Hunting for Vanishing Tracks express
In the ATLAS detector at the Large Hadron Collider, particles leave tracks like skiers in the snow. Some tracks break off after a couple of centimeters: that's what a particle would look like if it were born, flew a little way, and then decayed into something invisible. Physicists were looking for exactly such 'vanishing tracks' in the data from 13 TeV proton collisions.
The search relied on distinctive signatures: a track of three or four points in the first layers of the detector, a powerful jet of particles nearby, and a large missing transverse momentum (carried away by something invisible). To distinguish a real signal from random coincidences, machine learning and specialized background estimation methods were used.
This result is like combing a forest: no tracks of the sought-after beast, but the map of the area is now more accurate. Future searches will know where not to look and can focus on heavier or longer-lived particles.
In one nanosecond, light travels only 30 centimeters — about the length of a school ruler. The particles physicists were looking for live for less time than it takes light to cross the detector.
The idea of particles that leave a short track and then vanish is reminiscent of ghost stories where apparitions are visible only for a moment before dissolving into thin air.