Unraveling the Cosmic Dance: How Atomic Collisions Reveal the Universe’s Earliest Moments
In the heart of colossal particle accelerators, where the fundamental building blocks of matter are smashed together at energies mimicking the Big Bang, physicists are meticulously charting the secrets of the universe’s infancy. A groundbreaking study, published in The European Physical Journal C, delves into the intricate behavior of charged particles produced in these titanic collisions, offering a tantalizing glimpse into the exotic state of matter that prevailed mere microseconds after creation. This research, by K. Nayak and V. Bairathi, employs a sophisticated multi-phase transport model to simulate and analyze the directed flow of charged hadrons – the subatomic particles that emerge from these high-energy encounters. Their findings illuminate how the very size of the colliding systems influences the collective motion of these nascent particles, a crucial piece of the puzzle in understanding the emergence of our universe. The directed flow, a subtle yet powerful indicator of the system’s initial conditions and subsequent evolution, acts like a cosmic fingerprint, betraying the forces at play in those fleeting, primordial moments.
The energy scale at which these collisions are conducted, specifically $\sqrt{s{NN}} = 200$ GeV (where $\sqrt{s{NN}}$ represents the center-of-mass energy per nucleon-nucleon collision), is designed to recreate the conditions of the quark-gluon plasma (QGP), a state of matter thought to have existed for an infinitesimal fraction of a second before the familiar protons and neutrons formed. Imagine a soup so hot and dense that protons and neutrons themselves break down into their constituent quarks and gluons, swimming freely in a quantum fluid. The directed flow, often quantified by a parameter called the directed flow coefficient ($v_1$), measures any net deflection of these charged particles from the impact parameter plane – the imaginary plane defined by the collision trajectory. A non-zero $v_1$ signifies a systematic bias in the particles’ motion, a collective “push” in a particular direction, hinting at asymmetries in the initial collision or the subsequent expansion of the QGP.
The multi-phase transport (AMPT) model, a sophisticated computational tool, is central to this investigation. It meticulously simulates the entire lifecycle of a heavy-ion collision, from the initial geometrical overlap of the colliding nuclei to the final “hadronization” where quarks and gluons coalesce into observable particles. The AMPT model incorporates various theoretical components, including an initial state model to describe the distribution of nucleons within the colliding nuclei, a string-melting mechanism to represent the deconfined QGP phase, a partonic cascade to handle interactions within the plasma, and a hadronization and hadronic cascade to describe the subsequent formation and evolution of hadrons before they reach the detectors. This comprehensive approach allows researchers to connect the microscopic dynamics of the QGP to the macroscopic observables detected in experiments.
A pivotal aspect of this study is its exploration of the system size dependence. The researchers are not just looking at one type of collision; they are examining how the directed flow of charged hadrons changes as the size of the colliding nuclei varies. This means comparing collisions of different types of ions, such as gold-gold (Au-Au) and smaller systems like proton-lead (p-Pb) or even potentially smaller nucleus-nucleus collisions. The rationale is that the geometry and the initial energy density distribution within the system are strongly correlated with its size. Larger systems, with more nucleons involved, are expected to produce a denser and more extended QGP, potentially leading to different collective behaviors than smaller, more peripheral collisions.
The findings reveal a fascinating trend: the system size significantly influences the magnitude and behavior of the directed flow. As the size of the colliding system increases, the interplay of forces within the expanding QGP and the subsequent hadronic phase leads to discernible changes in the $v_1$ coefficient. This dependence is not a mere academic curiosity; it directly probes the interplay between the initial geometrical anisotropies of the collision and the hydrodynamic response of the QGP. Understanding how these initial anisotropies are translated into the final observed particle production is paramount to reconstructing the properties of the early universe’s matter.
Directed flow is particularly sensitive to the initial asymmetry of the collision. If the colliding nuclei are not perfectly aligned or if their internal structures are not uniform, the resulting overlap region will exhibit an initial shape that is not perfectly circular. As the QGP expands, this initial shape is “hydrodynamically” evolved, meaning it behaves like a fluid, carrying these initial geometric imperfections outwards. The directed flow, $v_1$, is a direct manifestation of this initial asymmetry being translated into a directed momentum of the produced particles. Studying how this translation changes with system size allows physicists to disentangle the contributions of different physical mechanisms.
