Abstract
A search is presented for long-lived particles decaying into an oppositely charged lepton pair,
+
−,
+
−, or
±
∓,
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 f b−1 of Run-2 data collected at √
= 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 10 000 mm. The first model is a generic ′ boson pair-produced by a new heavy
scalar, with the ′ decaying into lepton pairs. The remaining two models are -parity violating supersymmetric
models in which the lightest neutralino ̃
0
1
decays into
+
′− ( ,
′
= , ). The models differ by the mode
of production of the ̃
0
1
, which can be produced via the decay of pairs of gluinos or of pairs of charginos and
neutralinos ( ̃
±
1
̃
0
1
, ̃
±
1
̃
0
2
, or ̃
0
2
̃
0
1
). 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.