Speaker
Description
Measurements of correlations between two particles separated in pseudorapidity and azimuthal angles 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 in principle by requiring the presence of a hard-scattering process in the collision. This talk presents the first measurement of two-particle correlations in $pp$ collisions with a Z boson identified via its dimuon decay channel. The analysis uses ATLAS data recorded with nominal $pp$ luminosity, with high pileup, and is analyzed using a new procedure developed to correct for the contribution of tracks arising from pileup vertices. The multiplicity and transverse momentum dependence of the inclusive charged-particle $v_2$ measured in Z-tagged events at $\sqrt{s}$ = 8 and 13 TeV is compared to the $v_2$ measured in minimum-bias collisions. They are found to be of a similar magnitude to each other and to that measured in typical events.
