  The transverse energy ($E_\mathrm{T}^{\gamma}$) spectra of photons isolated
from other particles are measured using proton-proton (pp) and lead-lead (PbPb)
collisions at the LHC at $\sqrt{s_\mathrm{NN}} =$ 5.02 TeV with integrated
luminosities of 27.4 pb$^{-1}$and 404 $\mu$b$^{-1}$ for pp and PbPb data,
respectively. The results are presented for photons with 25 $<$
$E_\mathrm{T}^{\gamma}$ $<$ 200 GeV in the pseudorapidity range $|\eta|$ $<$
1.44, and for different centrality intervals for PbPb collisions. Photon
production in PbPb collisions is consistent with that in pp collisions scaled
by the number of binary nucleon-nucleon collisions, demonstrating that photons
do not interact with the quark-gluon plasma. Therefore, isolated photons can
provide information about the initial energy of the associated parton in
photon+jet measurements. The results are compared with predictions from the
next-to-leading-order JETPHOX generator for different parton distribution
functions (PDFs) and nuclear PDFs (nPDFs). The comparisons can help to
constrain the nPDFs global fits.

High Energy Physics - Experiment;Nuclear Experiment