ATLAS strategy for primary vertex reconstruction during Run-II of the LHC

Based on experience gained from run-I of the LHC, the ATLAS vertex reconstruction group has developed a refined primary vertex reconstruction strategy for run-II. With instantaneous luminosity exceeding 10^34 cm-2 s-1, an average of 40 to 50 pp collisions per bunch crossing are expected. Together with the increase of the center-of-mass collision energy from 8 TeV to 13 TeV, this will create a challenging environment for primary vertex pattern recognition. This contribution explains the ATLAS strategy for primary vertex reconstruction in high pile-up conditions. The new approach is based on vertex seeding with a medical-imaging algorithm, adaptive reconstruction of vertex positions, and iterative recombination of occasional split vertices. The mathematical foundation and software implementation of the method are described in detail. Monte Carlo-based estimates of vertex reconstruction performance for LHC run-II are presented.