Exclusive electroproduction of $\rho^0$ and $J/\psi$ mesons at HERA

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Exclusive production of $\rho^0$ and $J/\psi$ 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 $\rho^0$ data and $2 < Q^2 < 40$ GeV $^2$ , $50 < W < 150$ GeV for the $J/\psi$ data. Cross sections for exclusive $\rho^0$ and $J/\psi$ 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 $\rho^0$ production as well as $r^{04}_{00}$ and $r^{04}_{1-1}$ for exclusive $J/\psi$ production. The results are discussed in the context of theoretical models invoking soft and hard phenomena.

Received: 20 August 1998 / Published online: 14 January 1999