Vector Meson Photoproduction in Ultra-Peripheral Ion–Ion Collisions

Abstract The strong electromagnetic fields accompaning heavy ions accelerated at BNL and LHC lead to vector meson photoproduction in the ultra-peripheral collisions (UPC), where there is no overlap between the colliding nuclei.We point out the importance of ultra-peripheral photoproduction of vector meson in the Coulomb field of nuclei as a means of measuring the radius of the neutral vector meson due to possibility of two photon exchange. This new contribution to the production amplitude is small compared to the conventional diffractive amplitude, but because of large impact parameters inherent to the ultra-peripheral Coulomb mechanism its impact on the slope in the transfer momenta distribution is substantial. We predict appreciable and strongly energy dependent increase of the slope in the vector meson distribution towards very small momentum transfer.The magnitude of the effect is proportional to the mean radius squared of the vector meson and is within the reach of high precision photoproduction experiments, which gives a unique experimental handle on the size of vector meson.