Charged particle multiplicity and correlations in heavy ion collisions in the ATLAS experiment

Since the start of Large Hadron Collider (LHC) the ATLAS experiment has collected data from $p+p$, Pb+Pb and recently $p$+Pb collisions. In this paper two topics from the heavy ion programme are presented. The first of them is the multiplicity of charged particles in Pb+Pb collisions studied as a function of centrality. In such collisions very high pseudorapidity particle density is obtained, reaching 1738 $\pm 76$ at $\eta \approx0$. For the $p$+Pb collisions the latest analysis of two-particle correlations is presented. The "ridge" observed in pp events with large multiplicities is present also in central $p$+Pb collisions selected by large $\Sigma E_{T}$ measured at the Pb fragmentation side. In the azimuthal correlation for $2<|\Delta \eta|< 5$ two components can be extracted. Assuming that in the peripheral collisions only the correlation due to momentum conservation effects are present, in the central collisions additional quadruple modulation component can be identified.