Evidence for no shrinkage in elastic photoproduction of J/Ψ

The differential cross section dσ/dt of elastic photoproduction of J/Ψ at different γp center of mass energies W are used to determine the properties of the exchanged trajectory in the reaction γp→J/Ψp. In the region |t|< 1 GeV2, the resulting trajectory is α(t) = (1.153 ± 0.027) + (−0.001 ± 0.072)t. The vanishing value of the slope of the trajectory is an experimental evidence for no shrinkage and thus for the perturbative nature of the exchange.