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 $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV. 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 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, the $I_{\mathrm{pQCD}}$. 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 $I_{\mathrm{pQCD}}$ measurement is compared to $I_{\mathrm{AA}}$ calculations with an NLO pQCD model that includes energy loss, and to the CoLBT-hydro model. The $I_{\mathrm{pQCD}}$ results from central collisions are also compared with $I_{\mathrm{AA}}$ 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 $I_{\mathrm{AA}}$ measurements reported by the PHENIX and STAR Collaborations at RHIC.

26 pages, 7 captioned figures, 1 table, authors from page 21, published version, figures at http://alice-publications.web.cern.ch/node/13246