Photoproduction of J/psi in hadronic collisions
Heavy nuclei at ultrarelativistic speeds are sources of intense electromagnetic fields, interacting with the other nucleus and leading to photoproduction of charmonium states at very low transverse momenta. A very strong excess of J/psi mesons in this transverse momentum region has been observed by ALICE in peripheral (hadronic) to ultraperipheral Pb-Pb collisions, with and without nuclear overlap.
Detailed studies of this phenomenon might offer information on both hot and cold medium effects. The charmonium photoproduction cross section is sensitive to the gluon nuclear parton distribution function, which depends on initial-state CNM effects like nuclear shadowing and / or gluon saturation. Furthermore, photoproduced charmonia will interact with the plasma, and the differential yield of this very distinctive component as a function of transverse momentum and centrality, plus its elliptic flow, could be exploited as a relatively clean probe of final-state hot matter (QGP) effects.