⚛️ phenomenology

Examining hadronic resonance dynamics at energies available at the CERN Large Hadron Collider: Insights from EPOS4

This study utilizes the EPOS4 model to analyze hadronic resonance dynamics in LHC collisions, revealing how the hadronic phase lifetime increases with system size and how competing effects like rescattering, regeneration, and strangeness enhancement govern particle production across different transverse momentum regions.

Original authors: Vikash Sumberia, Dukhishyam Mallick, Sanjeev Singh Sambyal, Nasir Mehdi Malik

Published 2026-07-09

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Vikash Sumberia, Dukhishyam Mallick, Sanjeev Singh Sambyal, Nasir Mehdi Malik

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 Large Hadron Collider (LHC) as the world's most powerful particle accelerator, smashing protons and heavy lead atoms together at nearly the speed of light. When these collisions happen, they create a tiny, super-hot drop of "primordial soup" called the Quark-Gluon Plasma (QGP). As this soup cools down, it freezes into a gas of particles called hadrons (like protons, neutrons, and pions).

This paper is like a detective story investigating what happens to the "unstable guests" at this party. These guests are hadronic resonances—particles that exist for only a split second (a few femtoseconds, or $10^{-15}$ seconds) before they fall apart.

Here is the breakdown of the study using simple analogies:

1. The Two Scenarios: The "Afterburner" vs. The "Snapshot"

The researchers used a computer simulation called EPOS4 to model these collisions. To understand the physics, they ran the simulation in two modes:

Mode A (UrQMD OFF): This is like taking a snapshot of the party the moment the music stops. It shows what particles were created, but it ignores what happens next.
Mode B (UrQMD ON): This is like playing a video of the party after the music stops. It simulates the "hadronic phase," where the particles are still moving around, bumping into each other, and interacting before they finally fly away to the detectors.

The Analogy: Imagine a room full of people (particles) holding fragile balloons (resonances).

Mode A counts the balloons the moment they are inflated.
Mode B watches the room for a few seconds. Some people bump into the balloons, popping them (rescattering). Others might accidentally tie two pieces of string together to make a new balloon (regeneration).

2. The Main Discovery: The "Short-Lived" vs. The "Long-Lived"

The study looked at different types of balloons (particles) with different lifespans:

Short-lived: Like the $K^{*0}$ or $\rho^0$ . They pop almost instantly.
Long-lived: Like the $\phi$ meson. They hang around for a long time.

The Finding: In the "video" mode (Mode B), the short-lived balloons were missing from the final count, especially in the biggest, densest collisions (Lead-Lead).

Why? Because the room was so crowded that the decay products of the short-lived balloons bumped into other people before the scientists could identify them. The original balloon was "lost."
The Long-lived ones: The $\phi$ meson was fine because it lived long enough to escape the crowded room before anyone could bump into it.

3. The "Crowd Size" Matters

The researchers compared huge collisions (Lead-Lead) with smaller ones (Proton-Proton).

The Big Party (Lead-Lead): The room is packed. Short-lived particles get "popped" (rescattered) frequently.
The Small Party (Proton-Proton): You might think a small room means no interactions. However, the study found that in the busiest small collisions (high multiplicity), the room is still crowded enough to cause some "popping." This was a surprise, suggesting that even tiny collisions create a brief, dense phase where particles interact.

4. The "Heavy Hitters" (Baryons vs. Mesons)

The paper also looked at how heavy particles (baryons, like protons) behave compared to lighter ones (mesons).

The Flow: In the biggest collisions, the "soup" expands like a balloon being blown up. This expansion (radial flow) pushes heavier particles harder, giving them more speed.
The Result: The simulation showed that heavy particles get a bigger "boost" than light ones, which matches what real experiments see. However, the simulation struggled a bit to perfectly match the data for certain particle ratios, suggesting our understanding of how these particles form is still being tweaked.

5. The "Strangeness" Factor

The study also tracked particles containing "strange" quarks.

The Trend: In big, dense collisions, strange particles are produced much more efficiently than in small ones (Strangeness Enhancement).
The Twist: For protons (non-strange), the number actually decreased in the densest collisions. The authors suggest this is because protons and anti-protons are finding each other and annihilating (destroying each other) in the crowded room, while the heavier, multi-strange particles are too rare or too fast to get caught in this "annihilation trap."

