Exotic charmonium and light-by-light scattering in heavy ion collisions

International audienceThe internal structure of exotic charmonium (R) states isstill a subject of debate. Some of the popular pictures used to describe themare tetraquark, meson molecule or mixtures of those with charmonium.One way to shed light into the nature of these states is to study theirradiative decays. We expect that their decay widths intotwo photons will vary from one configuration to another. Thedirect experimental measurement of these widths in the reaction$R \to \gamma \gamma$ is difficult. However, they manifest themselvesalso indirectly in the production, through photon-photon fusion, of theresonance $R$ in ultraperipheral nucleus-nucleus collisions (UPHIC), where theproduction cross section is directly proportional to the decay width$\Gamma (R \to \gamma \gamma)$. Once produced, the resonance $R$ willdecay mostly in hadronic channels but it may also decay into a photon pair.In this case, the cross section will be proportional to thesquared $2 \gamma$ decay width, being even more sensitive tothe internal structure of the resonance. In this note we review thepredictions for exotic charmonium production in UPHIC and present,for the first time, estimatesof the production cross section of exotic states which decay into twophotons