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, \( \sqrt{s} \), 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 GeV2. The dependence on \( \sqrt{s} \) was studied by normalising to the high-statistics measurement at \( \sqrt{s}=318 \) 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 \( \sqrt{s} \) dependence of the data well.