Measurements of jet quenching via hadron+jet correlations in Pb-Pb and high-particle multiplicity pp collisions with ALICE

Abstract

The ALICE Collaboration presents measurements of jet quenching in the 0--10\% most central Pb--Pb collisions at $\sqrt{s_{\text{NN}}} = 5.02~\text{TeV}$ and high multiplicity pp collisions at $\sqrt{s} = 13~\text{TeV}$ by investigating hadron-jet acoplanarity. In Pb--Pb collisions, the obtained acoplanarity distribution exhibits a marked suppression and narrowing when compared to the pp reference spectrum obtained from PYTHIA simulations. Similar measurements for pp collisions show that the acoplanarity distributions obtained in high multiplicity events are significantly suppressed and broadened relative to the analogous distributions from minimum bias events. The observed features are not caused by jet quenching, since they can be reproduced by PYTHIA 8 event generator, which does not account for jet quenching. Analysis of the PYTHIA events reveals that the suppression and broadening of the hadron-jet acoplanarity distributions are the consequence of a bias induced by the ALICE high multiplicity trigger. This trigger increases the probability to measure a high-$p_{\rm{T}}$ recoil jet in the pseudorapidity acceptance of the forward trigger detectors, and biases toward multi-jet final states.