A pebble drops into a pond, and ripples spread across the water. But sometimes the pebble doesn't sink right away: it skims beneath the surface, travels a few meters, and only then surfaces, creating ripples in an unexpected spot. This is how physicists search for long-lived particles: they are born in proton collisions, fly through the detector unnoticed, and then decay into ordinary particles far from their birth point.

In the ATLAS experiment at the Large Hadron Collider, the hunt was precisely for such "delayed ripples"—decays into pairs of charged leptons (electrons or muons) away from the proton collision point. To do this, they sifted through data collected over four years of collider operation at an energy of 13 TeV. They looked for events where two charged particles emerge from a single point displaced from the beam axis. Such a point — a "vertex"—is exactly what reveals the decay of a long-lived particle.

The scientists tested several theoretical scenarios. In one, a new heavy particle decays into a pair of light long-lived particles, which then produce lepton pairs. In others, it's about supersymmetric variants where a neutralino (the hypothetical partner of the photon or Higgs boson) lives long enough to fly from the birth point and decays into leptons and neutrinos. The masses of such particles in the models ranged from 100 GeV to 2.2 TeV, and their "decay lengths" from a millimeter to ten meters.

The absence of a signal allowed setting new upper limits on the production probability of such particles. For several models, these limits are the world's most stringent, and part of the mass and lifetime range has been placed under direct experimental constraints for the first time. Now theorists will have to refine their predictions, and experimentalists will continue searching for long-lived particles in other decay channels or at future colliders.