Invisible Particles: The Hunt for Long-Lived Particles in ATLAS express
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.
Original abstract
as written by the authors arXiv:2601.05664
A search is presented for long-lived particles decaying into an oppositely charged lepton pair, $$\mu^{+}\mu^{-}$$, $$e^{+}e^{-}$$, or $$e^{\pm}\mu^{\mp}$$, that form a vertex within the inner tracking system of the ATLAS detector at the Large Hadron Collider, displaced from the primary proton-proton interaction region. The analysis uses the 140 fb$$^{-1}$$ of Run-2 data collected at $$\sqrt{s}=13$$ TeV by the ATLAS experiment in 2015-2018. The results of the analysis are interpreted in the context of three benchmark models covering masses from 0.1 to 2.2 TeV and a range of mean proper lifetimes times the speed of light from 1 to 10000 mm. The first model is a generic $$Z'$$ boson pair-produced by a new heavy scalar, with the $$Z'$$ decaying into lepton pairs. The remaining two models are $$R$$-parity violating supersymmetric models in which the lightest neutralino $$\tilde{\chi}^{0}_{1}$$ decays into $$\ell^{+}\ell^{'-} u$$ ($$\ell, \ell^{'} = e$$, $$\mu$$). The models differ by the mode of production of the $$\tilde{\chi}^{0}_{1}$$, which can be produced via the decay of pairs of gluinos or of pairs of charginos and neutralinos ($$\tilde{\chi}_{1}^{\pm}\tilde{\chi}_{1}^{0}$$, $$\tilde{\chi}_{1}^{\pm}\tilde{\chi}_{2}^{0}$$, or $$\tilde{\chi}_{2}^{0}\tilde{\chi}_{1}^{0}$$). Although each benchmark sample includes pair-produced LLPs, only a single vertex is required to be reconstructed. No dilepton displaced vertex candidate is observed and the results are presented as upper limits on the production cross-sections. This analysis sets leading limits on the production cross-sections for multiple models, including parameter space that has never been directly probed.
Long-lived particles can travel several meters through the detector before decaying, like a bullet fired from a rifle that bursts not at the muzzle but a hundred meters from the shooter.