ALICE shines light inside the proton and nuclei

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The strong electromagnetic fields generated by ultrarelativistic heavy ions provide the possibility to study photon-induced interactions at the LHC in new kinematic regions. Vector mesons are copiously produced in ultra-peripheral collisions where the impact parameter is larger than the sum of the radii of the two projectiles, implying that electromagnetic induced processes become dominant. Light vector meson photoproduction cross sections can be used to study Glauber-Gribov shadowing effects at the hadronic level while the photoproduction of heavy vector mesons is expected to be sensitive to the poorly known gluon structure function and gluon shadowing effects at low Bjorken $x$. Photon-induced reactions in ultra-peripheral collisions (UPCs) at the LHC have been studied using the ALICE detector for several years. The ALICE Collaboration has published the first measurements of the $\rho^0$, $J/\psi$ and $\psi(2S)$ photoproduction cross section in ultra-peripheral Pb-Pb collisions and that for exclusive $J/\psi$ photoproduction off protons in ultra-peripheral proton-lead collisions at the LHC and has obtained a substantially larger data set in 2015-2018 from LHC Run 2. These new data allow us to cover a larger kinematic domain with smaller experimental uncertainties than in the past, and to explore new observables.

First measurements of the $\rho^0$ photoproduction cross section in Xe-Xe UPCs and the $t$-dependent photonuclear cross section of $J/\psi$ in Pb-Pb UPCs, both at midrapidity, unveil new routes to disentangle the different $\gamma$-Pb contributions to the UPCs cross sections and to understand the high-energy limit of QCD via shadowing or saturation, respectively. Additionally, new ALICE results on rapidity-differential cross section measurements for different nuclear-breakup classes in Pb-Pb at $\sqrt{s_{NN}} = 5.02$ TeV will be presented. The results on $J/\psi$ and $\psi(2S)$ cross sections at midrapidity are complementary to previous findings in the…