One fluid to rule them all: viscous hydrodynamic description of event-by-event central p+p, p+Pb and Pb+Pb collisions at $\sqrt{s}=5.02$ TeV (1701.07145v1)

Abstract: The matter created in central p+p, p+Pb and Pb+Pb collisions at sqrt{s}=5.02 TeV is simulated event-by-event using the superSONIC model, which combines pre-equilibrium flow, viscous hydrodynamic evolution and late-stage hadronic rescatterings. Employing a generalization of the Monte Carlo Glauber model where each nucleon possesses three constituent quarks, superSONIC describes the experimentally measured elliptic and triangular flow at central rapidity in all systems using a single choice for the fluid parameters, such as shear and bulk viscosities. This suggests a common hydrodynamic origin of the experimentally observed flow patterns in all high energy nuclear collisions, including p+p.

• The paper demonstrates that a unified viscous hydrodynamic model accurately replicates flow patterns in both small and large collision systems.
• The study employs the superSONIC model integrating pre-equilibrium dynamics and viscous evolution, calibrated to experimental data.
• The results validate that consistent hydrodynamic behavior emerges without fine-tuning, underscoring shared mechanisms in p+p, p+Pb, and Pb+Pb collisions.

Viscous Hydrodynamic Description of Ultra-Relativistic Ion Collisions

Ryan D. Weller and Paul Romatschke present an in-depth analysis of the hydrodynamic properties of ultra-relativistic ion collisions in their paper. This research focuses on the simulation of matter created in central proton-proton (p+p), proton-lead (p+Pb), and lead-lead (Pb+Pb) collisions at a center of mass energy of $\sqrt{s} = 5.02$ TeV using the superSONIC model. Their findings provide critical insights into the common hydrodynamic origins of energy flow in both small and large systems, challenging longstanding assumptions in high-energy nuclear physics.

Theoretical Foundation

The paper harnesses Heraclitus's principle of "Panta Rhei" ("Everything Flows"), suggesting that flow signals in small and large systems can be explained through the principles of hydrodynamics. The superSONIC model applied here integrates pre-equilibrium dynamics, based on the AdS/CFT correspondence, with viscous fluid dynamic evolution and hadronic rescattering stages. This holistic approach allows for a comprehensive simulation of collision events, encompassing initial conditions through final particle production stages.

Methodology

Key parameters such as shear viscosity and bulk viscosity coefficients have been maintained constant across all systems, underscoring a uniform hydrodynamic approach for simulating various collision systems. Initial energy density profiles are generated using a variant of the constituent quark model to incorporate nucleon substructure, allowing for event-by-event variability and capturing the heterogeneous nature of ultra-relativistic ion collisions.

The model parameters are calibrated to experimental data, with simulations conducted over a wide range of grid resolutions to ensure robust results free from finite-volume artifacts. Additionally, simulations incorporate lattice QCD equations of state to convert initial entropy densities into detailed particle spectra.

Results

The paper's results show that superSONIC simulations can accurately replicate experimental data for elliptic, triangular, and quadrupolar flow across different collision systems. Specifically, the model's predictions align well with experimental measurements at mid-rapidity obtained by ALICE, CMS, and ATLAS. Importantly, no fine-tuning of hydrodynamic parameters was necessary, indicating that observed flow features in all collision systems are fundamentally hydrodynamic.

The superSONIC model simulates multiplicity distributions and mean transverse momenta that qualitatively match the experimental results, albeit with certain deviations in pion mean transverse momentum, attributed to the exclusion of bulk viscous corrections in simulations.

Implications and Future Directions

This research emphasizes that flow patterns observed in both small and large nuclear systems share a common hydrodynamic origin, challenging prevalent theoretical expectations that limited hydrodynamic behavior to large systems. The ability to describe these flows using a unified set of fluid parameters advances our understanding of quantum chromodynamics (QCD) phenomena in ultra-relativistic conditions, revealing the possibility of a coherent hydrodynamic behavior even in the smallest systems.

Future research should aim to include bulk corrections to further refine model predictions. Additionally, exploring alternative nucleon substructure models could provide further insights into the granular features of the nucleons that drive initial conditions in ultra-relativistic collisions. The paper serves as a significant step toward corroborating hydrodynamic frameworks for diverse collision systems, necessitating ongoing investigations to fully ascertain these mechanisms' intricacies.