled only down through the atmosphere, mostly did not.

How heavy is a neutrino?

Nobody knows yet. Oscillations measure only differences: the PDG lists Δm²₂₁ = (7.53 ± 0.18) × 10⁻⁵ eV² and |Δm²₃₂| ≈ 2.5 × 10⁻³ eV² (PDG 2024^[11]). The square root of the larger one, about 0.05 eV, means at least one neutrino must be at least that heavy. From above, the KATRIN experiment, which studies tritium beta decay near its endpoint, set “an upper limit of m_ν < 0.45 eV at 90 % confidence level” (KATRIN 2025^[12]). For scale, the electron’s mass is 0.511 MeV, or 511 000 eV (PDG 2024^[11]).

What is still unknown about neutrinos?

The PDG review lists “the issues which still require clarification”: “the mass ordering discrimination, the determination of θ23 and the leptonic CP phase” (PDG 2024 review^[10]). In plain words: we do not know whether the third mass state is the heaviest (normal ordering) or the lightest (inverted ordering); current analyses favour normal ordering, but not decisively. We also do not know the absolute masses, or whether neutrinos and antineutrinos oscillate differently. These are open questions, and our mass-ordering diagram below is labelled that way.

FAQ

Are neutrinos dangerous?

No. They interact so rarely that trillions pass through you each second without effect.

Where are the weak-force characters?

See the Weak Force Series.

References

1. NobelPrize.org, Popular information: The Nobel Prize in Physics 2015 (“Metamorphosis in the particle world”). https://www.nobelprize.org/prizes/physics/2015/popular-information/
2. NobelPrize.org, Frederick Reines – Facts (Nobel Prize in Physics 1995). https://www.nobelprize.org/prizes/physics/1995/reines/facts/
3. NobelPrize.org, The Nobel Prize in Physics 1995 (Martin L. Perl and Frederick Reines). https://www.nobelprize.org/prizes/physics/1995/summary/
4. G. Danby, J-M. Gaillard, K. Goulianos, L. M. Lederman, N. Mistry, M. Schwartz and J. Steinberger, “Observation of High-Energy Neutrino Reactions and the Existence of Two Kinds of Neutrinos”, Phys. Rev. Lett. 9, 36 (1962). https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.9.36
5. NobelPrize.org, The Nobel Prize in Physics 1988 (Lederman, Schwartz, Steinberger). https://www.nobelprize.org/prizes/physics/1988/summary/
6. K. Kodama et al. (DONUT Collaboration) (2001), “Observation of Tau Neutrino Interactions,” Phys. Lett. B 504, 218–224 (arXiv:hep-ex/0012035; DOI 10.1016/S0370-2693(01)00307-0). https://arxiv.org/abs/hep-ex/0012035
7. NobelPrize.org, The Nobel Prize in Physics 2015 (Takaaki Kajita and Arthur B. McDonald). https://www.nobelprize.org/prizes/physics/2015/summary/
8. Y. Fukuda et al. (Super-Kamiokande Collaboration) (1998), “Evidence for Oscillation of Atmospheric Neutrinos,” Phys. Rev. Lett. 81, 1562–1567 (DOI 10.1103/PhysRevLett.81.1562). https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.81.1562
9. Q. R. Ahmad et al. (SNO Collaboration) (2002), “Direct Evidence for Neutrino Flavor Transformation from Neutral-Current Interactions in the Sudbury Neutrino Observatory,” Phys. Rev. Lett. 89, 011301 (DOI 10.1103/PhysRevLett.89.011301). https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.89.011301
10. M. C. Gonzalez-Garcia & M. Yokoyama (2024), “Neutrino Masses, Mixing, and Oscillations,” in S. Navas et al. (Particle Data Group), Review of Particle Physics, Phys. Rev. D 110, 030001. https://pdg.lbl.gov/2024/reviews/rpp2024-rev-neutrino-mixing.pdf
11. S. Navas et al. (Particle Data Group) (2024), Review of Particle Physics, Phys. Rev. D 110, 030001: Lepton summary table (neutrino mixing). https://pdg.lbl.gov/2024/tables/rpp2024-sum-leptons.pdf
12. M. Aker et al. (KATRIN Collaboration) (2025), “Direct neutrino-mass measurement based on 259 days of KATRIN data,” Science 388, 180–185 (arXiv:2406.13516; DOI 10.1126/science.adq9592). https://arxiv.org/abs/2406.13516

Written by Pixelated Physics. Every factual claim is linked to the source we checked; points that are still debated are labelled open.