The ALICE Collaboration has measured the azimuthal correlation between trigger isolated-prompt photons and associated charged hadrons in Pb$-$Pb collisions at the CERN LHC, at a centre-of-mass energy per nucleon pair of \snnfive. The trigger isolated-prompt photons are measured in the transverse-momentum range $18<~ p_{\rm T}^γ <~ 40$ GeV/$c$ and pseudorapidity range $|η^γ| <~0.67$. The isolation selection is based on a charged particle isolation momentum threshold $p_{\rm T}^{\rm iso, ch} = 1.5$ GeV/$c$ within a cone of radius $R=0.2$. The associated charged particles are measured in the transverse-momentum ranges $p_{\rm T}^{\rm h} > 1.8$ GeV/$c$ and pseudorapidity $|η^{\rm h}| <~0.9$. The yield D$(z_{\rm T})$ of associated hadrons per trigger, with $z_{\rm T} = p_{\rm T}^{\rm h}/p_{\rm T}^γ$, is measured in three Pb$-$Pb collision centrality classes: central (0$-$30%), semicentral (30$-$50%), and peripheral (50$-$90%). An approximation to the standard $I_{\rm AA}$ is computed from the D$(z_{\rm T})$ conditional yields, using NLO pQCD predictions as pp reference. A strong suppression of this ratio is observed in central collisions compared to peripheral collisions. The result extends to a lower $p_{\rm T}^γ$ relative to those reported in previously published Pb$-$Pb collisions measurements at $\sqrt{s_{\rm NN}}=5.02$ TeV. The measurement is compared to NLO pQCD calculations that include energy loss, and to the CoLBT-hydro model. The results from central collisions are also compared with measurements of jets correlated with isolated-prompt photons and of hadrons correlated with Z$^0$ bosons, both reported by the CMS Collaboration at the LHC, as well as with direct photon$-$hadron correlation measurements reported by the PHENIX and STAR Collaborations at RHIC.
Submitted to: EPJC
e-Print: arXiv:2605.02342 | PDF | inSPIRE
CERN-EP-2026-127
Figure group
Figure 1
Azimuthal correlation distributions in the 30--50\% centrality class of \PbPb collisions at \snnfive and for the interval $0.20\zt0.30$, for isolated-narrow clusters triggered in the range $20 \ptT 25$ \GeVc, with charged-particle tracks in the same collision (``Same event'', black-open circles) or in a different collision (``Mixed event'', red without marker). The result of subtracting the mixed-event distribution is shown in blue-full circles. The vertical bars indicate the statistical uncertainties. |  |
Figure 2
\ Azimuthal correlation distributions in the 30--50\% centrality class of \PbPb collisions at \snnfive for the intervals $0.20\zt0.30$ and $20$ \ptT $25$ \GeVc. Each marker corresponds to a different trigger: isolated-narrow clusters (blue-full circles), also shown in Fig. \ref{fig:DPhi_samemix}; isolated-wide clusters (violet-open squares), scaled by $1-P$, where $P$ is the purity; isolated-photon triggers (orange-empty crosses), calculated using the equation in the text The vertical bars indicate the statistical uncertainties. The dashed-green line at $|\Delta \varphi|=3/5\pi$ indicates the lower limit of the integration used in the \Dzt calculation. |  |
Figure 3
\ \Dzt distributions for isolated-prompt photon-hadron correlations measured in \PbPb collisions at \snnfive for three centrality classes: 0--30\% (left), 30--50\% (middle), and 50--90\% (right). The boxes and vertical lines represent the systematic and statistical uncertainties, respectively. Different theory predictions are shown: an NLO pQCD calculation including energy loss (pink line and uncertainty band), and from CoLBT-hydro (cyan-dashed line and uncertainty band). Also, an NLO pQCD prediction for \pp collisions at \sfive is shown (green line and uncertainty band). |  |
Figure 4
\ Ratio of the measured \Dzt distributions in \PbPb collision at \snnfive to the NLO pQCD predictions for \pp collisions at \sfive, \ipqcd, all distributions in denominator and numerator are shown in Fig. \ref{fig:Dzt}. Each panel shows a different centrality class: 0--30\% (left), 30--50\% (middle), and 50--90\% (right) The grey-filled boxes and vertical-black lines represent the data systematic and statistical uncertainties, respectively. The green-open boxes represent the $\mu$ scale uncertainty for the denominator NLO pQCD calculation for \pp collisions. Different theory predictions for the \iaa, which include energy loss, are shown: an NLO pQCD calculation (pink line and uncertainty band), and from a CoLBT-hydro calculation (cyan-dashed line and uncertainty band, not for peripheral collisions). Also, an NLO pQCD prediction, which includes CNM effects only (no energy loss, no centrality dependence), is reported (light orange line and uncertainty bands. |  |
Figure 5
\ \icp\ ratio of the \Dzt distributions for \PbPb collisions at \snnfive in Fig. \ref{fig:Dzt} for data: 0--30\% over 50--90\% (red squares), and 30--50\% over 50--90\% (violet bullets). The boxes represent the systematic uncertainties, while the vertical bars indicate the statistical uncertainties. Also, the equivalent ratios from the NLO pQCD calculation for \PbPb collisions, including energy loss, are shown as a band indicating the theory uncertainty. |  |
Figure 6
\ \ipqcd\ for central \PbPb collisions at \snnfive measured in ALICE for isolated-prompt $\gamma$--hadron correlations, and \iaa\ measured in CMS for isolated-prompt $\gamma$--jet correlations and \zz--hadron correlations also in central collisions. The boxes and vertical lines represent the systematic and statistical uncertainties, respectively. For ALICE, the blue-open boxes represent the $\mu$ scale uncertainty for the denominator NLO pQCD calculation for \pp collisions. |  |
Figure 7
\ Isolated-prompt $\gamma$--hadron correlations \ipqcd\ for central \PbPb collisions at \snnfive measured by the ALICE Collaboration, and \iaa\ for central \AuAu collisions at $\sqrt{s_{\rm NN}} = 200$ GeV measured by the PHENIX and STAR Collaborations at RHIC, also in central collisions. The boxes represent the systematic uncertainties, while the vertical bars indicate the statistical uncertainties. For ALICE, the blue-open boxes represent the $\mu$ scale uncertainty for the denominator NLO pQCD calculation for \pp collisions. |  |