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 $\sqrt{s_{\mathrm{NN}}}$ = 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 ($p_{\mathrm{T}}^{\mathrm{trig}}$). The jet-like yield is defined by the particle density in the toward region ($|\Delta \phi |<\pi /3$) after subtracting that in the transverse region ($\pi /3<|\Delta \phi |<2\pi /3$). 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 $p_{\mathrm{T}}^{\mathrm{trig}}$ in the interval 0.5-20\,GeV/$c$. Calculations are performed for the $p_{\mathrm{T}}$ 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 $p_{\mathrm{T}}^{\mathrm{trig}}$ ($<8$\,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 $p_{\mathrm{T}}^{\mathrm{trig}}$ 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 $p_{\mathrm{T}}^{\mathrm{trig}}$ 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-$p_{\mathrm{T}}^{\mathrm{trig}}$ ($p_{\mathrm{T}}^{\mathrm{trig}}>$ 8 GeV/$c$) for minimizing underlying event biases in jet-related studies.

9 pages, 6 captioned figures

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