Measurements of long-range two-particle correlation over a wide pseudorapidity range in p$-$Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV

Correlations in azimuthal angle extending over a long range in pseudorapidity between particles, usually called the "ridge" phenomenon, were discovered in heavy-ion collisions, and later found in pp and p$-$Pb collisions. In large systems, they are thought to arise from the expansion (collective flow) of the produced particles. Extending these measurements over a wider range in pseudorapidity and final-state particle multiplicity is important to understand better the origin of these long-range correlations in small-collision systems. In this Letter, measurements of the long-range correlations in p$-$Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV are extended to a pseudorapidity gap of $\Delta\eta \sim 8$ between particles using the ALICE, forward multiplicity detectors. After suppressing non-flow correlations, e.g., from jet and resonance decays, the ridge structure is observed to persist up to a very large gap of $\Delta\eta \sim 8$ for the first time in p$-$Pb collisions. This shows that the collective flow-like correlations extend over an extensive pseudorapidity range also in small-collision systems such as p$-$Pb collisions. The pseudorapidity dependence of the second-order anisotropic flow coefficient, v2({\eta}), is extracted from the long-range correlations. The $v_{2}(\eta)$ results are presented for a wide pseudorapidity range of $-3.1 <~ \eta <~ 4.8$ in various centrality classes in p$-$Pb collisions. To gain a comprehensive understanding of the source of anisotropic flow in small-collision systems, the $v_{2}(\eta)$ measurements are compared to hydrodynamic and transport model calculations. The comparison suggests that the final-state interactions play a dominant role in developing the anisotropic flow in small-collision systems.

 

JHEP 01 (2024) 199
e-Print: arXiv:2308.16590 | PDF | inSPIRE
CERN-EP-2023-196
Figure group

Figure 1

The associated yield per trigger as a function of $\Delta\eta$ and $\Delta\varphi$ as measured for \TPC--\FMDab (left), \TPC--\FMDc (central), and \FMDab--\FMDc (right) correlations in the 0--5\% (top) and 60--100\% (bottom) \pPb collisions.

Figure 2

Projection of the correlation function of \TPC--\FMDab (left), \TPC--\FMDc (central), and \FMDab--\FMDc (right) correlations in 0--5\% \pPb collisions with the template fit using Eq. \eqref{eqtemplate}. The lines correspond to the various components of the fit; see text for details.

Figure 3

$p_{\mathrm T}$-integrated $v_{2}\{2\}$ as a function of $\eta$ in various centrality classes using the template fitting method. Boxes show the total systematic uncertainties.

Figure 4

\vtwo as a function of charged-particle pseudorapidity density for five different pseudorapidity regions.

Figure 5

The \Veta in central \pPb collisions compared with \Veta in peripheral \PbPb collisions with a compatible mean charged-particle multiplicity in Pb-going direction. The \vtwo \PbPb results were obtained using the Q-cumulant method .

Figure 6

Pseudorapidity dependence of \pt integrated \vtwo. Comparison of the measured data (black circles) with a calculation by the hydrodynamical model (blue band)  for the 0--5\% (top left), 5--10\% (top right), 10--20\% (bottom left), and 20--40\% (bottom right) centrality classes.

Figure 7

Pseudorapidity dependence of \pt integrated \vtwo. Comparison of the measured data (black circles) with a calculation by AMPT with the string-melting configuration (blue band) for 0--5\% (top left), 5--10\% (top right), 10--20\% (bottom left), and 20--40\% (bottom right) centrality class.

Figure A.1

$p_{\mathrm T}$-integrated $v_{2}\{2\}$ as a function of $\eta$ in various centrality classes using the improved-template-fit method, and the peripheral subtraction method.

Figure A.2

\vtwo as a function of charged particle density for five different pseudorapidity regions with the peripheral subtraction (left) and the improved template method (right).

Figure A.3

Pseudorapidity dependence of \pt-integrated \vtwo as measured in peripheral Pb--Pb collisions and central p--Pb collisions The Pb–Pb results were obtained with the Q-cumulant method . The p--Pb results were obtained with the improved-template-fit and peripheral subtraction methods.

Figure A.4

Pseudorapidity dependence of \pt-integrated \vtwo as measured in different p--Pb centrality classes and as obtained from the AMPT calculation with the string melting configuration. The \vtwo were extracted using the improved-template-fit method and the peripheral subtraction.