Measurement of D ∗ photoproduction at three different centre-of-mass energies at HERA

The photoproduction of D ∗± mesons has been measured with the ZEUS detector at HERA at three different ep centre-of-mass energies s $$\sqrt{s}$$ of 318, 251 and 225 GeV. For each data set D ∗± mesons were required to have transverse momentum p T D  ∗ and pseudo-rapidity η D ∗ in the ranges 1 . 9 < p T D  ∗ < 20 GeV and | η D ∗ | < 1 . 6. The events were required to have a virtuality of the incoming photon Q 2 of less than 1 GeV 2 . The dependence on s $$\sqrt{s}$$ was studied by normalising to the high-statistics measurement at s = 318 $$\sqrt{s}=318$$ GeV. This led to the cancellation of a number of systematic effects both in data and theory. Predictions from next-to-leading-order QCD describe the s $$\sqrt{s}$$ dependence of the data well