This Letter reports the first measurement of photonuclear D0 meson production in ultraperipheral
heavy ion collisions. The study is performed using lead-lead collision data, with an integrated luminosity
of 1.34 nb−1, collected by the CMS experiment at a nucleon-nucleon center-of-mass energy of 5.36 TeV.
Photonuclear events, where one of the colliding nuclei breaks up and the other remains intact, are selected
based on breakup neutron emissions and by requiring no particle activity in a large rapidity interval
in the direction of the photon-emitting nucleus. The D0 mesons are reconstructed via the D0 → K−πþ
decay channel, with the cross section measured as a function of D0 meson transverse momentum
and rapidity. The results are compared with next-to-leading-order perturbative QCD calculations that
employ recent parametrizations of the lead nuclear parton distribution functions, as well as with
predictions based on the color glass condensate framework. This measurement is the first photonuclear
collision study characterizing parton distribution functions of lead nuclei for parton fractional momenta
x (relative to the nucleon) ranging approximately from a few 10−4 to 10−2 for different hard energy
scale Q2 selections
