Abstract
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 $\rho'$ is of interest
as an intermediate mass state (between the $\rho$ and $J/\psi$) that is easier
to detect than the $\phi$.
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 $\rho'\rightarrow\pi^+\pi^-\pi^+\pi^-$ 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 $e$Pb data exhibit very similar $4\pi$ 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
$\rho'\rightarrow\pi^+\pi^-\pi^+\pi^-$ 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.