Study of D * (2010) ± production in ep collisions at HERA

Abstract We report the first observation of charmed mesons with the ZEUS detector at HERA using the decay channel D ∗+ → (D 0 → K − π + )π + (+c.c.) . Clear signals in the mass difference ΔM=M( D ∗ )−M( D 0 ) as well as in the M ( Kπ ) distribution at the D 0 mass are found. The ep cross section for inclusive D ∗± production with Q 2 2 in the γp centre-of-mass energy range 115 W −7 +4 ) nb in the kinematic region pt (D ∗ ) ≥ 1.7 GeV, |η( D ∗ )| . Extrapolating outside this region, assuming a mass of the charm quark of 1.5 GeV, we estimate the ep charm cross section to be σ(ep → c c X) = (0.45 ± 0.11 −0.22 +0.37 ) μb at s =296 GeV and 〈 W 〉 = 198 GeV. The average γp charm cross section σ(γp → c c X) is found to be (6.3 ± 2.2 −3.0 +6.3 ) μb at 〈 W 〉 = 163 GeV and (16.9 ± 5.2 −8.5 +13.9 ) μb at 〈 W 〉 = 243 GeV. The increase of the total charm photoproduction cross section by one order of magnitude with respect to low energy data experiments is well described by QCD NLO calculations using singular gluon distributions in the proton.