Vector Mesons and Heavy Flavors at Hera

On behalf of the H1 and ZEUS Collaborations At HERA 27.5 GeV positrons collide with 820 GeV protons. The relevant kine-matical parameters are an ep center-of-mass energy p s = 300 GeV and a photon virtuality in the range 0 < Q 2 < 10 5 GeV 2. Given the intense ux of photons emitted by the positron beam, photon-proton collisions can be studied for p center-of-mass energies 20 < Wp < 300 GeV. In the rst part results on diiractive production of vector mesons (from to) are presented. The second part is dedicated to the study of non-diiractive heavy avor production: results on inelastic J== photo-production and D photo-and electro-production are presented. 1 Diiractive vector meson production Elastic photo-production of vector mesons was studied in xed target experiments up to a center-of-mass energy of about 20 GeV and was successfully explained in the framework of the Vector Dominance Model (VDM) 1 and Regge theory 2. According to this model, the photon can uctuate in a vector meson (having the same quantum numbers) and the interaction, becoming purely hadronic, proceeds via the exchange of a soft Pomeron trajec-tory that in the Donnachie-Landshoo parameterization can be written as 3 IP (t) = IP (0) + 0 IP t = 1:08 + 0:25 t where t is the square of the momentum transfer at the proton vertex. Feature of the soft diiractive processes is the weak energy dependence of the cross section (besides the shrinkage of the forward peak and the s-channel helicity conservation that are not investigated in this paper). In presence of a hard scale QCD models can be applied to get quantitative predictions of diiractive processes. In these models the photon uctuates in a q q pair interacting with the proton via the exchange of a gluon ladder giving a stronger energy dependence of the cross-section, due to the rise in xg(x) when x becomes small, since x Q 2 =W 2 p and (p) / (xg(x)) 2. At HERA the investigation has been extended to higher energies, for different vector mesons and photon virtualities. The transition from soft to hard regime and the limits of applicability of pQCD have been investigated.