Search for jet quenching effects in high-multiplicity pp collisions at $\sqrt{s}$ = 13 TeV via di-jet acoplanarity

TL;DR

This study investigates jet quenching effects in high-multiplicity proton-proton collisions at 13 TeV by analyzing di-jet azimuthal distributions, finding that observed broadening is due to event selection bias rather than quenching.

Contribution

It demonstrates that azimuthal broadening in high-multiplicity pp collisions is caused by selection bias towards multi-jet events, not jet quenching.

Findings

Azimuthal broadening observed in high-multiplicity events.

Broadening also appears in PYTHIA 8 simulations without quenching.

Bias towards multi-jet events explains the broadening.

Abstract

The ALICE Collaboration reports a search for jet quenching effects in high-multiplicity (HM) proton$-$proton collisions at $\sqrt{s}$ = 13 TeV, using the semi-inclusive azimuthal-difference distribution $\Delta\varphi$ of charged-particle jets recoiling from a high transverse momentum (high-$p_{\mathrm{T,trig}}$) trigger hadron. Jet quenching may broaden the $\Delta\varphi$ distribution measured in HM events compared to that in minimum bias (MB) events. The measurement employs a $p_{\mathrm{T,trig}}$-differential observable for data-driven suppression of the contribution of multiple partonic interactions, which is the dominant background. While azimuthal broadening is indeed observed in HM compared to MB events, similar broadening for HM events is observed for simulations based on the PYTHIA 8 Monte Carlo generator, which does not incorporate jet quenching. Detailed analysis of these data and simulations show that the azimuthal broadening is due to bias of the HM selection towards events with multiple jets in the final state. The identification of this bias has implications for all jet quenching searches where selection is made on the event activity.