The AM PT model, in this context, is crucial for disentangling these contributions. It allows for the differentiation between the effects of the QGP phase and the subsequent hadronic interactions. For instance, it can help determine how much of the observed directed flow is generated during the hot, deconfined phase, and how much is influenced by the final-state interactions between the myriad of newly formed hadrons. This distinction is vital for accurately characterizing the properties of the QGP itself, such as its viscosity and equation of state. The model’s ability to simulate multiple phases of the collision grants it a unique advantage in this complex analysis.
The research highlights the importance of charged hadron directed flow as a sensitive probe of the QGP. Unlike neutral particles, charged particles can be easily detected and their momentum precisely measured by sophisticated detectors like those at the Relativistic Heavy Ion Collider (RHIC) or the Large Hadron Collider (LHC). The directed flow coefficient, $v_1$, is typically extracted by correlating the particle’s azimuthal angle (its direction of motion in the plane perpendicular to the beam) with the reaction plane (the plane containing the impact parameter and the beam axis). Even tiny asymmetries in the collision can lead to a measurable $v_1$.
Furthermore, the study delves into the dependence of directed flow on the transverse momentum ($p_T$) of the charged hadrons. This means examining how the directed flow changes for particles moving at different speeds or with different momenta. Generally, low-$p_T$ particles are considered to be more representative of the bulk collective expansion of the QGP, as they have had more time to equilibrate with the system. High-$p_T$ particles, on the other hand, are often thought to be more influenced by hard scattering processes that occur very early in the collision. Studying the $p_T$ dependence of $v_1$ provides further constraints on the theoretical models and helps to understand the different particle production mechanisms at play.
The quantitative results from the AMPT model show a systematic variation in the directed flow coefficients as the system size is varied. These variations are not random; they follow patterns that can be directly linked to theoretical predictions. For example, theoretical models predict that the shear viscosity to entropy density ratio ($\eta/s$) of the QGP, a measure of its fluidity, plays a significant role in shaping the collective flow. By comparing the model predictions with the experimental data for directed flow, physicists can constrain the value of $\eta/s$ for the QGP, a fundamental property of this exotic state of matter.
The implications of this research extend far beyond the experimental facilities. Understanding the physics of the early universe is a quest that drives fundamental advancements in our understanding of all fundamental forces and particles. The methods and tools developed to study the QGP are applicable to a wide range of physics problems, from the behavior of matter under extreme pressures to the search for new fundamental particles. The ability to simulate and interpret complex quantum phenomena, as demonstrated by this study, is a testament to the power of theoretical physics and computational modeling.
The directed flow coefficient can also shed light on the role of fluctuations. In smaller systems or peripheral collisions, initial state fluctuations – random variations in the distribution of nucleons within the colliding nuclei – can play a more dominant role in determining the initial geometry and hence the directed flow. The AMPT model can be used to isolate the effects of these fluctuations from the more deterministic hydrodynamic evolution. This allows researchers to probe the nature of these initial fluctuations and their impact on the subsequent development of the QGP.
The study’s focus on charged hadrons also allows for the investigation of particle-dependent directed flow. Different types of charged hadrons, such as pions, kaons, and protons, have different masses and compositions. Their directed flow may exhibit variations due to differences in their formation temperatures and interaction cross-sections during the hadronic phase. Examining these differences provides a more nuanced understanding of the hadronization process and the final-state effects.
Ultimately, this research contributes to a grander narrative: the quest to understand the origin and evolution of the universe. By recreating and studying the conditions that existed billions of years ago, physicists are not just performing abstract experiments; they are piecing together the cosmic story, one collision at a time. The intricate dance of subatomic particles, guided by the fundamental laws of physics, reveals the remarkable journey from a primordial fireball to the galaxies and stars we observe today. The precise measurements and sophisticated modeling employed in this study are essential steps in this profound exploration.
Subject of Research: System size dependence of charged hadrons directed flow at $\sqrt{s_{NN}} = 200$ GeV.
Article Title: System size dependence of charged hadrons directed flow at $\sqrt{s_{NN}}$ = 200 GeV using a multi-phase transport model.
Article References:
Nayak, K., Bairathi, V. System size dependence of charged hadrons directed flow at (\sqrt{s_{NN}}) = 200 GeV using a multi-phase transport model.
Eur. Phys. J. C 85, 1236 (2025). https://doi.org/10.1140/epjc/s10052-025-14966-5
Image Credits: AI Generated
DOI: https://doi.org/10.1140/epjc/s10052-025-14966-5
Keywords: Quark-gluon plasma, directed flow, multi-phase transport model, heavy-ion collisions, system size dependence
