Observation and study of the J/ψ ->μ+μ- in the ATLAS experiment at LHC

The high cross section for the J/ψ production and the wide pT spectrum accessible to ATLAS give to these events a central role in the calibration of the muon spectrometer. The data collected during the first months of the 2010 from √s = 7 TeV p-p collisions show a clear signal with two muons in the final state. The reconstructed invariant mass agrees with the PDG value and the peak width is compatible with MC expectations. The double differential cross section is measured with respect to the pT and rapidity.