Exclusive Electroproduction of $ρ^0$ and $J/ψ$ Mesons at HERA
ZEUS Collaboration
TL;DR
This ZEUS study measures exclusive electroproduction of ρ^0 and J/ψ mesons over broad Q^2 and W ranges, extracting cross sections, t-slope parameters, and decay angular distributions. By comparing Regge/VMD expectations with perturbative QCD and dipole models, it reveals a transition from soft to hard diffractive dynamics, with J/ψ production showing strong, gluon-density–driven W dependences and rho^0 exhibiting rising longitudinal contributions at higher Q^2. Spin-density matrix analyses support s-channel helicity conservation and natural-parity exchange, while forward longitudinal cross sections provide stringent tests for pQCD predictions and gluon density parameterisations. Overall, the results demonstrate the differing roles of Q^2 and M_V in setting the interaction scale and underscore the relevance of perturbative approaches for heavy vector mesons and high-Q^2 light-vector meson production.
Abstract
Exclusive production of $ρ^0$ and $J/ψ$ mesons in e^+ p collisions has been studied with the ZEUS detector in the kinematic range $0.25 < Q^2 < 50 GeV^2, 20 < W < 167 GeV$ for the $ρ^0$ data and $2 < Q^2 < 40 GeV^2, 50 < W < 150 GeV$ for the $J/ψ$ data. Cross sections for exclusive $ρ^0$ and $J/ψ$ production have been measured as a function of $Q^2, W$ and $t$. The spin-density matrix elements $r^{04}_{00}, r^1_{1-1}$ and $Re r^{5}_{10}$ have been determined for exclusive $ρ^0$ production as well as $r^{04}_{00}$ and $r^{04}_{1-1}$ for exclusive $J/ψ$ production. The results are discussed in the context of theoretical models invoking soft and hard phenomena.