Observation of medium-induced yield enhancement and acoplanarity broadening of low-$p_\mathrm{T}$ jets from measurements in pp and central Pb$-$Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV

TL;DR

This study measures how the quark-gluon plasma medium enhances low-$p_T$ jet yields and broadens acoplanarity in Pb-Pb collisions at 5.02 TeV, revealing medium response effects through recoil jet distributions.

Contribution

It provides the first detailed measurement of medium-induced yield enhancement and acoplanarity broadening of low-$p_T$ jets in heavy-ion collisions at this energy.

Findings

Observed significant low-$p_T$ jet yield enhancement in Pb-Pb collisions.

Detected broadening of jet acoplanarity relative to pp collisions.

Model comparisons suggest the enhancement is due to QGP medium response.

Abstract

The ALICE Collaboration reports the measurement of semi-inclusive distributions of charged-particle jets recoiling from a high transverse momentum (high $p_{\rm T}$) hadron trigger in proton$-$proton and central Pb$-$Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV. A data-driven statistical method is used to mitigate the large uncorrelated background in central Pb$-$Pb collisions. Recoil jet distributions are reported for jet resolution parameter $R=0.2$, 0.4, and 0.5 in the range $7 < p_{\rm T,jet} < 140$ GeV$/c$ and trigger$-$recoil jet azimuthal separation $\pi/2 < \Delta\varphi < \pi$. The measurements exhibit a marked medium-induced jet yield enhancement at low $p_{\rm T}$ and at large azimuthal deviation from $\Delta\varphi\sim\pi$. The enhancement is characterized by its dependence on $\Delta\varphi$, which has a slope that differs from zero by 4.7$\sigma$. Comparisons to model calculations incorporating different formulations of jet quenching are reported. These comparisons indicate that the observed yield enhancement arises from the response of the QGP medium to jet propagation.