Exclusive vector meson production at high energies and gluon saturation

We systematically study exclusiv2 diffractive (photo)production of vector mesons ($J/\ensuremath{\psi}$, $\ensuremath{\psi}(2s)$, $\ensuremath{\phi}$, and $\ensuremath{\rho}$) off protons in high-energy collisions and investigate whether the production is a sensitive probe of gluon saturation. We confront saturation-based results for diffractive $\ensuremath{\psi}(2s)$ and $\ensuremath{\rho}$ production at HERA and $J/\ensuremath{\psi}$ photoproduction with all available data, including recent work from HERA, ALICE, and LHCb, finding good agreement. In particular, we show that the $t$ distribution of differential cross sections of photoproduction of vector mesons offers a unique opportunity to discriminate among saturation and nonsaturation models. This is due to the emergence of a pronounced dip (or multiple dips) in the $t$ distribution of diffractive photoproduction of vector mesons at relatively large, but potentially accessible $|t|$ that can be traced back to the unitarity features of color dipole amplitude in the saturation regime. We show that in saturation models the dips in $t$ distribution recede towards lower $|t|$ with decreasing mass of the vector meson, increasing energy or decreasing Bjorken-$x$, and decreasing virtuality $Q$. We provide various predictions for exclusive (photo)production of different vector mesons including the ratio of $\ensuremath{\psi}(2s)/J/\ensuremath{\psi}$ at HERA, the LHC, and future colliders.