In a vast, dark hall, they're searching for a single person who makes almost no sound. That's how physicists hunt for sterile neutrinos — ghostly particles that pass through matter, leaving barely a trace.
A strange visitor came to light in a fleeting flash. A beam of photons struck a deuterium nucleus (heavy hydrogen), and in the cascade of produced particles, detectors noticed something unusual. At a certain combination of masses and momenta, the signal sharply intensified, as if a new particle had appeared for a brief moment.