Measurement of isolated-prompt photon$-$hadron correlations in Pb$-$Pb collisions at $\sqrt{{\text{s}}_{\mathrm{NN}}}=5.02$ TeV

Abstract

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 $18<p_{\mathrm{T}}^{\gamma }<40$ GeV/$c$ and pseudorapidity range $|{\eta }^{\gamma }|<0.67$. The isolation selection is based on a charged particle isolation momentum threshold $p_{\mathrm{T}}^{\mathrm{iso},\mathrm{ch}}=1.5$ GeV/$c$ within a cone of radius $R=0.2$. The associated charged particles are measured in the transverse-momentum ranges $p_{\mathrm{T}}^{\mathrm{h}}>1.8$ GeV/$c$ and pseudorapidity $|{\eta }^{\mathrm{h}}|<0.9$. The yield D$(z_{\mathrm{T}})$ of associated hadrons per trigger, with $z_{\mathrm{T}}=p_{\mathrm{T}}^{\mathrm{h}}/p_{\mathrm{T}}^{\gamma }$, is measured in three Pb$-$Pb collision centrality classes: central (0$-$30%), semicentral (30$-$50%), and peripheral (50$-$90%). An approximation to the standard $I_{\mathrm{AA}}$ is computed from the D$(z_{\mathrm{T}})$ 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 $p_{\mathrm{T}}^{\gamma }$ relative to those reported in previously published Pb$-$Pb collisions measurements at $\sqrt{s_{\mathrm{NN}}}=5.02$ 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.

Comments: 25 pages, 7 captioned figures, 1 table, authors from page 20, submitted to EPJC, figures at http://alice-publications.web.cern.ch/node/13246