ATLAS Experiment Status and First Results

A Toroidal LHC ApparatuS (ATLAS) is a multi-purpose experiment that aims to explore the energy frontier of particle physics. It is made of successive detector layers, starting from the interaction points, of inner tracking detectors immersed in a solenoidal magnetic field, liquid argon electromagnetic calorimetry, hadronic calorimetry and muon detectors immersed in a toroidal magnetic field. The first collision run of the LHC took place in December of 2009 at center-of-mass energies of 900 GeV and 2.36 TeV. We assess the detector performance during run and present the first ATLAS physics measurement: the charged particle multiplicity in 900 GeV $pp$ collisions