Fluctuating Glasma initial conditions and flow in heavy ion collisions (1202.6646v2)

Abstract: We compute initial conditions in heavy-ion collisions within the Color Glass Condensate (CGC) framework by combining the impact parameter dependent saturation model (IP-Sat) with the classical Yang-Mills description of initial Glasma fields. In addition to fluctuations of nucleon positions, this IP-Glasma description includes quantum fluctuations of color charges on the length-scale determined by the inverse nuclear saturation scale Q_s. The model naturally produces initial energy fluctuations that are described by a negative binomial distribution. The ratio of triangularity to eccentricity is close to that in a model tuned to reproduce experimental flow data. We compare transverse momentum spectra and v_(2,3,4)(p_T) of pions from different models of initial conditions using relativistic viscous hydrodynamic evolution.

• The paper presents the IP-Glasma model that integrates IP-Sat with classical Yang-Mills dynamics, naturally producing negative-binomial-distributed energy fluctuations.
• The study employs lattice gauge theory to compute event-by-event energy densities, yielding accurate predictions for ellipticity and triangularity in collision events.
• The research refines QGP simulations by addressing initial state quantum fluctuations, thereby reconciling discrepancies in predicted flow harmonics such as v3.

Overview of "Fluctuating Glasma Initial Conditions and Flow in Heavy Ion Collisions"

The paper discusses the development of the IP-Glasma model to comprehend the initial conditions in heavy-ion collisions using the Color Glass Condensate (CGC) framework. The paper integrates the impact parameter dependent saturation model (IP-Sat) with the classical Yang-Mills description, introducing fluctuations of color charges on scales dictated by the inverse of the nuclear saturation scale, $Q_s$.

A prominent advancement noted in the paper is the natural emergence of initial energy fluctuations that align with a negative binomial distribution. This model leads to predictions of a ratio of triangularity to eccentricity $\varepsilon_3/\varepsilon_2$ that closely mirrors models accurately reflecting experimental flow data. The research involves a comparative analysis of transverse momentum spectra and harmonics of pion flow $v_{2,3,4}(p_T)$ across different initial state models implemented with relativistic viscous hydrodynamics.

Key Findings and Computational Approach

One pivotal aspect of the paper is its treatment of quantum fluctuations in nuclear wavefunctions, which are essential for enhancing predictions of hydrodynamic flow observables. The paper employs a lattice implementation of Glasma initial conditions to compute the event-by-event energy density distributions at $\tau=0$. This model successfully captures the energy density fluctuations through the discretization of color charges and solves the classical Yang-Mills equations using lattice gauge theory methods.

The initial energy density on the lattice is ascertained to follow a negative binomial distribution, displaying excellent agreement with experimental multiplicity distributions from p+p and A+A collisions at various colliders, a noteworthy metric for validating theoretical models in high-energy nuclear physics.

Furthermore, the paper performs an exhaustive analysis of the participant ellipticity and triangularity across the implemented initial conditions. The authors demonstrate that the calculated ellipticity and triangularity metrics lie between existing Monte-Carlo Glauber and MC-KLN models, thereby offering a balance between event-by-event fluctuations and phenomenologically accurate descriptions of particle multiplicities.

Implications and Future Prospects

The implications of this work extend to improving the understanding of initial conditions prior to the formation of the Quark-Gluon Plasma (QGP) in heavy-ion collisions. The IP-Glasma model contributes to more realistic simulations of the QGP evolution, which are critical for extracting precise properties of this state of matter, such as its low shear viscosity to entropy density ratio $\eta/s$.

The paper posits that discrepancies in triangularity results between the IP-Glasma model and MC-KLN models directly impact predicted flow harmonics, particularly the $v_3$ coefficient, which is predominantly driven by initial state fluctuations. This suggests potential refinements in initial condition models could lead to more precise experimental agreements, especially when contrasting results from different collision energies and centralities.

For future research, the authors recommend incorporating JIMWLK evolution equations to handle rapidity and energy distributions more effectively, alongside tackling the thermalization process from a first-principles perspective. This paper highlights the critical nature of accurately modeling initial state fluctuations, shaped by the outcome of quantum chromodynamics dynamics and their connections to hydrodynamic simulations.

Overall, the IP-Glasma model and its methodological advancements offer a robust gateway to fine-tuning the theoretical frameworks employed to simulate and analyze high-energy nuclear collision events, providing a foundation for further explorations in understanding the complex dynamics of QGP formation and behavior.