Antonio MarroneTheoretical Physics · Bari

Interactive neutrino physics · Detectors

What does a neutrino look like in a water-Cherenkov detector?

The neutrino itself is invisible: the ring of light comes from the charged lepton it produces.

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Interactive figure — detector response

Water-Cherenkov event

The neutrino emits no Cherenkov light: it is neutral, it crosses the water unseen, and its direction is only inferred after the fact. Light appears when the charged lepton produced at the vertex travels faster than the phase velocity of light in water, c/n, which is well below c. The cone opens at cos θC = 1/(βn), and its intersection with the sensor wall is the ring.

SECTIONAL VIEW · WATER VOLUME SENSOR WALL, FACE ON SENSOR WALL VERTEX inferred ν direction — not an observed track — βn ≤ 1 — NO CHERENKOV LIGHT EARLY LATE RELATIVE ARRIVAL TIME · ILLUSTRATIVE, NOT PHYSICAL TIME
Event topology

A muon travels as a single straight track, so the ring edge stays sharp.

0.851.00 — threshold1.34

State

βn
—
cos θC
—
θC
—
stage
—
hit PMTs
—

Nothing here moves faster than light in vacuum. The charged lepton exceeds only c/n, the phase velocity of light in water, which is why the wavefronts pile up into a cone; below βn = 1 the condition fails and no light is produced. Distances are schematic and the sequence is paced for reading, not to scale in physical time.

About this figure

Water-Cherenkov event

Level. Undergraduate: special relativity and the refractive index.

A neutral neutrino crosses the water unseen and its direction is only inferred afterwards. The charged lepton produced at the vertex radiates because it travels faster than the phase velocity of light in water, c/n; the cone opens at cos θC = 1/(βn), and its intersection with the sensor wall is the ring.

Assumptions

  • Default βn = 1.32 (water, n ≈ 1.33, β → 1). Below βn = 1 there is no Cherenkov light, and the figure says so instead of drawing a degenerate ring.
  • The colouring of the sensors by arrival time is illustrative, as labelled on the figure.
  • One schematic event, reproducible from load to load — not a detector simulation. Nothing in it travels faster than c.

Where it fits

Cite and reuse

A. Marrone, Water-Cherenkov event, interactive figure (2026), https://home.ba.infn.it/~marrone/interactive-cherenkov.html.

@misc{Marrone:cherenkov,
  author       = {Marrone, Antonio},
  title        = {Water-Cherenkov event},
  howpublished = {Interactive figure, \url{https://home.ba.infn.it/~marrone/interactive-cherenkov.html}},
  year         = {2026}
}

Text and figures: CC BY 4.0. Code (assets/js/cherenkov.js): MIT licence. Use it in lectures and talks, adapt it, redistribute it — with attribution.