Methods of multiplicity reconstruction in heavy ion collisions in the ATLAS experiment

The Large Hadron Collider started to accelerate lead beams and collide them at the highest energy available in laboratory. One of the most important global characteristics of such collisions is the multiplicity of produced particles. The ATLAS detector registers passage of charged particles using the silicon detectors in about 5 pseudorapidity units. Several methods can be used to reconstruct the multiplicity and angular distribution of charged particles. These methods range from the full reconstruction of particles and their momenta to estimate of the number of charged particles from hits registered in a single layer of pixel detector. The studies of performance of the methods based on Monte Carlo simulations and first results obtained from experimental data are discussed.