6. Timing the "Party"

By counting how many short-lived balloons were missing, the researchers estimated how long the "crowded room" (the hadronic phase) lasted.

The Result: The longer the collision system, the longer the room stays crowded.
The Surprise: Even in high-multiplicity proton-proton collisions, the room stayed crowded for about 0.5 to 1 femtometer/c (a tiny fraction of a second), proving that a "hadronic phase" exists even in small systems.

Summary

This paper uses a sophisticated computer model to show that what we see in the detectors isn't exactly what was created.

In the densest collisions, the "crowded room" effect (rescattering) hides or destroys short-lived particles.
Even in small collisions, if there are enough particles, this "crowded room" effect still happens.
The model helps us understand that the final count of particles depends on a tug-of-war between rescattering (hiding the particles) and regeneration (making new ones), as well as the lifetime of the particles themselves.

Essentially, the paper confirms that to understand the universe's building blocks, we have to account for the chaotic "after-party" where particles bump into each other before we can take their picture.

Technical Summary: Examining Hadronic Resonance Dynamics at LHC Energies with EPOS4

Problem Statement
High-energy heavy-ion collisions create a Quark-Gluon Plasma (QGP) that evolves into a hadron resonance gas (HRG) as it cools. The period between chemical freeze-out (inelastic collisions cease) and kinetic freeze-out (elastic collisions cease) is the hadronic phase. During this phase, short-lived hadronic resonances (lifetimes $\sim$ 1–12 fm/c) decay within the dense medium. Their decay products can undergo rescattering (altering momentum and preventing reconstruction) or regeneration (reforming the parent resonance). Understanding the interplay between these processes is crucial for characterizing the hadronic phase. While experimental data from the ALICE Collaboration at the LHC shows modifications in resonance yields, theoretical models must accurately reproduce these dynamics to disentangle the effects of system size, multiplicity, and collision energy.

Methodology
This study utilizes the EPOS4 model, a general-purpose framework for relativistic collisions ranging from proton-proton (pp) to nucleus-nucleus (Pb–Pb) collisions. The analysis focuses on:

Collision Systems: pp collisions at $\sqrt{s} = 13.6$ TeV and Pb–Pb collisions at $\sqrt{s}_{NN} = 5.36$ TeV.
Model Configuration: The study employs EPOS4 with the UrQMD (Ultra-Relativistic Quantum Molecular Dynamics) hadronic afterburner switched ON and OFF. This toggle allows for the isolation of final-state hadronic interactions (rescattering and regeneration) from the initial production and hydrodynamic evolution.
Event Generation: 5 million events for pp and 1.5 million for Pb–Pb were generated. Events were categorized by charged-particle multiplicity ( $\langle dN_{ch}/d\eta \rangle$ ) to study system-size dependence.
Particle Selection: Resonances were identified via unique EPOS IDs. A "reconstructible" flag was applied: if a decay daughter underwent elastic scattering altering its momentum, the parent resonance was marked as non-reconstructible, mimicking experimental reconstruction limitations.
Observables: The study analyzes transverse momentum ( $p_T$ ) spectra, yield ratios (resonance-to-stable, baryon-to-meson, hadron-to-pion), average transverse momentum ( $\langle p_T \rangle$ ) scaled by reduced mass, and the estimated duration ( $\tau$ ) of the hadronic phase.

