Speaker
Description
Title: Evidence of the medium response to hard probes with Z-hadron correlations in PbPb and pp collisions at \sqrt{s_{NN}} = 5.02 TeV
Abstract: The first measurement of low transverse momentum (p_{T}) charged hadron pseudorapidity and azimuthal angle distributions relative to Z bosons in PbPb collisions at \sqrt{s_{NN}} = 5.02 TeV is presented. This study utilizes PbPb collision data recorded in 2018 with an integrated luminosity of $1.67 \pm 0.03$ nb^{-1}, as well as pp collision data acquired in 2017 with an integrated luminosity of 301 \pm 6 pb^{-1}. For the first time in PbPb collisions, the azimuthal angle and pseudorapidity distributions of charged hadrons relative to Z bosons are measured in bins of charged hadron p_{T} to search for in-medium parton shower modifications and medium recoil effects. The analysis focuses on events containing at least one Z boson with 40<p_{T} <350 GeV. In events with more than one Z, the highest p_{T} boson is used for correlation analysis.
A significant modification in the azimuthal angle and pseudorapidity distributions for charged hadrons in the low p_{T} range, around 1 to 2 GeV, is observed compared to reference measurements from pp collisions. The results are consistent with expectations from phenomenological models, including medium recoil and the medium response to hard probes traversing the quark-gluon plasma. The data provide significant new information about the correlation between hard
and soft particles in heavy ion collisions, which can be used to test predictions of various jet quenching models. In data comparisons with models, the first evidence for medium recoil and hole effects caused by a hard probe is found.
About the speaker:
Yen-Jie Lee is an experimental particle physicist specializing in proton-proton and heavy-ion collisions. Using the Large Hadron Collider, Lee explores matter under extreme conditions to shed light on the early universe and neutron stars. His research on jets and heavy-flavor particle production enhances understanding of quark-gluon plasma and heavy nuclei structures. He also pioneered studies of high-density QCD with electron-position annihilation data. Lee completed his undergraduate and Master’s degrees at National Taiwan University and earned his Ph.D. from MIT in 2011. After a fellowship at CERN, he joined the MIT Physics faculty in 2013. He has held leadership roles in the CMS collaboration and received prestigious awards, including the DOE Early Career Award, Sloan Fellow, and the Presidential Early Career Award for Scientists and Engineers. He was promoted to Professor of Physics in 2024.
