First observation of the J/ψ ->μ+μ- in the ATLAS experiment at LHC

The high cross section value for the J/ψ production and the wide transverse momentum spectrum accessible to ATLAS give to these events a central role in the calibration of the muon spectrometer and to the analysis software tools. The first data collected during the first part of the 2010 from √s = 7 TeV proton-proton collisions show a clear signal characterized by the presence of two muons in the final state. The reconstructed invariant mass agrees with the PDG value within statistical uncertainties, and the peak width is compatible with Monte Carlo expectations.