Measurement with the ATLAS detector of multi-particle azimuthal correlations in p+Pb collisions at sqrt(s_NN)=5.02 TeV

TL;DR

This study addresses whether ridge-like, long-range correlations in $p$+$Pb$ collisions arise from initial-state effects or final-state collective behavior. It leverages the ATLAS detector to measure the second-order azimuthal anisotropy $v_2$ using two- and four-particle cumulants, with event activity categorized by forward $\Sigma E_T^{\mathrm{Pb}}$ and a detailed track selection in $|\eta|<2.5$; a flow-vector based method yields both reference and differential $v_2$ values, with extensive corrections for nonuniform acceptance. The results show a significant $v_2$ from multi-particle cumulants, with $v_2\{4\}$ aligned with hydrodynamic predictions in the accessible $\Sigma E_T^{\mathrm{Pb}}$ range, supporting final-state collectivity even in the small $p$+$Pb$ system. This finding strengthens the interpretation that ridge-like correlations in such collisions reflect collective flow phenomena rather than purely initial-state effects, providing important constraints for theoretical models of small-system QCD dynamics.

Abstract

In order to study further the long-range correlations ("ridge") observed recently in p+Pb collisions at sqrt(s_NN) =5.02 TeV, the second-order azimuthal anisotropy parameter of charged particles, v_2, has been measured with the cumulant method using the ATLAS detector at the LHC. In a data sample corresponding to an integrated luminosity of approximately 1 microb^(-1), the parameter v_2 has been obtained using two- and four-particle cumulants over the pseudorapidity range |eta|<2.5. The results are presented as a function of transverse momentum and the event activity, defined in terms of the transverse energy summed over 3.1<eta<4.9 in the direction of the Pb beam. They show features characteristic of collective anisotropic flow, similar to that observed in Pb+Pb collisions. A comparison is made to results obtained using two-particle correlation methods, and to predictions from hydrodynamic models of p+Pb collisions. Despite the small transverse spatial extent of the p+Pb collision system, the large magnitude of v_2 and its similarity to hydrodynamic predictions provide additional evidence for the importance of final-state effects in p+Pb reactions.

Figures (4)

• Figure 1: Upper plot: the $\Sigma E^{\mathrm{Pb}}_{\mathrm{T}}$ distribution with the six activity intervals indicated. Lower plot: the distribution of $N_{\mathrm{ch}}^{\mathrm{rec}}$ for each activity interval. The leftmost distribution corresponds to the interval with the lowest $\Sigma E^{\mathrm{Pb}}_{\mathrm{T}}$, etc.
• Figure 2: The two-particle (upper plot) and four-particle (lower plot) cumulants calculated using the reference flow particles as a function of $\Sigma E^{\mathrm{Pb}}_{\mathrm{T}}$ for data (circles), the fully simulated HIJING events (open squares) and the large generator-level HIJING sample (filled squares). For clarity, the points for the fully simulated (generated) HIJING events are slightly shifted to the left (right).
• Figure 3: The second-order harmonic calculated with the two-particle (circles) and four-particle (stars) cumulants as a function of transverse momentum in four different activity intervals. Bars denote statistical errors; systematic uncertainties are shown as shaded bands. The $v_2$ derived from the Fourier decomposition of two-particle correlations ridge_atlas is shown by squares.
• Figure 4: The second-order harmonic, $v_2$, integrated over $\pT$ and $\eta$, calculated with two- and four-particle cumulants (circles and stars, respectively), as a function of $\Sigma E^{\mathrm{Pb}}_{\mathrm{T}}$. Systematic uncertainties are shown as shaded bands. Also shown is $v_2\{2PC\}$ (squares) and predictions from the hydrodynamic model bozek2 (triangles) for the same selection of charged particles as in the data.