Measurement of long-range correlations in $Z$-tagged $pp$ events with ATLAS

Measurements of azimuthal correlations between two particles separated in pseudorapidity have shown striking similarities between results obtained in $pp$, $p$+A and A+A collision systems. In $pp$ collisions, unlike in $p$+A and A+A, the strength of the correlations, quantified by the anisotropy parameter $v_2$, shows little dependence on the observed charged-particle multiplicity. Recent theoretical models suggest that this can result from an intrinsically weaker correlation between the charged-particle multiplicity and the impact parameter of the $pp$ collision. An independent handle on the impact parameter can be obtained by requiring the presence of a hard-scattering process in the collision. In this report the measurement of two-particle correlations in $pp$ collisions containing a $Z$ boson identified in dimuon decay channel is presented. The analysis uses ATLAS data recorded with nominal $pp$ luminosity, with a high pileup. A novel procedure was developed to correct the two-particle correlation function for the contribution of tracks arising from pileup . The multiplicity dependence of the charged-particle $v_2$ measured in $Z$-tagged events at the energy $\sqrt{s}$ = 8~TeV is compared to the $v_2$ measured in inclusive collisions. They are found to be of a similar magnitude.