ATLAS measurements of J/ψ events at s=7TeV

The first observation of the J/ψ→μ+μ− resonance 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 background estimated from Monte Carlo, or reconstructed same sign muon pairs, matches well with observations. The muon trigger and reconstruction efficiencies in this very low transverse momentum region are studied using the J/ψ candidates.