Quantum Decoherence Model Explains Early Universe Nuclear Collision Dynamics

Research calculates entropy production from the decoherence of a coherent state, modelling interactions typical of high-energy nuclear collisions. Findings demonstrate that decoherence alone does not produce sufficient entropy to fully thermalise the initial state into a gluon bath, except in specific proton-nucleus collisions.

The behaviour of quantum systems interacting with their surroundings is central to understanding the emergence of classicality from the quantum realm, and is particularly relevant in the extreme conditions following high-energy particle collisions. Researchers now investigate how entropy, a measure of disorder, arises from the process of decoherence, where a quantum state loses its coherence due to interaction with the environment. Gabriele Coci, Gabriele Parisi, and colleagues from the Department of Physics and Astronomy at the University of Catania detail their analysis in the article, “Entropy from decoherence: a case study using glasma-based occupation numbers”. Their work employs an analytical approach to model the entropy generated when a coherent quantum state, characterised by initial conditions mirroring those found in ‘glasma’ fields – transient states created in the earliest moments of heavy-ion collisions – interacts with vacuum fluctuations, revealing limitations in the ability of decoherence alone to fully thermalise the system.

The study of entropy generation and information loss represents a central challenge in modern physics, bridging cosmology and nuclear physics. Cosmological models posit that the early universe underwent rapid expansion, followed by a transition to a thermalised state intrinsically linked to entropy production. In heavy-ion collisions, physicists recreate conditions resembling these early epochs, generating a state of matter known as the Quark-Gluon Plasma, where entropy production serves as a key observable. Quantum decoherence, the process by which a quantum system loses coherence through environmental interaction, is understood as a fundamental source of entropy; a pure quantum state evolves into a mixed state, increasing entropy as information about the system’s initial state is lost. This evolution is mathematically described by the density operator, ρ, and quantified by the von Neumann entropy, S, which measures the degree of mixedness, providing a crucial framework for understanding thermalisation processes.

Theoretical frameworks for understanding decoherence rely on the theory of open quantum systems, which models the system of interest interacting with a larger environment. This interaction is often described using quantum master equations, tracking the evolution of the system’s density operator over time. Approximations, such as the Born-Markov approximation – assuming weak coupling between the system and environment and neglecting memory effects – simplify these calculations, leading to the Lindblad equation, a widely used master equation guaranteeing a physically realistic, trace-preserving evolution. The Lindblad equation allows researchers to estimate the timescale of decoherence and the total entropy produced, providing insights into quantum dynamics.

Specific models, like phase-damping and amplitude-damping, represent simplified interactions between the system and environment, offering tractable frameworks for studying decoherence and its impact on entropy production in various physical scenarios. Phase-damping models focus on the loss of phase information without altering particle number, while amplitude-damping models involve changes in particle number, allowing for nuanced investigations into the mechanisms driving decoherence and entropy generation.

Research into the extreme conditions created in heavy-ion collisions presently focuses on understanding the initial stages of these events, specifically the formation of a pre-equilibrium state known as the Glasma. This fleeting, dense state of gluonic fields arises immediately after the collision of heavy ions, setting the stage for the creation of the quark-gluon plasma, a state of matter thought to have existed shortly after the Big Bang. Current investigations employ sophisticated theoretical frameworks, such as the Color Glass Condensate model, to describe the Glasma’s properties and evolution.

A key methodological innovation lies in the application of open quantum system approaches to model this decoherence, moving beyond treating the colliding ions in isolation and explicitly accounting for the interaction between the system—the Glasma—and its environment—the quantum vacuum. This interaction is often represented using a phase-damping model, simulating continuous measurements on the system without altering its fundamental energy or particle number, allowing researchers to capture realistic processes that drive the Glasma towards thermalisation.

