Global observables and identified-hadron production in pp, O-O and Pb-Pb collisions at LHC Run 3 energies with EPOS4

This paper presents EPOS4 model predictions for pp, O-O, and Pb-Pb collisions at LHC Run 3 energies, demonstrating that the dynamical core-corona approach and hadronic-phase effects (via UrQMD) are essential for describing the non-universal scaling of transverse momentum, identified-hadron spectra, and nuclear modification factors across different system sizes.

The Gist

Imagine you are trying to understand how a massive, roaring bonfire behaves compared to a tiny, flickering candle, or even a single spark from a match.

In the world of particle physics, scientists do something similar. They smash tiny particles (protons) together, or larger "clumps" of particles (like Oxygen or Lead), at nearly the speed of light. When these collisions happen, they create a "soup" of energy so hot and dense that it mimics the conditions of the universe just microseconds after the Big Bang. This soup is called the Quark-Gluon Plasma (QGP).

This paper uses a sophisticated computer model called EPOS4 to predict what will happen in these collisions at the Large Hadron Collider (LHC). Here is the breakdown of their findings using everyday analogies.

1. The "Core and Corona" Concept: The City and the Suburbs

The most important idea in this paper is how the model divides a collision into two parts: the Core and the Corona.

• The Core (The Busy City Center): Imagine a massive, crowded city center. Everything is packed together, people are bumping into each other, and there is a collective "flow" to the crowd. In a collision, this is the high-density center where a "liquid" (the QGP) forms and expands outward like a single, unified wave.
• The Corona (The Quiet Suburbs): Now imagine the outskirts of the city. The houses are spread out, and people move independently. In a collision, these are the low-density edges where particles just fly off individually, like sparks from a fire, without interacting with a "crowd."

The researchers found that as you move from small collisions (proton-proton) to medium (Oxygen-Oxygen) to huge (Lead-Lead), the "City" (the Core) gets bigger and more dominant, while the "Suburbs" (the Corona) shrink.

2. The "System Size" Mystery: The Bridge

For a long time, scientists thought you needed a massive collision (like Lead) to create this "liquid" state. But they started seeing "liquid-like" behavior in tiny collisions (protons), too. This was a huge puzzle.

The researchers used Oxygen-Oxygen collisions as a "bridge." If a tiny spark (proton) shows some flow, and a massive bonfire (Lead) shows massive flow, what does a medium-sized campfire (Oxygen) do?

• The finding: Oxygen acts as the perfect middle ground. It shows that the transition from "individual sparks" to "flowing liquid" isn't a sudden jump; it’s a smooth, continuous evolution.

3. The "Hardness" of the Soup: The Heavy vs. Light Particles

The paper looks at different types of particles (pions, kaons, and protons) to see how they move.

• The Analogy: Imagine a gust of wind blowing through a park. A leaf (a light particle) will be blown very easily and fast. A heavy bowling ball (a heavy particle) will barely move.
• The finding: In the "City" (the Core), the collective expansion acts like a powerful wind. It pushes the heavy "bowling balls" (protons) much harder than you’d expect. By measuring how much these heavy particles "speed up," scientists can calculate exactly how strong the "wind" (the pressure of the QGP) actually is.

4. The "After-Party": The Hadronic Phase

Even after the "soup" cools down, the particles don't just stop. They enter a phase called the hadronic cascade (modeled by a tool called UrQMD).

• The Analogy: Think of this as the "after-party" after a big concert. The main show (the QGP) is over, but people are still bumping into each other in the parking lot, exchanging words, and moving around before they finally go home.
• The finding: This "after-party" is crucial. It changes the final count of certain particles (like protons). If you don't account for these final bumps and collisions, your predictions won't match the real-world data.

Summary: Why does this matter?

By using this "City vs. Suburbs" model, the researchers have created a roadmap for the next phase of experiments at the LHC. They have shown that we can predict how the "liquid" of the early universe forms, grows, and eventually cools down, whether we are looking at a tiny spark or a massive cosmic explosion.

Technical Summary

This technical summary provides a detailed overview of the research paper titled "Global observables and identified-hadron production in pp, O–O and Pb–Pb collisions at LHC Run 3 energies with EPOS4."

