Search for long-lived particles using displaced vertices of oppositely charged leptons in 140 fb$^{-1}$ of pp collisions at $\sqrt{s} = 13$ TeV with the ATLAS detector

TL;DR

The paper reports a search for long-lived neutral particles decaying to dilepton pairs with displaced vertices inside the ATLAS inner tracker, using the full Run-2 dataset of $140.1~\mathrm{fb}^{-1}$ at $\sqrt{s}=13~\mathrm{TeV}$. It interprets results in three benchmark models (a scalar-produced $Z'$ pair and two RPV SUSY scenarios with neutralino decays) across masses $0.1$–$2.2~\mathrm{TeV}$ and lifetimes $c\tau$ from $1$ to $10^4$ mm, requiring only a single DV per event. No signal DV is observed, and the study sets upper limits on production cross-sections, including regions of parameter space not previously probed. The results provide leading constraints on LLP production in these channels and contribute to the exploration of high-mass, long-lived neutral particles at the LHC.

Abstract

A search is presented for long-lived particles decaying into an oppositely charged lepton pair, $μ^{+}μ^{-}$, $e^{+}e^{-}$, or $e^{\pm}μ^{\mp}$, 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 $\tildeχ^{0}_{1}$ decays into $\ell^{+}\ell^{'-}ν$ ($\ell, \ell^{'} = e$, $μ$). The models differ by the mode of production of the $\tildeχ^{0}_{1}$, which can be produced via the decay of pairs of gluinos or of pairs of charginos and neutralinos ($\tildeχ_{1}^{\pm}\tildeχ_{1}^{0}$, $\tildeχ_{1}^{\pm}\tildeχ_{2}^{0}$, or $\tildeχ_{2}^{0}\tildeχ_{1}^{0}$). 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.

Figures (7)

• Figure 1: Diagrams for (a) a pair of $Z'$ produced via a scalar and for a pair of $\tilde{\chi}_{1}^{0}$, each produced via (b) a gluino or (c) an electroweakino. For the benchmark models used, the $Z'$ and $\tilde{\chi}_{1}^{0}$ are the LLPs while the scalars, gluinos, and higher-mass electroweakinos ($\tilde{\chi}_{1}^{\pm}$ and $\tilde{\chi}_{2}^{0}$) decay promptly.
• Figure 2: The efficiency for reconstructing and correctly identifying a simulated lepton from an LLP decay as a function of $d_{0}$ for (left) electrons and (right) muons for ST and ST+LRT, with standard and modified working points. The efficiencies were determined using gluino-produced $\tilde{\chi}_{1}^{0}$ samples with a $\tilde{g}$ mass of 2000 $\text{Ge V}$ and a $\tilde{\chi}_{1}^{0}$ mass of 1000 $\text{Ge V}$, averaged over the five $c\tau$ from 10 to 1000 mm.
• Figure 3: Total signal efficiency for an LLP decay as a function of $c\tau$ for the gluino-produced $\tilde{\chi}_{1}^{0}$ with a $\tilde{g}$ mass of 2000 $\text{Ge V}$ and $\tilde{\chi}_{1}^{0}$ mass of 100, 500, 1000, or 1700 $\text{Ge V}$ decaying into (left) $ee\nu$, (middle) $e\mu\nu$, and (right) $\mu\mu\nu$ final states. The colored bands represent the total uncertainty at one standard deviation.
• Figure 4: Systematic uncertainties in the signal DV selection efficiency extrapolated as a function of $c\tau$ for the gluino-produced $\tilde{\chi}_{1}^{0}$ with a $\tilde{g}$ mass of 2000 $\text{Ge V}$ and $\tilde{\chi}_{1}^{0}$ mass of 1000 $\text{Ge V}$ decaying into (left) $ee\nu$, (middle) $e\mu\nu$, and (right) $\mu\mu\nu$ final states.
• Figure 5: Observed and expected limits on the product of cross-section and ${B}^{2}$ as a function of $c\tau$ for models with an $S$ mass of 1000 $\text{Ge V}$ and $Z'$ masses between 200 and 450 $\text{Ge V}$ decaying into (left) $e^+e^-$, (middle) $e^{\pm}\mu^{\mp}$, and (right) $\mu^{+}\mu^{-}$ final states. The branching ratio $B$ is assumed to be 100% to the given final state for both $Z'$.