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The Large Hadron Collider (LHC) of the European Organisation for NuclearResearch (CERN) started its operation in Autumn of 2009. The initial run at acentre-of-mass energy of 900 GeV, has been followed by the on-going run at theenergy of 7 TeV. While initially the probability of several proton-protoncollisions to happen within the same bunch-crossing was approximately 10^-5,the level of the pile-up grows steadily with better focusing and squeezing ofthe LHC beams at collision point. Presented in this contribution is theperformance of the primary vertex reconstruction algorithms used for analysisof the first collisions at the LHC. Different approaches used for thereconstruction of primary vertices in 900 GeV and 7 TeV collisions arepresented. The efficiencies of the primary vertex reconstruction used for thefirst physics analyses of ATLAS are shown. The resolutions on positions of thereconstructed primary vertices are investigated by studying the distributionsof pulls of distance between artificially created half-vertices. Implicationsof the ATLAS performancewith respect to primary vertex reconstruction for theon-going and future physics analyses are discussed.

Reconstruction of primary vertices in $pp$ collisions at energies of 900 GeV and 7 TeV with the ATLAS detector