Abstract Several models of elementary particle physics beyond the Standard Model, predict the existence of neutral particles that can decay in jets of leptons and light hadrons named Lepton-Jets (LJ). The results on this kind of jets search with the ATLAS experiment at the proton-proton LHC collider at s=8 TeV during 2012 data taking (20 fb-1) is presented. No excess of events have been observed over the expected background and the exclusion limits for two different models, that predict the Higgs boson to decay in Lepton-Jets, have been computed. A similar analysis will be performed with the Run II data taking (s=13 TeV) including other signatures. Abstract Several models of elementary particle physics beyond the Standard Model, predict the existence of neutral particles that can decay in jets of leptons and light hadrons named Lepton-Jets (LJ). The results on this kind of jets search with the ATLAS experiment at the proton-proton LHC collider at s=8 TeV during 2012 data taking (20 fb-1) is presented. No excess of events have been observed over the expected background and the exclusion limits for two different models, that predict the Higgs boson to decay in Lepton-Jets, have been computed. A similar analysis will be performed with the Run II data taking (s=13 TeV) including other signatures. ATLAS detector and particle ID for LJ Lepton-Jet production mechanism Displaced LJ definition Displaced LJ selection Inner detector (ID): Si detectors and Transition Radiation Tracker Sampling-based calorimetry 2T superconductive solenoid surrounding the ID Muon Spectrometer: one barrel and 2 end-cap 4 T air-core toroid Muon identification MS track vetoed by ID Jet identification Calibrated calorimeter clusters with low EM energy fraction The portal to hidden sector can be Higgs or super-symmetric particles Lepton-Jet Dark-photon lifetime depends on the size of the kinetic-mixing : small values correspond to long lifetime and then displaced decay vertex. The BR of the dark-photon to leptons depends on its mass. If the produced hidden particles have mass O(1GeV) (e.g. the dark-photon) they can decay back to SM to collimated pair of leptons (LJ) Excess of positron flux in cosmic rays (not anti- proton) if DM annihilates to a hidden sector it would produce leptons Some motivations (g-2) anomaly. Comparing theory to experiment there is a 3.2 discrepancy. The anomaly can be explained including corrections from an hidden photon The model independent search strategy for non- prompt LJs starts from an experimental signature: N neutral light dark photons, highly isolated in ID (small ), in a narrow angular cone (R) decaying far from the primary vertex to pairs of electrons/muons/pions. 2 good muons clustered in a cone opening R=0.5 and NO jets in the cone 2 good muons + good jets clustered in a cone opening R=0.5 good jets with low EM fraction and narrow width and NO jmuons in a cone opening R=0.5 A Lepton-Jet gun MC generator has been developed to optimize search criteria and to produce detection efficiency curves muon trigger requires at least 3 standalone muons with pT> 6GeV (it selects displaced LJ with muonic content) calorimeter trigger requires a 35 GeV scale) jet with EM fraction less than 0.07 (it selects displaced LJ containing only electrons/pions into HCAL) Triggers for non prompt LJ (unprescaled during 2012 data taking) Cut flow for event selection Discriminating variables Only two reconstructed LJs: || between the two LJ of the event and max(pT) Backgrounds QCD multi-jet is reduced using isolation in the ID (less dependent to pile-up) and jet EM fraction/width cuts. The isolation variable is the sum of the pT of the ID tracks associated to the PV in a cone R=0.5 around the LJ line of flight (pT tracks > 0.5 GeV). For the remain QCD di- jet contribution the data-driven matrix method has been used. As uncorrelated variables the || and max(pT) are used. The signal region is defined as ||>1rad and max(pT) 2 + X d Higgs -> 4 + X d 95% CL upper limits for FRVZ models The used statistical method is the likelihood-based ABCD: simultaneous fit for signal and multi-jet bkg in both signal and control regions; other bkg and systematics included via nuisance parameters Upper limits are given as function of lifetime. Yields (c) obtained by pseudo experiments using detector efficiencies function of the decay length of the d and pT distributions of the models. d xBR(H-> s) assuming SM gg->H cross section value 1.2 pb d All LJ pairs TYPESTYPE2-TYPE2 removed Results and plan for the future More details: [arXiv: ] JHEP 11 (2014) 088 and ATLCOMPHYS2015248 Way to unify the LJ limits from various experiments and from different searches in the same experiment Dark photon exclusion plot for H->2d + X Plan for the future Repeat the LJ search on the 13 TeV Run II data including new triggers, new TYPEs with EM clusters, the RPC timing for CR rejection, the LJ vertexing and the multivariate statistical analysis. Efficiency tables for reconstruction in ATLAS d Within 20% discrepancy is it possible to predict the number of expected reconstructed events for different models and lifetime, starting from a single MC with a specific c. This table refers to TYPE2 in the barrel region calculated from the LJ gun MC. d