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

TL;DR

Understanding jet–medium interplay in small systems, this work compares three state-of-the-art event generators to ALICE data by analyzing regionally defined jet-like yields from $p_{\mathrm{T}}^{\mathrm{trig}}$-dependent charged-particle densities. It shows that medium-like mechanisms (CR, hydro, string melting) induce notable modifications in the jet-like signal at low to intermediate $p_{\mathrm{T}}^{\mathrm{trig}}$, with AMPT's string-melting yielding effects across the full range and reproducing the pp/p--Pb dip observed in data. The study emphasizes that high-$p_{\mathrm{T}}^{\mathrm{trig}}$ triggers minimize underlying-event biases in small systems and provides guidance for future jet studies and trigger choices, including potential identified triggers and higher-energy analyses.

Abstract

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 ($|Δ\varphi|<π/3$) after subtracting that in the transverse region ($π/3<|Δ\varphi|<2π/3$). 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.

Figures (6)

• Figure 1: Illustration of the toward ($|\Delta\phi| < 60^\circ$), transverse ($60^\circ < |\Delta\phi| < 120^\circ$), and away ($|\Delta\phi| > 120^\circ$) regions in the azimuthal plane with respect to the leading particle direction, showing example particles in toward, transverse and away regions.
• Figure 2: Charged-particle number density as a function of $p_{\rm T}^{\rm trig}$ measured in pp collisions at $\sqrt{s}$ = 5.02 TeV for three topological regions; toward side (left panel), transverse side (middle panel) and away side (right panel). We have compared results from PYTHIA8, AMPT and EPOS4 with the ALICE experimental data ALICE:2022fnb.
• Figure 3: Leading particle transverse momentum spectra in pp collisions at $\sqrt{s}$ = 5.02 TeV for Pythia8 (left panel), AMPT (middle panel) and EPOS4 (right panel). The green solid line in the lower pads show the fit to the ratio in the range $8 \leq p_{\rm T}^{\rm trig} \leq 20$ GeV/c.
• Figure 4: Charged-particle number density for jet-like signal in the toward region as a function of $p_{\rm T}^{\rm trig}$ for pp collisions at $\sqrt{s}$ = 5.02 TeV estimated from Pythia8 (left panel), AMPT (middle panel) and EPOS4 (right panel). The green solid line in the lower pads show the fit to the ratio in the range $8 \leq p_{\rm T}^{\rm trig} \leq 20$ GeV/c.
• Figure 5: Charged-particle number density as a function of $p_{\rm T}^{\rm trig}$ measured in p--Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV for three topological regions; toward side (left panel), transverse side (middle panel) and away side (right panel). We have compared results from AMPT with the ALICE experimental data ALICE:2022fnb.