First observation of the J/ψ → μ+μ− resonance in ATLAS pp collisions at √s = 7 TeV

The first J/ψ → μ+μ− observation using the ATLAS detector, in 7 TeV proton-proton collisions at the LHC, is reported for an integrated luminosity of approximately 6.4 nb−1. The reconstructed invariant mass agrees with the PDG value within statistical uncertainties, and the peak width is compatible with Monte Carlo expectations. The kinematic properties of the J/ψ candidates are presented and compared with Monte Carlo data. The muon trigger and reconstruction efficiencies in this very low transverse momentum region are studied using the J/ψ candidates.