Key Contributions and Results

Resonance Suppression and Rescattering:
- The inclusion of UrQMD leads to significant suppression of short-lived mesonic resonances ( $\rho^0$ , $K^{*0}$ ) at low $p_T$ in central Pb–Pb and high-multiplicity pp collisions. This is attributed to the rescattering of decay daughters, which prevents reconstruction.
- Long-lived resonances (e.g., $\phi$ ) show minimal modification, consistent with their decay occurring after the hadronic phase.
- A clear ordering of suppression is observed based on lifetime: $\rho^0 < \Delta^{++} < K^{*0} < \Sigma^{*\pm} \sim \Lambda^* < \Xi^{*0} < \phi$ .
- Notably, $\Sigma^{*\pm}$ (shorter lifetime) shows suppression comparable to $\Lambda^*$ (longer lifetime). The authors attribute this to regeneration effects counteracting rescattering for $\Sigma^{*\pm}$ , whereas $\Lambda^*$ experiences less regeneration, leading to net stronger suppression despite its longer lifetime.
Baryon-to-Meson Ratios and Radial Flow:
- In central Pb–Pb collisions, baryon-to-meson ratios (e.g., $\Lambda/K_S^0$ , $\Sigma^{*\pm}/K_S^0$ ) exhibit a mass-dependent enhancement at intermediate $p_T$ , driven by radial flow.
- The study compares particles of similar mass (e.g., $p$ vs. $\phi$ vs. $K^{*0}$ ). While $p/\phi$ remains relatively flat, $p/K^{*0}$ shows a decreasing trend due to the specific hadronic phase modification of the short-lived $K^{*0}$ .
- The EPOS4 model, particularly with UrQMD, is required to reproduce the experimental $p/K^{*0}$ behavior and the intermediate $p_T$ enhancement in $p/\pi$ , which is driven by collective flow and microcanonical hadronization.
Average Transverse Momentum ( $\langle p_T \rangle$ ) Scaling:
- $\langle p_T \rangle$ increases with charged-particle multiplicity and hadron mass.
- When $\langle p_T \rangle$ is plotted against reduced mass (mass divided by valence quark number), stable mesons and resonances generally follow a linear trend. However, short-lived resonances ( $\rho^0$ , $K^{*0}$ , $\Delta^{++}$ ) deviate from this linearity, particularly in central Pb–Pb collisions.
- This deviation is attributed to hadronic phase effects (rescattering) increasing the $\langle p_T \rangle$ of short-lived resonances.
- Baryons and mesons form distinct, parallel groups in the reduced mass scaling, suggesting that hadronization dynamics (core-corona picture, exact conservation) affect them differently, rather than a simple coalescence mechanism.
Hadronic Phase Duration ( $\tau$ ):
- Using the exponential decay law applied to resonance-to-stable yield ratios, the study estimates the time duration between chemical and kinetic freeze-outs.
- $\tau$ increases with system size and multiplicity.
- Crucially, the model predicts a non-zero hadronic phase duration ( $\sim$ 0.5–1 fm/c) even in high-multiplicity pp collisions, comparable to peripheral Pb–Pb collisions. This supports the existence of a hadronic phase in small systems.
Strangeness Enhancement and Annihilation:
- Strangeness enhancement (yield ratios of strange hadrons to pions) increases with multiplicity and saturates in central Pb–Pb collisions, consistent with grand-canonical equilibrium.
- The proton-to-pion ratio ( $p/\pi$ ) decreases at high multiplicities in Pb–Pb collisions. The authors attribute this to baryon-antibaryon annihilation in the dense hadronic phase.
- Multi-strange baryons ( $\Xi$ , $\Omega$ ) show no such decreasing trend, likely due to their lower abundance and earlier freeze-out, which reduces their exposure to annihilation processes.

Significance and Claims
The paper claims that EPOS4 with UrQMD provides a robust framework for understanding the late-stage dynamics of high-energy collisions. Key findings include:

System Size Dependence: The evolution of resonance yields is governed primarily by charged-particle multiplicity rather than the colliding system (pp vs. Pb–Pb) or energy, provided the multiplicity is comparable.
Hadronic Phase in Small Systems: The observation of a non-zero hadronic phase duration in high-multiplicity pp collisions challenges the notion that such phases are exclusive to large heavy-ion systems.
Complexity of Baryonic Resonances: The study highlights that resonance yield modification is not solely determined by vacuum lifetime. It is a complex interplay of lifetime, decay product cross-sections, regeneration probabilities, and annihilation effects. Specifically, the behavior of baryonic resonances ( $\Lambda^*$ vs. $\Sigma^{*\pm}$ ) cannot be explained by lifetime alone.
Model Validation: The results qualitatively reproduce ALICE experimental data across various observables, validating the EPOS4 core-corona approach and the necessity of the UrQMD afterburner for describing final-state interactions.

The authors conclude that while the model captures the qualitative trends, precise quantitative agreement for certain ratios (e.g., $p/\phi$ at high $p_T$ ) requires further refinement. Future differential studies and measurements of additional resonances are necessary to fully constrain the microscopic origins of resonance suppression and the dynamics of the hadronic phase.