1. The Problem: The "Small System" Puzzle

The central challenge in modern high-energy nuclear physics is understanding the minimal requirements for the formation of the Quark-Gluon Plasma (QGP). While heavy-ion collisions (like Pb–Pb) clearly exhibit collective, fluid-like behavior, recent observations of similar "collective" signatures (such as mass-dependent $p_T$ hardening and anisotropic flow) in high-multiplicity proton-proton (pp) collisions have challenged the traditional view.

The core scientific questions are:

• Is the observed collectivity in small systems a signature of genuine QGP formation (tiny droplets of fluid)?
• How do these signatures evolve continuously from fragmentation-dominated systems (pp) to hydrodynamics-dominated systems (Pb–Pb)?
• How can we decouple initial-state nuclear geometry from final-state medium dynamics?

2. Methodology: The EPOS4 Framework

The authors utilize EPOS4, the latest iteration of a full-chain Monte Carlo event generator. The methodology relies on several sophisticated theoretical components:

• Core–Corona Separation: This is the model's defining feature. It dynamically separates the collision into two regions based on local energy density:
  • The Core: High-density regions that undergo 3+1D viscous hydrodynamic evolution (modeling the QGP).
  • The Corona: Low-density regions that hadronize via independent string fragmentation (modeling the non-collective baseline).
• Microcanonical Statistical Hadronization: A refined approach to hadronization that ensures exact conservation of baryon number, charge, and strangeness, which is critical for accurately modeling small systems.
• UrQMD Afterburner: The authors perform simulations both with and without the Ultra-relativistic Quantum Molecular Dynamics (UrQMD) model. This allows them to isolate the effects of the "hadronic phase"—the period of elastic and inelastic re-scattering that occurs after the QGP has transitioned into hadrons.
• Systematic Scan: The study covers a wide range of energies and systems: pp at $\sqrt{s} = 13.6$ TeV, and O–O and Pb–Pb at $\sqrt{s_{NN}} = 5.36$ TeV.

3. Key Contributions

• Bridging the Gap with O–O: The paper provides one of the first comprehensive theoretical predictions for Oxygen–Oxygen (O–O) collisions. This system acts as a crucial "bridge" between the small pp and large Pb–Pb systems.
• Dissecting Collectivity: By using the core–corona tag, the authors quantify exactly how much of the observed particle production comes from the thermalized medium versus string fragmentation.
• Baseline for LHC Run 3: The study provides a robust theoretical baseline for upcoming experimental measurements at the Large Hadron Collider.

4. Key Results

• Non-Universal Scaling: The mean transverse momentum $\langle p_T \rangle$ does not follow a universal scaling with multiplicity. At a fixed multiplicity, pp collisions show higher $\langle p_T \rangle$ than Pb–Pb, indicating that high multiplicity in small systems is driven by hard/semi-hard processes, whereas in large systems, it is driven by soft, collective expansion.
• Core–Corona Dynamics:
  • The "core fraction" (the proportion of particles coming from the hydrodynamic medium) increases with multiplicity across all systems.
  • There is a species-dependent hierarchy: Kaons ($K$) and Protons ($p$) show higher core fractions than Pions ($\pi$), explaining why their spectra harden more significantly with multiplicity.
• Hadronic Phase Effects: The inclusion of UrQMD is essential to describe the $p/\pi$ ratio in central Pb–Pb collisions. The suppression of the $p/\pi$ ratio at high multiplicities is driven by baryon-antibaryon annihilation in the hadronic phase.
• Nuclear Modification Factor ($R_{AA}$):
  • Pb–Pb shows strong suppression (jet quenching).
  • O–O shows "substantial" but weaker suppression compared to Pb–Pb, confirming that medium-induced energy loss is already present in intermediate-sized systems.
• Freeze-out Dynamics: Blast-wave fits reveal an anti-correlation between kinetic freeze-out temperature ($T_{kin}$) and transverse flow velocity ($\langle \beta_T \rangle$). The hadronic afterburner (UrQMD) shifts these parameters toward lower temperatures and higher flow velocities.

5. Significance

This research demonstrates that the transition from "vacuum-like" particle production to "medium-like" collective behavior is a continuous process governed by the interplay of core and corona components. By successfully modeling the intermediate O–O system, the authors provide a roadmap for experimentalists to determine the minimum volume or energy density required to create a QGP. The study reinforces that while the microscopic origins of high multiplicity differ between pp and Pb–Pb, the EPOS4 core–corona mechanism provides a unified theoretical language to describe them both.