Small System Collectivity in Relativistic Hadron and Nuclear Collisions (1801.03477v2)

Abstract: The bulk motion of nuclear matter at the ultra-high temperatures created in heavy-ion collisions at the Relativistic Heavy Ion Collider and the Large Hadron Collider is well described in terms of nearly inviscid hydrodynamics, thereby establishing this system of quarks and gluons as the most perfect fluid in nature. A revolution in the field is underway, spearheaded by the discovery of similar collective, fluid-like phenomena in much smaller systems including p+p, p+A, d+Au, and $^3$He$+$Au collisions. We review these exciting new observations and their implications.

• The paper demonstrates that fluid-like collective phenomena, typically seen in large A+A collisions, also manifest in small p+p and p+A systems.
• It details how nearly inviscid hydrodynamic models, supported by multiparticle cumulants and mass-ordering observations, describe the observed flow patterns.
• The analysis challenges traditional views by showing that initial state geometry and fluctuations can drive collective behavior even in minimal hadronic systems.

Small System Collectivity in Relativistic Hadronic and Nuclear Collisions

The paper "Small System Collectivity in Relativistic Hadronic and Nuclear Collisions" by Nagle and Zajc provides a comprehensive overview of the burgeoning interest in collective phenomena observed in high-energy collisions involving small systems. The authors explore the surprising collective behavior characteristic of quark-gluon plasma (QGP) formation, historically associated with large-scale nucleus-nucleus (A+A) collisions, seen in much smaller systems like proton-nucleus (p+A) and even proton-proton (p+p) interactions. This phenomenon challenges the existing frameworks and expands our understanding of the hydrodynamics applicability in nuclear physics.

Summary of Key Concepts

1. Hydrodynamic Descriptions: Traditionally, QGP in heavy ion collisions is described using nearly inviscid hydrodynamics. This model has successfully depicted the QGP as a near-perfect fluid with minimal viscosity. However, the extension of such descriptions to small systems was initially unexpected, given the presumption of inadequate thermalization due to smaller system sizes.
2. Observations in Small Systems: The discovery of fluid-like collective behavior in smaller systems has led to significant advancements in the field. Evidence points toward the same hydrodynamic principles being applicable in small systems, with experimental data revealing long-range azimuthal correlations and elliptic flow coefficients similar to those observed in large nuclei collisions.
3. Experimental Insights: The authors highlight several experimental measurements that validate the extension of hydrodynamic models to small systems. These include the observation of multiparticle cumulants, mass-ordering in flow patterns, and higher moments of flow harmonics. Such metrics implicate a strong undercurrent of collective behavior likely influenced by initial state geometry.
4. Hydrodynamic Regime and Initial Conditions: The paper discusses the role of initial state fluctuations and the importance of initial geometry in influencing the final state momentum distributions. In small systems, sub-nucleonic degrees of freedom become significant in modeling the initial conditions due to the reduced size scale.
5. Challenges and Alternatives: Contradictory observations, such as the lack of jet quenching in small systems (a hallmark of QGP in large systems), invite scrutiny of alternative explanations like parton scatter models and initial-state momentum correlations. These alternatives provide a contrasting lens through which the data can be interpreted.

Implications and Future Directions

The findings provoke reconsideration of the minima required for QGP-like behavior, suggesting that even systems as small as hadronic clusters can exhibit collectivity under certain conditions. This results in broader implications for the theoretical understanding and model-dependent estimates of transport coefficients like shear viscosity (η/s). Furthermore, discriminating between hydrodynamic scaling and other theoretical models remains a pivotal question.

Future research is likely to focus on refining initial state models and further testing the limits of hydrodynamics in terms of system size and collision energy. This work suggests an ongoing necessity to reconcile small system collectivity with theoretical models, offering potential cross-disciplinary insights relevant to other domains where strongly coupled systems are studied.

In conclusion, the extension of fluid-like descriptions to small-scale systems marks a significant shift in nuclear physics paradigms, urging deeper inspection of the transition from microscopic interactions to macroscopic flows and challenging the very boundaries of hydrodynamic applicability.