Reconstruction of J/$\psi$ mesons in $t{\bar t}$ final states in proton-proton collisions at $\sqrt{s}=8$ TeV with the ATLAS detector

This note presents the reconstruction of $t{\bar t}$ events with final states containing a J/$\psi(\rightarrow\mu^+\mu^-)$ meson using the ATLAS detector. The analysis is performed with the ATLAS 2012 proton-proton collision data sample, collected at the LHC at $\sqrt{s}=8$ TeV and corresponding to an integrated luminosity of 20.3 $\mathrm{fb}^{-1}$. The yields are estimated from a fit of the $m(\mu^+\mu^-)$ mass distributions in the mass range [2.9;3.3] GeV. In the lepton+jets (dilepton) channel a total of 350 (34) J/$\psi$ candidates are found with 220 (24) background events. The kinematic properties of the J/$\psi$ candidates are studied and found in reasonable agreement, within uncertainties, with the Monte Carlo prediction as given by the POWHEG+PYTHIA6+EVTGEN generator. Modelling of the fragmentation and decay of heavy flavour hadrons is compared for two Monte Carlo generators: PYTHIA6 and HERWIG. The effects predicted by different generators (POWHEG and MC@NLO) are also explored. These results are expected to contribute to future constraints on the systematic uncertainties in top quark studies and to prepare alternative methods to measure the top quark mass.