⚛️ phenomenology

Pinning down the origin of "strangeness enhancement" in the jet-like region of pp collisions with Monte Carlo simulations

This study utilizes Monte Carlo simulations to demonstrate that the trigger transverse momentum dependence of strangeness enhancement in high-multiplicity proton-proton collisions serves as a critical discriminator between the core-contribution suppression mechanism in EPOS-LHC and the local string-overlap mechanism in PYTHIA8 with color ropes.

Original authors: Antonio Ortiz, Dushmanta Sahu, Victor Vázquez

Published 2026-10-08

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CC BY 4.0

Original authors: Antonio Ortiz, Dushmanta Sahu, Victor Vázquez

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

In the subatomic world, physicists have long searched for a state of matter so hot and dense that the building blocks of protons and neutrons melt into a free-flowing soup of quarks and gluons. This state, known as a quark-gluon plasma, is believed to have existed for a fleeting moment just after the Big Bang. One of the clearest signs that this plasma has formed is an unexpected abundance of "strange" particles—exotic forms of matter containing strange quarks that are usually rare in ordinary collisions. For decades, this "strangeness enhancement" was seen only in massive crashes between heavy atomic nuclei, where the energy is high enough to create a large, collective fireball. However, recent observations have shaken up this understanding. Scientists found that even in tiny, high-energy collisions between single protons, strange particles appear in greater numbers when the collision is particularly violent. This discovery raised a difficult question: are these small collisions creating a miniature version of the Big Bang's plasma, or is there a different, simpler mechanism at work that mimics the effect without the extreme heat?

To answer this, a team of researchers led by Antonio Ortiz, Dushmanta Sahu, and Victor Vázquez at UNAM in Mexico City turned to powerful computer simulations to dissect the aftermath of these proton collisions. They focused on a specific region of the collision debris, known as the "toward-leading" region, which is dominated by a high-speed jet of particles shooting out from the initial impact. In previous experiments, scientists measured strange particles in this jet-like area and found that their numbers increased as the overall violence of the collision grew. The new study asked a critical follow-up question: does this increase persist if we look only at the most energetic, hardest-hitting particles within that jet? By systematically changing the minimum energy threshold of the "trigger" particle used to start the analysis, the researchers could separate the effects of the main jet from the softer, underlying activity of the collision.

The team ran millions of simulated collisions using two different theoretical frameworks. The first, called PYTHIA8, models the collision as a series of string-like connections between particles that can overlap and interact locally. The second, EPOS-LHC, treats the collision as having a dense, collective core that expands like a fluid, surrounded by a thinner outer layer. Both models were tested against real data from the ALICE experiment at the Large Hadron Collider. When the researchers set the energy threshold for their trigger particle to a moderate level, both models reproduced the observed increase in strange particles, matching the experimental data. However, the story changed dramatically when they raised the energy threshold to select only the most powerful particles, specifically those with a transverse momentum greater than 10 GeV/c.

In the simulations using the EPOS-LHC model, which relies on a collective, fluid-like core, the enhancement of strange particles in the jet-like region vanished as the trigger energy increased. The signal essentially disappeared, suggesting that the strange particles seen at lower energies were likely coming from the softer, underlying environment rather than the hard jet itself. In contrast, the PYTHIA8 model, which relies on local interactions between overlapping strings, showed no such drop-off. Even when selecting the hardest, most energetic particles, the model continued to predict a strong increase in strange particles as the collision became more violent. This divergence in behavior is the key finding of the work. It suggests that the mechanism driving the strangeness enhancement in the jet-like region is highly sensitive to the energy of the particles being observed.

The results imply that the "strangeness enhancement" seen in small collisions is not a single, uniform phenomenon. Instead, it appears to be a mix of different processes. The collective, fluid-like behavior that resembles a quark-gluon plasma seems to fade away when looking at the most energetic particles, pointing toward a reduced contribution from a dense core in those specific cases. Meanwhile, the persistence of the effect in the local string-interaction model suggests that other mechanisms, such as the overlapping of color fields between particles, remain active even in the most energetic parts of the collision. By using the energy of the trigger particle as a precise dial, the researchers have provided a new way to distinguish between these competing explanations. The study does not definitively prove which mechanism is the sole cause, but it demonstrates that the energy of the particles involved is a powerful tool for peeling back the layers of these complex collisions, helping scientists understand whether the strange particles are born from a collective fireball or from the intricate, local tangles of the subatomic strings.

Technical Summary: Pinning down the origin of "strangeness enhancement" in the jet-like region of pp collisions with Monte Carlo simulations

Problem Statement
Recent measurements by the ALICE Collaboration have observed a finite strangeness enhancement in the "toward-leading" region of high-multiplicity proton-proton (pp) collisions. This region is typically associated with jet fragmentation and hard scattering processes, distinct from the underlying event (UE). While strangeness enhancement is a hallmark signature of Quark-Gluon Plasma (QGP) formation in heavy-ion collisions, its presence in small systems like high-multiplicity pp has sparked debate. The central question addressed by this work is whether the enhancement observed in the jet-like region reflects a genuine modification of jet fragmentation or arises from residual medium-like contributions associated with the underlying event. Previous analyses utilized a trigger transverse momentum ( $p_T^{trigg}$ ) threshold of $>3$ GeV/c, a range where particles may still receive substantial contributions from multiparton interactions (MPI) and the UE. Consequently, it remains unclear if the enhancement persists when selecting increasingly hard trigger particles that are more dominantly associated with hard processes.

