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

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 ($|\Delta\varphi|<\pi/3$) after subtracting that in the transverse region ($\pi/3<|\Delta\varphi|<2\pi/3$). 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 ($|\eta| < 0.8$). 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.