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.
Original abstract
as written by the authors arXiv:2603.08315
This paper reports a search for decays of long-lived charginos or $$\tau$$-sleptons to final states containing a short disappearing track, a single high-energy jet, and missing transverse momentum. The search uses 137 fb$$^{-1}$$ of data from 13 TeV proton-proton collisions recorded by the ATLAS detector during Run 2 of the LHC. Multiple search regions are defined, all requiring the presence of a track reconstructed from either three or four measurements in the innermost layers of the ATLAS detector. Regions with tracks having only three measurements are further characterised by the absence or presence of a low-energy charged pion reconstructed using a dedicated algorithm, leveraging machine learning. Data-driven methods are used to estimate the background contributions in the search regions. No significant excesses are found and 95% CL lower limits are placed on the masses of charginos and $$\tau$$-sleptons in the lifetime range $$0.01{-}10$$ ns. Observed (expected) mass limits of up to 225 GeV (250 GeV) are set for pure-higgsino charginos in scenarios with lifetimes below 0.03 ns, where the electroweakino mass splitting is entirely due to loop corrections involving the Standard Model bosons, and up to 720 GeV (840 GeV) for charginos with a lifetime of around 1 ns. For wino production, charginos with masses up to 880 GeV (1020 GeV) are excluded for lifetimes of around 1 ns. For $$\tau$$-sleptons with lifetimes of around 1 ns, masses are excluded up to 320 GeV (390 GeV) in Constrained Minimal Supersymmetric Standard Model scenarios and 300 GeV (380 GeV) in Gauge-Mediated Supersymmetry-Breaking scenarios.
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.