Long-range angular correlations of $\rm π$, K and p in p-Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV

Abstract: Angular correlations between unidentified charged trigger particles and various species of charged associated particles (unidentified particles, pions, kaons, protons and antiprotons) are measured by the ALICE detector in p-Pb collisions at a nucleon-nucleon centre-of-mass energy of 5.02 TeV in the transverse-momentum range $0.3 < p_{\rm T} < 4$ GeV/$c$. The correlations expressed as associated yield per trigger particle are obtained in the pseudorapidity range $|\eta_{\rm lab}|<0.8$. Fourier coefficients are extracted from the long-range correlations projected onto the azimuthal angle difference and studied as a function of $p_{\rm T}$ and in intervals of event multiplicity. In high-multiplicity events, the second-order coefficient for protons, $v_2^p$, is observed to be smaller than that for pions, $v_2^\pi$, up to about $p_{\rm T} = 2$ GeV/$c$. To reduce correlations due to jets, the per-trigger yield measured in low-multiplicity events is subtracted from that in high-multiplicity events. A two-ridge structure is obtained for all particle species. The Fourier decomposition of this structure shows that the second-order coefficients for pions and kaons are similar. The $v_2^p$ is found to be smaller at low $p_{\rm T}$ and larger at higher $p_{\rm T}$ than $v_2^pi$, with a crossing occurring at about 2 GeV. This is qualitatively similar to the elliptic-flow pattern observed in heavy-ion collisions. A mass ordering effect at low transverse momenta is consistent with expectations from hydrodynamic model calculations assuming a collectively expanding system.

• The paper reveals that long-range angular correlations exhibit ridge-like structures indicative of collective flow in p–Pb collisions.
• It employs two-particle correlation and scalar-product methods to extract Fourier coefficients quantifying anisotropic particle flow.
• Results show mass ordering of v2 coefficients, challenging existing models and refining our understanding of QCD matter under extreme conditions.

Analysis of Long-range Angular Correlations of Identified Particles in p–Pb Collisions at 5.02 TeV

The study under discussion provides a comprehensive investigation into the angular correlations between unidentified charged trigger particles and various species of charged associated particles (notably pions, kaons, protons, and antiprotons) in p–Pb collisions at a nucleon–nucleon center-of-mass energy of 5.02 TeV. Conducted by the ALICE Collaboration, this research is crucial for understanding the underlying mechanisms of particle production in high-energy hadronic and nuclear collisions.

Key Findings and Analysis

The experiment utilizes the ALICE detector's capabilities to measure the correlations within a pseudorapidity range of |η| < 0.8 and a transverse momentum (pT) range from 0.3 to 4.0 GeV/c. These correlations are expressed in terms of the associated yield per trigger particle. By employing a Fourier decomposition of these long-range correlations projected onto azimuthal angle differences, the study quantifies the presence of ridge-like structures, which are indicative of collective behavior in the system.

The results highlight a mass ordering effect at low transverse momenta consistent with predictions from hydrodynamic models of a collectively expanding system. Specifically, it is observed that the v2 coefficient of protons (v2) is significantly smaller than that of pions (v2) up to about 2 GeV/c, after which the reverse occurs. This behavior is qualitatively similar to the elliptic flow patterns observed in larger A–A collisions, suggesting the emergence of collective phenomena in smaller systems like p–Pb, which typically are not expected to manifest such characteristics.

Methodological Approaches

To isolate the effects not contributed by jets and to enhance signal observability, the authors deployed an extraction method where the per-trigger yield measured in low-multiplicity events was subtracted from that in high-multiplicity events. By doing so, a two-ridge structure was discerned for all particle species, and the Fourier analyses yielded key insights into the second-order coefficients that reflect the anisotropic nature of the observed particle flows.

The analysis employed two main methods: two-particle correlations and the scalar-product method. Both methods yielded consistent results across various multiplicity classes and particle species, except at very low transverse momenta and in low-multiplicity events where variances were noted. This consistency underscores the robustness of the correlations extracted as a measure of collective behavior in these collision systems.

Implications and Future Directions

The findings of this study have profound implications for the understanding of QCD matter under extreme conditions. The observed mass ordering and crossing behavior of the flow coefficients v2 suggest a degree of collective behavior in p–Pb collisions akin to that in larger systems like Pb–Pb, raising important questions about the conditions required for such collective phenomena to manifest.

From a theoretical standpoint, these results challenge existing paradigms concerning the scale at which collective behavior can arise and push for more refined models that can account for these observations in smaller collision systems. Practically, these insights compel future experimental endeavors to focus on a broader scope of collision energies and systems to map out the parameter space within which such collective phenomena can arise.

In conclusion, by meticulously analyzing the angular correlations in p–Pb collisions at a significant center-of-mass energy, this study provides essential insights into the possible collective behavior of hadronic matter in small-scale systems. These findings open new avenues for both experimental and theoretical research in the field of high-energy nuclear physics, particularly within the context of understanding the continuum between distinct nuclear collision regimes.