Evidence of medium response to hard probes using correlations of Z bosons with hadrons in heavy ion collisions

TL;DR

This study probes how the quark-gluon plasma responds to energy loss by a hard scattering by measuring Z-boson–tagged hadron correlations in PbPb collisions at 5.02 TeV, with pp data serving as a reference. By correlating a high-pT Z boson with charged hadrons in bins of hadron pT and angular differences, the authors observe significant modifications in PbPb relative to pp, including a depletion around Δφ ≈ 0 for 1–2 GeV hadrons and suppression on the jet side for 4–10 GeV hadrons, signaling medium-induced wake and recoil effects. Comparisons with jet-quenching models that include medium response (e.g., Jewel with recoil, Hybrid, Co-LBT) favor scenarios with medium recoil and diffusion-like holes, providing the first direct evidence of probe-induced medium response in Z-tagged events and offering new constraints on QGP transport and energy-deposition mechanisms.

Abstract

The first measurement of pseudorapidity and azimuthal angle distributions relative to the momentum vector of a Z boson for low transverse momentum ($p_\mathrm{T}$) charged hadrons in lead-lead (PbPb) collisions is presented. By studying the hadrons produced in an event with a high-$p_\mathrm{T}$ Z boson (40 $\lt$ $p_\mathrm{T}$ $\lt$ 350 GeV), the analysis probes how the quark-gluon plasma (QGP) medium created in these collisions affects the parton recoiling opposite to the Z boson. Utilizing PbPb data at a nucleon-nucleon center-of-mass energy $\sqrt{s_{_\mathrm{NN}}}$ = 5.02 TeV from 2018 with an integrated luminosity of 1.67 nb$^{-1}$ and proton-proton (pp) data at the same energy from 2017 with 301 pb$^{-1}$, the distributions are examined in bins of charged-hadron $p_\mathrm{T}$. A significant modification of the distributions for charged hadrons in the range 1 $\lt$ $p_\mathrm{T}$ $\lt$ 2 GeV in PbPb collisions is observed when compared to reference measurements from pp collisions. The data provide 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. When compared to theoretical predictions, the results are consistent with expectations of a hydrodynamic wake created when the QGP is depleted of energy by the parton propagating through it. Therefore, this Letter presents the first evidence of probe-induced energy depletion and the resulting response by the QGP medium.

Figures (4)

• Figure 1: The $\Delta\phi_{\mathrm{ch,Z}}\xspace$ spectra for events with Z boson $p_{\mathrm{T}}\xspace^{{ \mathup{{{Z}}{} _{ {}} ^{ {}}} }\xspace}\xspace > 40\,\text{Ge\spaceV}\xspace$ in ${ \mathup{{{p}}{} _{ {}} ^{ {}}} }\xspace{ \mathup{{{p}}{} _{ {}} ^{ {}}} }\xspace$ and $\text{PbPb}$ collisions. The filled circles (squares) are the $\text{PbPb}$ (${ \mathup{{{p}}{} _{ {}} ^{ {}}} }\xspace{ \mathup{{{p}}{} _{ {}} ^{ {}}} }\xspace$) data and the open circles (squares) are reflected data. The ${ \mathup{{{p}}{} _{ {}} ^{ {}}} }\xspace{ \mathup{{{p}}{} _{ {}} ^{ {}}} }\xspace$ data are identical across panels with different centrality intervals. The vertical bars and shaded boxes represent the statistical and systematic uncertainties, respectively. The results are presented in centrality intervals of 0--30% (top row, red), 30--50% (middle row, purple), and 50--90% (bottom row, yellow) and in the charged-hadron $p_{\mathrm{T}}\xspace^\text{ch}$ intervals of 1--2 (left column), 2--4 (middle column) and 4--10$\,\text{Ge\spaceV}$ (right column).
• Figure 2: The $\Delta y_{\mathrm{ch,Z}}\xspace$ spectra in the Z boson side ($\lvert \Delta\phi_{\mathrm{ch,Z}}\xspace \rvert<\pi/2$) for events with Z boson $p_{\mathrm{T}}\xspace^{{ \mathup{{{Z}}{} _{ {}} ^{ {}}} }\xspace}\xspace > 40\,\text{Ge\spaceV}\xspace$ in ${ \mathup{{{p}}{} _{ {}} ^{ {}}} }\xspace{ \mathup{{{p}}{} _{ {}} ^{ {}}} }\xspace$ and $\text{PbPb}$ collisions. The filled circles (squares) are the $\text{PbPb}$ (${ \mathup{{{p}}{} _{ {}} ^{ {}}} }\xspace{ \mathup{{{p}}{} _{ {}} ^{ {}}} }\xspace$) data and the open circles (squares) are reflected data. The vertical bars and shaded boxes represent the statistical and systematic uncertainties, respectively. The results are presented in centrality intervals of 0--30% (top row, red), 30--50% (middle row, purple), and 50--90% (bottom row, yellow) and in the charged-hadron $p_{\mathrm{T}}\xspace^\text{ch}$ intervals of 1--2 (left column), 2--4 (middle column) and 4--10$\,\text{Ge\spaceV}$ (right column). The ${ \mathup{{{p}}{} _{ {}} ^{ {}}} }\xspace{ \mathup{{{p}}{} _{ {}} ^{ {}}} }\xspace$ data are identical across panels with different centrality intervals.
• Figure 3: Upper row: Distributions of $\Delta\phi_{\mathrm{ch,Z}}$ in 0--30% centrality $\text{PbPb}$ collisions in the charged hadron $p_{\mathrm{T}}\xspace^\text{ch}$ intervals of 1--2 (left column), 2--4 (middle column) and 4--10$\,\text{Ge\spaceV}$ (right column). Lower row: The differences between the $\text{PbPb}$ and ${ \mathup{{{p}}{} _{ {}} ^{ {}}} }\xspace{ \mathup{{{p}}{} _{ {}} ^{ {}}} }\xspace$ distributions. The filled circles are $\text{PbPb}$ data and the open circles are reflected data. The vertical bars and shaded boxes represent the statistical and systematic uncertainties, respectively. The results are compared to predictions from theoretical models.
• Figure 4: Upper row: Distributions of $\Delta y_{\mathrm{ch,Z}}$ in 0--30% centrality $\text{PbPb}$ collisions in the charged hadron $p_{\mathrm{T}}\xspace^\text{ch}$ intervals of 1--2 (left column), 2--4 (middle column) and 4--10$\,\text{Ge\spaceV}$ (right column). Lower row: The differences between the $\text{PbPb}$ and ${ \mathup{{{p}}{} _{ {}} ^{ {}}} }\xspace{ \mathup{{{p}}{} _{ {}} ^{ {}}} }\xspace$ distributions. The filled circles are $\text{PbPb}$ data and the open circles are reflected data. The vertical bars and shaded boxes represent the statistical and systematic uncertainties, respectively. The results are compared to predictions from theoretical models.