Abstract
Results are presented from the measurement by ATLAS of long-range (|Delta eta |>2) dihadron angular correlations in sqrt{s}=8 and 13 TeV pp collisions containing a Z boson. The analysis is performed using 19.4 mathrm {fb}^{-1} of sqrt{s}=8 TeV data recorded during Run 1 of the LHC and 36.1 mathrm {fb}^{-1} of sqrt{s}=13 TeV data recorded during Run 2. Two-particle correlation functions are measured as a function of relative azimuthal angle over the relative pseudorapidity range 2<|Delta eta |<5 for different intervals of charged-particle multiplicity and transverse momentum. The measurements are corrected for the presence of background charged particles generated by collisions that occur during one passage of two colliding proton bunches in the LHC. Contributions to the two-particle correlation functions from hard processes are removed using a template-fitting procedure. Sinusoidal modulation in the correlation functions is observed and quantified by the second Fourier coefficient of the correlation function, v_{2,2}, which in turn is used to obtain the single-particle anisotropy coefficient v_{2}. The v_{2} values in the Z-tagged events, integrated over 0.5<p_{mathrm {T}}<5 GeV, are found to be independent of multiplicity and sqrt{s}, and consistent within uncertainties with previous measurements in inclusive pp collisions. As a function of charged-particle p_{mathrm {T}}, the Z-tagged and inclusive v_{2} values are consistent within uncertainties for p_{mathrm {T}}<3 GeV.
Highlights
The modulation of the single-particle azimuthal angle distributions is typically characterized using a set of Fourier coefficients vn, called flow harmonics, that describe the relative amplitudes of the sinusoidal components of the single-particle distributions: dN dφ 1 + 2 vn cos n=1 n(φ − Φn) where the vn and Φn denote the magnitude and orientation of the nth-order single-particle anisotropies
This transfer of the spatial anisotropies in the initial collision geometry to anisotropies in the final particle distributions is well described by relativistic hydrodynamics [12,13,14]
The small residual deviations from this linear fit are taken into account while estimating systematic uncertainties; they are primarily present in the regions of (μ, zvtx) that are not used in the analysis
Summary
Recent studies by the ATLAS Collaboration demonstrate that these long-range correlations are consistent with the presence of a cosine modulation of the single-particle azimuthal angle distributions [2, 3], similar to that seen in Pb+Pb [4,5,6,7] and p+Pb collisions [3, 8,9,10,11]. As in previous ATLAS analyses of long-range correlations in p+Pb and pp collisions [2, 3, 10, 11], the measured charged-particle multiplicity, uncorrected for detector efficiency, is used to quantify the event activity.
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