Methodology
The authors investigate the $p_T^{trigg}$ dependence of strange-hadron production using Monte Carlo event generators to disentangle the mechanisms driving strangeness enhancement. The analysis follows the correlation framework established by ALICE, defining three angular regions relative to the highest- $p_T$ charged particle (the trigger):

Toward-leading: $|\Delta\eta| < 0.86$ and $|\Delta\phi| < 1.1$ (jet-associated).
Transverse-to-leading: $0.86 < |\Delta\eta| < 1.2$ and $0.96 < \Delta\phi < 1.8$ (UE-dominated).
Full region: Encompassing both.

The study systematically varies the minimum trigger momentum ( $p_T^{trigg}$ ) from the nominal ALICE selection ( $>3$ GeV/c) to higher thresholds ( $>5$ and $>10$ GeV/c), while keeping multiplicity and correlation selections fixed. The analysis focuses on the per-trigger yield ratio of multi-strange baryons to strange mesons ( $\Xi^\pm/K_S^0$ ) as a function of charged-particle multiplicity.

Three theoretical frameworks are compared:

PYTHIA8 (Monash tune): A baseline model using standard Lund string fragmentation without color reconnection effects beyond the standard tune.
PYTHIA8 (with Color Ropes): Incorporates color-rope formation, where overlapping strings combine into higher-color representations with enhanced effective string tension, a local mechanism proposed to enhance strangeness.
EPOS-LHC: A framework incorporating a dense core undergoing collective hydrodynamic expansion and a dilute corona undergoing string fragmentation, representing a collective medium-like scenario.

Calculations were performed for pp collisions at $\sqrt{s} = 13$ TeV. Yields are calculated at the generator level, integrating over the full transverse momentum range ( $p_T > 0$ ) to avoid extrapolation uncertainties inherent in experimental measurements.

Key Results

Baseline ( $p_T^{trigg} > 3$ GeV/c): Both EPOS-LHC and PYTHIA8 with color ropes reproduce the ALICE observation of a multiplicity-dependent increase in the $\Xi^\pm/K_S^0$ ratio in the toward-leading region. The rope-free PYTHIA8 Monash baseline shows a significantly weaker dependence, consistent with a lack of strangeness enhancement mechanisms in standard fragmentation.
High Trigger Momentum ( $p_T^{trigg} > 10$ GeV/c): A distinct divergence in behavior emerges between the models as the trigger becomes harder:
- EPOS-LHC: The strangeness enhancement in the toward-leading region is strongly suppressed. The ratio becomes nearly independent of multiplicity. This is interpreted as a reduced contribution from the collective core for increasingly hard triggers, suggesting the enhancement in this model is tied to the soft, collective sector.
- PYTHIA8 with Color Ropes: The enhancement remains approximately unchanged compared to the lower momentum trigger case. The multiplicity dependence persists even for hard triggers. This behavior is consistent with a local string-overlap mechanism that remains active regardless of the trigger hardness.
Transverse Region: In the transverse-to-leading region (UE-dominated), the enhancement remains significant across all models and trigger selections, reinforcing the major role of the underlying event in strangeness production.
Baryon Ratios: Analysis of $\Xi/\Lambda$ and $\Omega/\Lambda$ ratios in PYTHIA8 with ropes confirms that the hierarchy of multi-strange production is less pronounced in the toward region compared to the transverse region, further highlighting the sensitivity of the toward region to the relative contributions of jet and UE production.

Significance and Conclusions
The paper concludes that the trigger transverse momentum serves as a model-sensitive probe for the mechanisms contributing to strangeness enhancement in the jet-like region of pp collisions. The contrasting responses of EPOS-LHC and PYTHIA8 with color ropes to increasing trigger hardness demonstrate that the origin of the observed enhancement is not unique across different theoretical frameworks.

Specifically, the results suggest that:

If the enhancement arises from a collective core (as in EPOS-LHC), it is strongly suppressed when selecting hard triggers, implying the effect is tied to the soft underlying event.
If the enhancement arises from local string-density effects (as in PYTHIA8 with ropes), it persists even for hard triggers, suggesting a mechanism intrinsic to the fragmentation process itself that is active in high-density environments.

The authors assert that while the trigger momentum provides a discriminating test between these specific model responses, it cannot yet serve as a model-independent separation of collective and non-collective mechanisms, as the generators differ in multiple aspects of particle production and hadronization. Nevertheless, the study establishes that the persistence of strangeness enhancement under increasingly hard trigger selections is a critical observable for testing the microscopic origins of this phenomenon in small collision systems.

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