In a dark room, you might notice a faint blue glow around some radiation sources. This is Cherenkov light: it appears when a charged particle travels through a medium faster than light can propagate in it. The result is a light shock wave, similar to the sonic boom from a supersonic jet, but made of photons.
In the new study, scientists used this effect to identify electrons. They built a detector from many tiny silicon cells — avalanche photodiodes that can count individual photons. As an electron flew through the air and produced a flash of Cherenkov light, the detector caught the photons and, from their number, reconstructed the particle's properties.
The tests were conducted on a particle beam at CERN, where electrons were accompanied by other particles — pions and other hadrons. It turned out that Cherenkov light distinguishes electrons very clearly: they can be separated from background particles with high reliability over a wide range of momenta. A simple computer model reproduced the results, meaning the method can be confidently applied in future experiments.
Such a detector could become a cheap and compact replacement for bulky particle identification systems. Instead of tons of iron and gas, just a few silicon wafers that count photons. And next time an electron zips through the air, it will be betrayed by a brief blue flash.
Cherenkov glow can be seen with the naked eye in spent nuclear fuel pools, the water glows blue.
In Stanisław Lem's novel "Solaris," the planet's ocean emits a mysterious glow — somewhat reminiscent of Cherenkov light, though its nature is entirely different.