Understanding the medium-like effects in the jet-like yield in pp and p-Pb collisions using event generators

To understand the dynamics of jet-medium interaction in small systems such as proton-proton (pp) and proton-lead (p-Pb) collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV, particle production is studied in three distinct topological regions defined with respect to the charged particle with the highest transverse momentum in the event ($p_{\rm T}^{\rm trig}$). The jet-like yield is defined by the particle density in the toward region ($|Δφ|<π/3$) after subtracting that in the transverse region ($π/3<|Δφ|<2π/3$). The activity on the transverse side is used as a proxy for medium-like effects. Three different Monte Carlo event generators – \textsc{Pythia8}, a multiphase transport (AMPT) model, and EPOS4 – are employed to investigate particle yields as a function of $p_{\rm T}^{\rm trig}$ in the interval 0.5-20,GeV/$c$. Calculations are performed for the $p_{\rm T}$ threshold of 0.5 GeV/$c$ at mid-rapidity ($|η| < 0.8$). The jet-like yield in the toward region for pp collisions show interesting dynamics; they are significantly affected by the medium-like effects in the low to intermediate $p_{\rm T}^{\rm trig}$ ($<8$,GeV/$c$) which is studied through color reconnection and hydrodynamics in \textsc{Pythia8} and EPOS4, respectively. However, the results from AMPT show that the jet-like yield is medium-like modified throughout the entire $p_{\rm T}^{\rm trig}$ range. The jet-like yield in p-Pb collisions using AMPT is also studied. Notably, a dip structure that is observed in the jet-like signal ratio of pp to p-Pb at low $p_{\rm T}^{\rm trig}$ in ALICE data, is reproduced by AMPT model with string melting on, pointing to possible medium-like behavior in small systems. The results of this article also underscore the importance of high-$p_{\rm T}^{\rm trig}$ ($p_{\rm T}^{\rm trig} >$ 8 GeV/$c$) for minimizing underlying event biases in jet-related studies.

💡 Research Summary

The paper investigates how jet‑like signals are modified by medium‑like effects in small collision systems—proton‑proton (pp) and proton‑lead (p‑Pb) at √sNN = 5.02 TeV—using three state‑of‑the‑art Monte‑Carlo event generators: Pythia 8, AMPT, and EPOS 4. The analysis follows the standard underlying‑event (UE) methodology: events are divided into three azimuthal regions relative to the leading charged particle (the “trigger”) with the highest transverse momentum (pT^trig). The “toward” region (|Δφ| < π/3) contains the leading jet, the “away” region (|Δφ| > 2π/3) contains the recoil jet, and the “transverse” region (π/3 < |Δφ| < 2π/3) is most sensitive to UE activity. The jet‑like yield is defined as the charged‑particle density in the toward region after subtracting the density measured in the transverse region, thereby isolating the jet contribution from the UE background.

The study scans pT^trig from 0.5 GeV/c up to 20 GeV/c, applying a common particle‑level cut of pT > 0.5 GeV/c and |η| < 0.8. For each generator, two configurations are examined: one that includes a mechanism mimicking collective, medium‑like behavior (color reconnection in Pythia 8, string melting in AMPT, and viscous hydrodynamics in EPOS 4) and a baseline without it. Large event samples (10 M minimum‑bias pp events for Pythia 8, 1 M pp events for EPOS 4, and 7 M pp plus 3 M p‑Pb events for AMPT) ensure statistical robustness.

Key findings:

1. Toward region – All three generators reproduce the ALICE data reasonably well, indicating that the leading‑jet fragmentation is captured adequately.

2. Transverse region – A saturation of the charged‑particle density is observed around pT^trig ≈ 5 GeV/c for Pythia 8 (with CR) and EPOS 4 (with hydro), matching the experimental trend that UE activity becomes independent of the trigger momentum at higher pT. AMPT does not show this saturation; instead, its transverse density continues to rise, reflecting the model’s older HIJING‑based treatment of MPI and the influence of the string‑melting phase.

3. Jet‑like yield (toward – transverse) – In the low‑to‑intermediate pT^trig range (< 8 GeV/c), the jet‑like yield is significantly reduced when medium‑like mechanisms are active. In Pythia 8, color reconnection shortens strings, decreasing overall multiplicity but producing harder particles, which lowers the jet‑like signal after subtraction. EPOS 4’s hydrodynamic core similarly drains energy from the transverse region, leading to a reduced jet‑like yield. AMPT, however, shows a persistent medium‑like modification across the entire pT^trig range because the string‑melting phase creates a deconfined partonic medium that enhances radial flow and coalescence, affecting both toward and transverse regions.

4. p‑Pb collisions (AMPT only) – When the string‑melting option is enabled, AMPT reproduces the dip structure observed by ALICE in the ratio of jet‑like yields (pp/p‑Pb) at low pT^trig. This dip is absent without string melting, suggesting that partonic rescattering and collective expansion in the small p‑Pb system can generate observable medium‑like signatures.

5. High‑pT regime – For pT^trig > 8 GeV/c, all generators converge: the UE contribution saturates, and the jet‑like yield becomes largely insensitive to the presence or absence of collective mechanisms. This confirms that high‑pT jet studies can safely neglect UE‑induced biases in small systems.

The comparative study highlights the distinct ways in which each generator implements “collectivity”: Pythia 8 relies on phenomenological color reconnection, EPOS 4 embeds a full viscous hydrodynamic evolution for dense cores, and AMPT builds a partonic phase via string melting followed by quark coalescence. While Pythia 8 and EPOS 4 agree well with experimental data in both pp and p‑Pb for most observables, AMPT’s older MPI description leads to discrepancies in the transverse region but offers a unique capability to reproduce the low‑pT dip in the pp/p‑Pb ratio, underscoring the relevance of partonic rescattering even in the smallest collision systems.

In conclusion, the paper demonstrates that medium‑like effects are non‑negligible in low‑to‑intermediate pT jet analyses of pp and p‑Pb collisions. Properly accounting for UE activity—especially by selecting trigger particles with pT > 8 GeV/c—mitigates these biases. The results provide a benchmark for future experimental studies aiming to disentangle genuine jet quenching from UE‑driven modifications and suggest that collective phenomena, traditionally associated with heavy‑ion collisions, may also manifest in small systems when examined with sufficient sensitivity.