When two particles fly at almost the same speed in the same direction, their signals in the detector merge, like two voices in unison. Telling a duet from a soloist is an art in itself. At the Large Hadron Collider, an electron and its antipode, the positron, are sometimes born with such momentum that their energies add up into a single blob in the calorimeter — the device that measures particle energy. Previously, such a blob was easily mistaken for a single particle, and valuable information was lost.
Physicists from the CMS collaboration have devised two detective algorithms. The first works when both tracks are visible in the tracker — the detector that records the particle's path. It finds such pairs in 80% of cases. The second comes into play when only one track remains: it catches the blob by indirect clues, and its success rate is about 60%. For testing, they used already familiar particles: J/ψ mesons (short-lived particles consisting of a charm quark and an anti-charm quark) that decay into an electron and a positron, and events where a photon converts into an electron-positron pair.
Such merged pairs are markers of very energetic processes. For example, they arise in the decays of heavy particles, similar to those that could point to new physics beyond the Standard Model. By learning to recognize them, scientists get a cleaner signal and lose less data.
The electron and positron are a particle and an antiparticle. When they meet, they annihilate, turning into pure energy. But in the detector, they haven't met yet: they just fly so close that their signals merge.