Search for quasi-particle scattering in the quark-gluon plasma with jet splittings in pp and Pb$-$Pb collisions at $\mathbf{\sqrt{\textit{s}_{\rm NN}} = 5.02}$ TeV

TL;DR

This study investigates jet substructure in proton-proton and lead-lead collisions at 5.02 TeV to explore quasi-particle scattering effects in the quark-gluon plasma, revealing medium-induced narrowing contrary to some theoretical predictions.

Contribution

It provides the first detailed comparison of jet substructure measurements with models including and excluding quasi-particle scattering effects in the quark-gluon plasma.

Findings

Observation of medium-induced narrowing of high-$k_T$ splittings in Pb-Pb collisions.

Contradiction of expected yield enhancement from quasi-particle scattering.

Comparison with theoretical models enhances understanding of jet quenching mechanisms.

Abstract

The ALICE Collaboration reports measurements of the large relative transverse momentum ($k_{\text{T}}$) component of jet substructure in pp and Pb$-$Pb collisions at center-of-mass energy per nucleon pair $\sqrt{s_{\text{NN}}}=5.02$ TeV. Enhancement in the yield of such large-$k_{\text{T}}$ emissions in head-on Pb$-$Pb collisions is predicted to arise from partonic scattering with quasi-particles of the quark-gluon plasma. The analysis utilizes charged-particle jets reconstructed by the anti-$k_{\text{T}}$ algorithm with resolution parameter $R=0.2$ in the transverse-momentum interval $60 < p_{\text{T,ch jet}} < 80$ GeV/$c$. The soft drop and dynamical grooming algorithms are used to identify high transverse momentum splittings in the jet shower. Comparison of measurements in Pb$-$Pb and pp collisions shows medium-induced narrowing, corresponding to yield suppression of high-$k_{\text{T}}$ splittings, in contrast to the expectation of yield enhancement due to quasi-particle scattering. The measurements are compared to theoretical model calculations incorporating jet modification due to jet-medium interactions ("jet quenching"), both with and without quasi-particle scattering effects. These measurements provide new insight into the underlying mechanisms and theoretical modeling of jet quenching.