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Investigation of hadronic effects on resonance productions in small collision systems using the EPOS4 model

This study utilizes the EPOS4 model to demonstrate that hadronic interactions, specifically the competition between rescattering and regeneration, significantly modify resonance production yields and distributions in both small (pp, p-O) and large (O-O, Pb-Pb) collision systems at LHC energies, highlighting the critical role of the hadronic phase even in small systems.

Original authors: Hyunji Lim, Bong-Hwi Lim, Minjung Kim, Sanghoon Lim

Published 2026-07-17

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CC BY 4.0

Original authors: Hyunji Lim, Bong-Hwi Lim, Minjung Kim, Sanghoon Lim

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the universe as a giant, cosmic kitchen. When you smash two tiny particles together at nearly the speed of light, it's like dropping a super-hot, super-dense drop of soup into a cold room. For a split second, this soup is so hot and energetic that the usual rules of matter don't apply; it becomes a "quark-gluon plasma," a state where the tiny building blocks of atoms (quarks and gluons) float freely instead of being stuck together. This is the stuff of stars and the early universe.

As this cosmic soup expands and cools down, it eventually solidifies back into the particles we know, like protons and neutrons. But here's the tricky part: right after it solidifies, it doesn't just sit there. It forms a "hadron gas," a bustling crowd of particles bumping into each other. Some of these particles are very short-lived "resonances"—think of them as fleeting bubbles in the soup that pop almost instantly. The big question for physicists is: what happens to these bubbles while they are in the crowd? Do they get squashed by the other particles before we can see them? Or do the other particles crash together and accidentally create new bubbles? Understanding this helps scientists figure out how long this "crowded" phase lasts and how it behaves, even in the smallest collisions.

This paper dives into that mystery using a powerful computer simulation called EPOS4. The researchers wanted to see how these short-lived bubbles (resonances) behave when particles smash together in different sizes of collisions, from tiny proton-proton hits to the massive collisions of lead nuclei. They focused on two main forces at play in the hadron gas crowd: rescattering and regeneration.

Think of rescattering like a game of musical chairs where the music stops too early. A resonance particle (a bubble) pops, and its pieces (decay products) fly out. If they bump into other particles in the crowd before they can be caught by a detector, the original bubble is "lost." It's as if the bubble popped, but the pieces got rearranged into something else, so you can't tell a bubble was ever there. This makes the signal weaker.

On the other hand, regeneration is like a magic trick where the crowd accidentally rebuilds the bubble. Two particles in the crowd crash into each other and, by pure chance, form a new resonance bubble. This adds to the count, making the signal stronger.

The paper's main finding is that the final number of bubbles we see isn't just about how long the bubble lasts or how big the crowd is. It's a tug-of-war between these two forces. For some particles, the "loss" from rescattering wins, and we see fewer bubbles. But for others, especially certain heavy particles called baryonic resonances, the "gain" from regeneration is so strong that it actually overpowers the loss, leading to more bubbles than we started with.

The researchers ran simulations for different types of collisions: tiny proton-proton (pp) collisions, medium-sized oxygen-oxygen (O-O) collisions, and huge lead-lead (Pb-Pb) collisions. They found that even in the tiny proton-proton collisions, which were once thought to be too small to have a "crowded" phase, these effects are still happening. The "crowd" in small collisions is short-lived (lasting only about 2 femtometers/c), but it's still enough to scramble the signals of very short-lived particles.

A key discovery is that you can't just look at one type of particle to understand the whole picture. Short-lived particles like the $K^*(892)^0$ (which lives for about 4.2 fm/c) are very sensitive to being squashed (rescattering), so their numbers drop as the collision gets bigger. However, longer-lived particles like the $\phi(1020)$ (which lives for about 46.3 fm/c) are so long-lived that they often escape the crowd before it gets messy, so their numbers stay steady. But for some baryonic resonances, the regeneration effect is so powerful that their numbers actually go up in larger collisions.

The team also looked at the "speed" (transverse momentum) of these particles. They found that regeneration tends to create new bubbles that are moving slowly, while rescattering tends to wipe out the slow-moving ones. This changes the average speed of the particles we see, giving scientists another clue about what's happening inside the soup.

By simulating these collisions from the smallest (proton-proton) to the largest (lead-lead) and including the new oxygen-oxygen collisions, the paper shows that the behavior of these particles changes smoothly as the system gets bigger. It suggests that the "crowded" phase of the universe behaves consistently whether the collision is tiny or huge, driven mostly by how many particles are produced. However, the way the particles move (their speed) depends on the specific shape and geometry of the collision, not just the number of particles.

In short, this study suggests that to understand the life and death of these fleeting particles, we have to stop looking at them as just "survivors" of a chaotic crowd. We have to realize that the crowd is also a factory, constantly rebuilding them. The final count is a balance between how many get destroyed and how many get made, a balance that shifts depending on the type of particle and the size of the collision. This helps scientists better measure how long this "hadronic phase" lasts, a crucial piece of the puzzle in understanding how the universe evolved from a hot soup into the matter we see today.

Technical Summary: Investigation of Hadronic Effects on Resonance Productions in Small Collision Systems Using the EPOS4 Model

Problem Statement
Recent experimental observations in high-multiplicity proton-proton (pp) collisions suggest the emergence of collective behavior and medium-like effects previously associated only with heavy-ion collisions. Resonance particles serve as sensitive probes of the hadronic phase—the period between chemical and kinetic freeze-out—because their yields and transverse-momentum ( $p_T$ ) distributions are modified by interactions occurring within this phase. Specifically, two competing mechanisms influence the final measurable resonance signal:

Rescattering: Decay products of short-lived resonances interact with surrounding hadrons, altering their momenta and preventing the reconstruction of the parent resonance, thereby suppressing the observed yield.
Regeneration: Pseudo-elastic scattering among hadrons in the medium can recreate resonances, potentially enhancing yields.

