J/ψ Performance of the ATLAS Inner Detector

A performance study of J/ψ → μ+μ− decays is presented from 7 TeV proton-proton collisions using the ATLAS detector at the LHC with an integrated luminosity of 78 nb−1. The J/ψ mass was extracted from the reconstructed di-muon invariant mass spectrum using track parameters determined from the ATLAS Inner Detector. The agreement with the PDG value of the reconstructed J/ψ mass is within statistical uncertainties and provides a validation of the momentum scale determination in the low mass region. The mass resolution is compatible with Monte Carlo expectations. The kinematic properties of the J/ψ candidates are presented and compared with Monte Carlo predictions. The reconstructed J/ψ mass and mass resolution are compared in different detector regions.