Measurement of long-range azimuthal correlations in $Z$-boson tagged $pp$ collisions at $\sqrt{s}=8$~TeV

This analysis is the first to study long-range hadron correlations in $pp$ collisions with a constraint on collision geometry. The constraint is implemented by requiring events in which a $Z$ boson is produced, which is a high-Q$^{2}$ hard scattering process related to the impact parameter of the $pp$ collision. The analysis is performed using 19.4~$fb^{-1}$ of $\sqrt{s}=8$~TeV $pp$ data obtained by the ATLAS detector during the physics Run-1 of the LHC. The sample contains approximately $6.2\times10^{6}$ selected $Z$-boson candidate events. The correlations between the charged-particle pairs in relative azimuthal angle over the transverse momentum range from 0.5~GeV to 5~GeV are studied as a function of the charged particle multiplicity of the event. The average number of interactions per bunch crossing in the data sampleis about 20, therefore the number of charged particle tracks and the correlation functions are corrected to account for the significant pileup contribution present in the events. The correlations between particle pairs is quantified by the second Fourier coefficient, $v_{2,2}$, which in turn is used to obtain the single-particle anisotropy coefficient $v_2$. The $v_2$ coefficient in the $Z$-tagged events is found to be independent of multiplicity as previously observed in inclusive $pp$ events, and its magnitude is found to be $8\pm6$\% larger than that in inclusive $pp$ events.