Abstract
We report the first observation of charmed mesons with the ZEUS detector at HERA using the decay channel D*(+) --> (D-0 --> K(-)pi(+)) pi(+) (+ c.c.). Clear signals in the mass difference Delta M=M(D*)-M(D-0) as well as in the M(K pi) distribution at the D-0 mass are found. The ep cross section for inclusive D*(+/-) production with Q(2) < 4 GeV2 in the gamma p centre-of-mass energy range 115 < W < 275 GeV has been determined to be (32+/-7(-7)(+4)) nb in the kinematic region {pr(D*) greater than or equal to 1.7 GeV, \pi(D*)\ < 1.5}. Extrapolating outside this region, assuming a mass of the charm quark of 1.5 GeV, we estimate the ep charm cross section to be sigma(ep --> c $($) over bar$$ cX)=(0.45+/-0.11(-0.22)(+0.37)) mu b at root s=296 GeV and [W]=198 GeV. The average gamma p charm cross section sigma(gamma p --> c $($) over bar$$ cX) is found to be (6.3+/-2.2(-3.0)(+6.3)) mu b at [W]=163 GeV and (16.9+/-5.2(-8.5)(+13.9)) mu b 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.