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

The Large Hadron Collider accelerates and collides both proton beams and lead beams at the highest energies available in the 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 of reconstruction of the charged particle multiplicity and angular distribution were developed. These methods range from the full reconstruction of particles and their momenta to an estimate of the number of charged particles based on counting hits registered in a single layer of the Pixel Detector. First results obtained from experimental data for p+p interactions and performance studies for heavy ion collisions based on Monte Carlo simulations are discussed.