Initial state geometry and the role of hydrodynamics in proton-proton, proton-nucleus and deuteron-nucleus collisions (1304.3403v3)

Abstract: We apply the successful Monte Carlo Glauber and IP-Glasma initial state models of heavy ion collisions to the much smaller size systems produced in proton-proton, proton-nucleus and deuteron- nucleus collisions. We observe a significantly greater sensitivity of the initial state geometry to details of multi-particle production in these models compared to nucleus-nucleus collisions. In particular, we find that the size of the system produced in p+A collisions is very similar to the one produced in p+p collisions, and predict comparable Hanbury-Brown-Twiss radii in the absence of flow in both systems. Differences in the eccentricities computed in the models are large, while differences amongst the generated flow coefficients v_2 and v_3 are smaller. For a large number of participants in proton-lead collisions, the v_2 generated in the IP-Glasma model is comparable to the value obtained in proton-proton collisions. Viscous corrections to flow are large over characteristic lifetimes in the smaller size systems. In contrast, viscous contributions are significantly diminished over the longer space-time evolution of a heavy ion collision.

• The paper demonstrates that initial state geometries profoundly affect hydrodynamic flow, with models predicting distinct flow coefficients.
• It employs Monte Carlo Glauber and IP-Glasma simulations to compare spatial eccentricities and reveals similar v2 values in p+p and p+A interactions.
• The study challenges the robustness of hydrodynamics in small systems and calls for precise experimental data to address model limitations.

Analysis of Initial State Geometry and Hydrodynamics in p+p, p+A, and d+A Collisions

The study by Bzdak et al. investigates the complexities of initial state geometry and the application of hydrodynamics in proton-proton (p+p), proton-nucleus (p+A), and deuteron-nucleus (d+A) collisions. They employ Monte Carlo Glauber and IP-Glasma models to parse the differences in multiparticle production dynamics within these smaller systems relative to the larger nucleus-nucleus (A+A) collisions. Their results reveal notable differences in system size and flow coefficients and highlight the importance of initial state configurations in subsequent flow phenomena.

Initial State Models and System Sensitivity

This paper underscores a heightened sensitivity of initial state geometries to multiparticle production specifics in p+p and p+A collisions, contrasting with A+A interactions where sheer nucleon numbers predominately determine geometry. The authors apply the IP-Glasma model, leveraging the Color Glass Condensate (CGC) effective field theory to capture gluon saturation effects, providing a robust theoretical framework for examining these initial states.

The IP-Glasma model predicts similar system sizes for p+p and p+Pb collisions, insightfully indicating comparable Hanbury-Brown-Twiss radii. This suggests that differential hydrodynamic evolution could be probed through radius analysis. Furthermore, the dependence of spatial eccentricities on dynamical assumptive models is demonstrated, with diverse outcomes from Glauber variants and the IP-Glasma model offering distinct initial state predictions.

Comparative Analysis of Eccentricities and Flow

Despite inherent model variances in eccentricities, flow coefficients like $v_2$ (elliptic flow) and $v_3$ (triangular flow) are less divergent. In p+Pb interactions, the results show $v_2$ discrepancies between Monte Carlo Glauber and IP-Glasma models, with the latter emphasizing considerable sensitivity to initial state assumptions. The paper highlights the surprising finding that for high participant numbers, $v_2$ values in p+A interactions mirror those in p+p collisions, indicating a unique dynamical similarity that provokes further exploration of hydrodynamics in small systems.

Hydrodynamics Applicability and Model Limitations

Significant discussion is devoted to the validity of applying hydrodynamics to these small systems. The work challenges the assumption of hydrodynamic usability, noting substantial viscous corrections in p+A and d+A systems comparable over their entire lifetime, as opposed to the minimal correction period in A+A systems. This casts doubt on the robustness of hydrodynamic models for p+/d+A collisions, particularly given the brief timescales at play.

Implications and Future Outlook

The implications of this researched distinction between p+A and A+A dynamics shape our grasp of hydrodynamic boundaries, potentially redefining the understanding of Quark-Gluon Plasma transport properties in smaller systems. The research anticipates that future work should rigorously examine initial state models alongside centrality and system lifetime to further elucidate the mechanisms leading to an isotropic flow in these smaller, dense systems.

Additionally, this paper calls attention to the need for more comprehensive data from high-energy collider results to refine these model predictions. These studies bridge the understanding from non-equilibrium quantum field dynamics to macroscopic flow phenomena, underpinning the ongoing quest to harness the full explanatory power of quantum chromodynamics in describing exotic phases of matter.