While these effects are well-studied in large systems (e.g., Pb-Pb), their interplay in small collision systems (pp, p-Pb) remains less understood. Furthermore, the recent availability of oxygen-oxygen (O-O) and proton-oxygen (p-O) collision data at the LHC offers a unique opportunity to investigate system-size dependence in a continuous manner, bridging the gap between small and large systems.

Methodology
The study utilizes the EPOS4 event generator, which incorporates a dynamical core-corona separation framework and viscous hydrodynamic evolution for high-density regions. To model final-state hadronic interactions, EPOS4 is coupled with the UrQMD (Ultra-relativistic Quantum Molecular Dynamics) transport approach.

The analysis employs a systematic comparison of three specific configurations to disentangle production mechanisms:

UrQMD OFF: No hadronic cascade; resonances and stable particles are identified immediately after hadronization. This serves as the baseline for primary production.
UrQMD reg: Includes the full UrQMD cascade, accounting for both resonance regeneration and rescattering. This represents the total yield after hadronic interactions.
UrQMD reg+res: Includes the UrQMD cascade but excludes resonances whose decay daughters have undergone rescattering. This configuration approximates the experimentally observable signal, isolating the loss of reconstructible resonances.

Simulations were performed for:

pp collisions at $\sqrt{s} = 13.6$ TeV.
p-O collisions at $\sqrt{s} = 9.62$ TeV.
O-O and Pb-Pb collisions at $\sqrt{s}_{NN} = 5.36$ TeV.

The study covers a wide range of mesonic and baryonic resonances (e.g., $\rho(770)^0$ , $K^*(892)^0$ , $\phi(1020)$ , $\Delta(1232)^{++}$ , $\Sigma(1385)^\pm$ , $\Lambda(1520)$ , $\Xi(1530)^0$ ) with varying lifetimes and quark contents. Observables include $p_T$ -integrated yields, resonance-to-ground-state particle ratios, and mean transverse momentum ( $\langle p_T \rangle$ ) as functions of charged-particle multiplicity density ( $\langle dN_{ch}/d\eta \rangle$ ).

Key Results

Competition Between Regeneration and Rescattering: The final resonance yield is not determined solely by resonance lifetime but by the competition between regeneration (yield enhancement) and rescattering (signal suppression).
- For short-lived mesonic resonances like $\rho(770)^0$ and $K^*(892)^0$ , rescattering effects dominate, leading to a net suppression of reconstructible signals compared to the UrQMD OFF baseline.
- For several baryonic resonances (e.g., $\Delta(1232)^{++}$ , $\Sigma(1385)^\pm$ ), regeneration effects are comparable to or exceed rescattering losses, resulting in a net yield enhancement even when UrQMD is active.
System-Size Dependence:
- In pp collisions, the separation between UrQMD configurations is generally smaller than in Pb-Pb collisions due to the shorter duration and smaller volume of the hadronic phase. However, significant differences remain, particularly for very short-lived resonances.
- p-O and O-O collisions act as a continuous bridge between pp and Pb-Pb systems. Integrated yields and yield ratios show a smooth, multiplicity-driven evolution across all system sizes.
- Conversely, $\langle p_T \rangle$ exhibits a stronger dependence on the specific collision system (geometry and initial dynamics) rather than just final-state multiplicity. For instance, O-O results at lower multiplicities resemble Pb-Pb trends more closely than pp trends, suggesting different underlying mechanisms for generating similar multiplicities in different systems.
Lifetime Estimation: The study estimates the hadronic-phase lifetime ( $\tau_{had}$ ) using resonance-to-ground-state ratios. The extracted $\tau_{had}$ generally increases with system size and multiplicity. However, the authors note that in pp and p-O systems, $\tau_{had}$ tends to plateau at higher multiplicities, unlike the linear increase seen in O-O and Pb-Pb.
Model Performance: The EPOS4 model with the UrQMD reg+res configuration reasonably describes experimental data for mesonic resonances. However, it tends to underestimate yields for certain baryonic resonances, suggesting that the balance of primary production, regeneration, and rescattering for baryons may require further tuning.

Significance and Claims
The paper claims that hadronic interactions play a significant role even in small collision systems (pp), challenging the notion that such effects are negligible outside heavy-ion collisions. A primary contribution is the explicit separation of regeneration and rescattering within a unified model framework, demonstrating that:

Rescattering is strongly dependent on resonance lifetime and system size.
Regeneration is strongly dependent on particle species and interaction channels.
The interpretation of resonance observables (yields, ratios, $\langle p_T \rangle$ ) cannot rely solely on resonance lifetime; the specific balance between regeneration and rescattering must be considered simultaneously.

The study emphasizes that estimating the hadronic-phase lifetime from resonance ratios requires accounting for regeneration, as ignoring it leads to an incomplete picture. The inclusion of p-O and O-O systems provides a crucial intermediate regime for disentangling multiplicity-driven effects from genuine system-size and geometry-dependent dynamics. The authors conclude that measuring resonances with diverse lifetimes and quantum numbers is essential to constrain the dynamics and lifetime of the hadronic phase across all collision systems.

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