Identify photon emitter and target nucleus in coherent UPC photoproduction

Determine, in coherent photonuclear vector meson photoproduction during ultraperipheral heavy-ion collisions, which of the two colliding nuclei emits the quasi-real photon and which nucleus acts as the scattering target, in order to clarify the origin of femtometer-scale quantum mechanical interference between the production amplitudes.

Background

Ultraperipheral heavy-ion collisions feature intense electromagnetic fields described as fluxes of quasi-real photons, enabling photonuclear production of vector mesons. In the coherent case, the photon couples to the nucleus as a whole, producing a meson with low transverse momentum.

Because either nucleus can serve as the photon source or as the target, there is an intrinsic ambiguity that leads to quantum mechanical interference between the two production amplitudes. This ambiguity is central to femtometer-scale interference effects and underlies the azimuthal anisotropies investigated in this measurement.