Search for long-lived neutral particles decaying into lepton-jets with the ATLAS detector in proton-proton collision data at $\sqrt{s}=13$ TeV.

Del Gaudio M. per la Collaborazione ATLAS
  Lunedì 11/09   14:30 - 18:30   Aula A208   I - Fisica nucleare e subnucleare
Several models of elementary particle physics beyond the Standard Model predict the existence of neutral particles that can also be long lived and decay in collimated jets of light leptons and hadrons (lepton-jets, LJs). The search for LJs extends from the radius 12 cm up to the first trigger plane of the muon spectrometer, and includes pairs of quasi back-to-back LJs, mono-LJs and pairs of LJs produced in association with a vector boson. The present contribution describes the various strategies adopted to search for displaced lepton-jets and to reduce the background coming from beam-induced background and cosmic rays in proton-proton collision data sample recorded at $\sqrt{s}=13$ TeV during 2015 and 2016 data taking periods. To reduce the di-jet contamination of the signal sample the search is limited to non-$\sqrt{s}$ combined muons and jets with very low electromagnetic energy fraction. The selected events are then compared with the Standard Model expectations and with various BSM predictions.