Two- and Multi-particle Azimuthal Correlations in Small Collision Systems with the ATLAS Detector

ATLAS measurements of two-particle correlations in $\Delta\phi$ and $\Delta\eta$ are presented for $pp$ collisions at 2.76, 5.02 and 13~TeV, and for $p$+Pb collisions at 5.02 TeV. A template fitting procedure is used to subtract the dijet contribution and to extract the genuine long-range ridge correlations. This template procedure was previously used for 2.76 TeV and 13 TeV pp collisions, but is now extended to pp and $p$+Pb collisions at 5.02 TeV. In all collision systems, the ridge correlations are shown to be present even in events with a low multiplicity of produced particles, implying that the long-range correlations are not unique to rare high-multiplicity events. The properties of the correlation are shown to exhibit only a weak energy dependence and are remarkably similar to that observed in $p$+Pb collisions. Another new aspect of this talk is a detailed study of ridge properties in collisions containing hard processes, characterized by large four-momentum transfer. This may help answering the question whether the ridge arises from hard or semi-hard processes, or if it is the result of mechanisms unrelated to the initial hardness scale. ATLAS measurements of multi-particle azimuthal correlations for produced charged particles in small collision systems (5.02 and 13 TeV $pp$, 5.02 TeV $p$+Pb and low-multiplicity 2.76 TeV Pb+Pb collisions) are also presented. The correlations are expressed in terms of cumulants $c_n$, which can be directly related to Fourier harmonics $v_n$. A comparison across different collision systems is presented as a function of the charged particle multiplicity $N_{\mathrm{ch}}$. These measurements aim to assess the collective nature of multi-particle production. While collectivity is well established in $p$+Pb and Pb+Pb collisions, its evidence in $pp$ collisions is still a matter of debate. The presented measurements of multi-particle cumulants $c_2 \{2–8\}$ confirm the evidence for collective phenomena in $p$+Pb and low-multiplicity PbPb collisions. For $pp$ collisions the same conclusion can be derived from two-particle cumulants calculated with the requirement of a large pseudorapidity separation, $|\Delta\eta|>2$. However, the measurements of $c_\{4\}$ cumulants with a method that is not susceptible to event-by-event multiplicity fluctuations, but is biased by higher order non-flow correlations, do not yet provide clear evidence for collectivity. A modified cumulant method is used to suppress both the contribution of multiplicity fluctuation and non-flow effects. The results from this method are presented for pp and $p$+Pb collisions.