Photoproduction and detection of '^+^-^+^- decays in ultra-peripheral collisions and at an electron-ion collider

Vector meson photoproduction is an important probe of nuclear structure. Light vector mesons are most sensitive to low-x structure, as long as they are not too light for perturbative QCD calculations. The ' is of interest as an intermediate mass state (between the and J/) that is easier to detect than the. Using HERA data on proton targets, we make projections for lead/gold targets in UPCs at the Large Hadron Collider and RHIC, and for ep and eA collisions at a future Electron-Ion Collider (EIC). These projections for ion targets depend on the largely-unknown '^+^-^+^- branching ratio, and use existing data to constrain that branching ratio. Current data points to a relatively low branching ratio, less than 50\%. The HERA ep and ALICE UPC ePb data exhibit very similar 4 mass spectra, indicating that, if the system is composed of two resonances, the products of their photon couplings with their four-pion branching ratios are similar. The predicted rates are high for both UPCs and the EIC. The '^+^-^+^- decay can be observed at the EIC with high efficiency. In ep collisions at the highest energy, the forward B0 detector is needed to observe this channel down to the lowest achievable Bjorken-x values.