Search for displaced vertices of oppositely charged leptons from decays of long-lived particles in pp collisions at s=13 TeV with the ATLAS detector

A search for long-lived particles decaying into an oppositely charged lepton pair, μμ, ee, or eμ, is presented using 32.8fb−1of ppcollision data collected at √s=13 TeV by the ATLAS detector at the LHC. Candidate leptons are required to form a vertex, within the inner tracking volume of ATLAS, displaced from the primary ppinteraction region. No lepton pairs with an invariant mass greater than 12 GeV are observed, consistent with the background expectations derived from data. The detection efficiencies for generic resonances with lifetimes (cτ) of 100–1000 mm decaying into a dilepton pair with masses between 0.1–1.0 TeV are presented as a function of pTand decay radius of the resonances to allow the extraction of upper limits on the cross sections for theoretical models. The result is also interpreted in a supersymmetric model in which the lightest neutralino, produced via squark–antisquark production, decays into +′−ν(, ′=e, μ) with a finite lifetime due to the presence of R-parity violating couplings. Cross-section limits are presented for specific squark and neutralino masses. For a 700 GeV squark, neutralinos with masses of 50–500 GeV and mean proper lifetimes corresponding to cτvalues between 1mm to 6m are excluded. For a 1.6 TeV squark, cτvalues between 3mm to 1m are excluded for 1.3 TeV neutralinos.©2019 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP3.1. IntroductionMany extensions to the Standard Model (SM) predict the pro-duction of weakly-coupled, long-lived particles (LLPs). In particu-lar, several models, including supersymmetry (SUSY) [1–6]with R-parity violation (RPV) [7,8]or with gauge-mediated supersym-metry breaking (GMSB) [9–11], Hidden Valley models [12], dark-photon models [13]or models with long-lived right-handed neu-trinos [14], predict the existence of LLPs that can decay into a pair of leptons. If the LLP has a lifetime of picoseconds to nanoseconds then its decay may be observed as a displaced vertex in the inner tracking volume of the ATLAS detector at the LHC.This letter presents a search for displaced dilepton vertices orig-inating from decays of LLPs into an oppositely charged μμ, ee, or eμpair, with an invariant mass of more than 12 GeV. The analysis uses data from proton–proton (pp) collisions recorded by the AT-LAS experiment in 2016 at a center-of-mass energy of 13 TeV. Two signal models are used to study the sensitivity of the ATLAS detec-tor to such LLPs. The first is a simplified RPV SUSY model in which a squark–antisquark pair is produced, each decaying into a long-E-mail address:atlas .publications @cern .ch .lived neutralino which results in a pair of charged leptons and a neutrino. The second is a toy model where a LLP, denoted by Z′, is produced in q ̄qannihilations and decays into a pair of charged leptons. These models were selected to study how the kinematic properties of a three- or two-body decay affect the signal effi-ciencies. The search is intended to be as model-independent as possible and is not optimized for these particular signal models.Previously, the ATLAS Collaboration searched for displaced dilepton vertices in the inner tracking volume of the ATLAS de-tector at √s=8TeV[15] and for oppositely charged dimuons using only muon-spectrometer tracks at √s=13 TeV [16]. ATLAS also searched for LLPs with mass of less than 2GeV at √s=8TeV by considering pairs of highly collimated leptons [17]. The CMS Collaboration searched for displaced dilepton vertices in the in-ner tracking volume of the CMS detector at √s=8TeV[18]and for electrons and muons with large impact parameters at √s=8TeV[19].2. ATLAS detectorThe ATLAS experiment [20– 22]at the LHC is a multipurpose particle detector with a cylindrical geometry and a near 4πcov