Neutrinos are electrically neutral, extremely light elementary particles that interact only through the weak force and gravity. Because they almost never interact with matter, enormous numbers pass through Earth (and us) constantly.
Why they matter
- They are among the most abundant particles in the universe.
- They are produced in the Sun, radioactive decay, supernovae, and high-energy cosmic collisions.
- They let us probe environments that light cannot easily escape.
Detection
Detecting neutrinos is difficult because interactions are rare. Experiments use huge detectors (deep underground, underwater, or in ice) to catch occasional interaction events.
Key physics insight
For years neutrinos were assumed massless in early formulations of the Standard Model. Neutrino oscillation experiments showed they change flavor in flight, which implies they have non-zero mass.
That result was major evidence that the original Standard Model description was incomplete.