The research leverages occupation numbers derived from the Glasma fields generated in high-energy proton-nucleus and nucleus-nucleus collisions, providing a direct link between theoretical predictions and experimental observations. Findings suggest that decoherence, while contributing to entropy production, may not be sufficient on its own to achieve complete thermalisation. Comparisons with the entropy expected from a two-dimensional thermal bath of ultrarelativistic gluons reveal that the entropy generated by decoherence is often lower, indicating that additional mechanisms, such as interactions between the gluons themselves, are likely necessary to complete the thermalisation process.

Investigations actively seek to understand the behaviour of matter at extreme energy densities, recreating conditions thought to have existed shortly after the Big Bang. Current research concentrates on the initial stages of these collisions, specifically the formation and evolution of the Glasma, a transient state arising from the interaction of the colliding nuclei. This work demonstrates a focus on quantifying entropy production within the Glasma and assessing the mechanisms driving the system towards thermalisation. Researchers utilise open quantum system approaches, modelling the interaction of a coherent initial state – representing the Glasma fields – with the vacuum, inducing decoherence and contributing to entropy generation. The study employs a phase-damping model, simulating continuous measurements on the system without altering its energy or particle number, to describe this decoherence.

Analyses reveal that the entropy-per-particle generated through decoherence alone is insufficient to fully thermalise the initial state into a two-dimensional bath of ultrarelativistic gluons, except in proton-nucleus collisions at low values of a specific parameter, suggesting that additional mechanisms, beyond the studied decoherence process, are necessary to account for the observed rapid thermalisation in heavy-ion collisions. The research highlights the importance of understanding the interplay between quantum coherence, decoherence, and the dynamics of the Glasma in establishing the conditions for thermal equilibrium. Investigations employ heavy quarks – particles containing charm or bottom quarks – as sensitive probes of the initial conditions and early dynamics. These particles experience the Glasma environment directly, and their behaviour provides insights into the properties of the strongly coupled medium. By modelling the interaction of heavy quarks with the Glasma, researchers aim to refine their understanding of the system’s evolution and the mechanisms driving thermalisation.

The research elucidates how the initial conditions impact the subsequent evolution towards a thermalised state of matter, employing both quantum distribution functions and semi-classical approximations to model the transition from a quantum to a classical description. Results indicate that decoherence alone, arising from interactions with vacuum fluctuations, does not generate sufficient entropy to fully transform the initial coherent state into a thermalised gluon bath. Heavy quarks, particularly charm and bottom quarks, serve as crucial diagnostic tools, as their interactions with the evolving medium provide insights into the initial conditions and the dynamics of the Glasma, and analysis of anisotropic fluctuations in heavy quark distributions further refines understanding of the pre-equilibrium stage, offering a more detailed picture of the system’s evolution. The research consistently employs models such as the Impact-Parameter dependent Saturation model and the MV model to accurately describe these initial conditions.

This interdisciplinary approach, combining quantum field theory, statistical mechanics, and computational modelling, continues to refine our understanding of the quark-gluon plasma and the fundamental properties of matter at extreme energies. Future work should prioritise refining the semi-classical approximations used to bridge the quantum-classical divide. Investigating the influence of more complex environmental interactions beyond simple vacuum fluctuations could reveal additional sources of entropy production, and expanding the range of observables considered, and incorporating data from ongoing experiments at facilities such as the Relativistic Heavy Ion Collider and the Large Hadron Collider, will be crucial for validating theoretical predictions and refining the models employed. Exploring the potential for incorporating hydrodynamic evolution following the initial Glasma stage would provide a more complete picture of the system’s evolution, and detailed comparisons between theoretical predictions and experimental data, particularly concerning heavy quark diffusion and anisotropic flow, will be essential for constraining the parameters of the models and improving their predictive power. A deeper investigation into the interplay between decoherence and other dissipative processes could also yield valuable insights into the thermalisation mechanism, and this ongoing research promises to further illuminate the fundamental properties of matter under extreme conditions and deepen our understanding of the early universe.