Search for jet quenching effects in high-multiplicityproton-proton collisions at $\sqrt {s}$ = 13 TeV

The ALICE Collaboration reports results of a novel approach to jet-quenching measurements in high-multiplicity pp collisions at $\sqrt{s}=13$ TeV, searching for broadening of the acoplanarity distribution measured by the semi-inclusive distribution of jets recoiling from a high-$p_{\rm T}$ hadron. Charged-particle jet reconstruction is carried out using the anti-$k_{\rm T}$ algorithm with $R=0.4$. A data-driven statistical method is used to correct the recoil jet yield for uncorrelated background, including multi-partonic interactions. High-multiplicity (HM) pp events are selected based on charged-particle multiplicity registered in forward scintillator detectors, and their acoplanarity distributions are compared to those for Minimum Bias (MB) events. Significant broadening is observed in the acoplanarity distribution of HM events, consistent with jet quenching. However, qualitatively similar features are also seen in a simulated population of pp collisions generated by PYTHIA 8 Monash, which does not incorporate jet quenching. We utilize these PYTHIA simulations to explore the origin of the observed effect. The PYTHIA simulations suggest that the enhanced acoplanarity results from the bias induced by the HM selection towards multi-jet final states.