The powerful photon fluxes of relativistic nuclei provide a possibility
to study photonuclear and two-photon interactions in ultra-peripheral collisions (UPC)
where the nuclei do not overlap and no strong nuclear interactions occur.
Within Vector-meson Dominance Model (VDM), $\rho^0$ contribution prevails in QCD photon structure
function and gamma+A -> $\rho^0$ +A process in heavy-ion UPC is a tool to test, so-called, black disk regime
where the target nuclei appears like a black disk and the total $\rho^0$ +A cross section reaches its limit.
RHIC and first LHC results have deviated from some Glauber+VDM calculations, which thus call for new data.
ALICE reports measurements on $\rho^0$ photoproduction cross sections
in Pb-Pb UPC with data taken at sqrt(NN)=2.76 TeV and new measurements with the data
taken at sqrt(NN)=5.02 TeV. The mid-rapidity cross section of coherent $\rho^0$ photoproduction is measured,
and it is compared to theoretical models. The measurements are presented for various nuclei
breakup scenarios - without electromagnetic dissociation (EMD) of nuclei and with single or mutual EMD as well.
|Preferred Track||Initial State Physics and Approach to Equilibrium|